3 Minerals

Learning Objectives

By the end of this chapter, students should be able to:

  • Define .
  • Describe the basic structure of the atom.
  • Derive basic atomic information from the Periodic Table of Elements.
  • Describe chemical related to .
  • Describe the main ways form.
  • Describe the and how it forms common .
  • List common non- in , , , and groups.
  • Identify using physical properties and identification tables.

The term “minerals” as used in nutrition labels and pharmaceutical products is not the same as a in a geological sense. In geology, the classic definition of a is: 1) naturally occurring, 2) inorganic, 3) solid at room , 4) regular crystal structure, and 5) defined chemical . Some natural substances technically should not be considered , but are included by exception. For example, water and mercury are liquid at room . Both are considered because they were classified before the room- rule was accepted as part of the definition. is quite often formed by organic processes, but is considered a because it is widely found and geologically important. Because of these discrepancies, the International Mineralogical Association in 1985 amended the definition to: “A is an or chemical compound that is normally crystalline and that has been formed as a result of geological processes.” This means that the in the shell of a clam is not considered a . But once that clam shell undergoes burial, , or other geological processes, then the is considered a . Typically, substances like , pearl, opal, or that do not fit the definition of are called mineraloids.

A rock is a substance that contains one or more or mineraloids. As is discussed in later chapters, there are three types of rocks composed of : (rocks crystallizing from molten material), sedimentary (rocks of products of (sand, gravel, etc.) and (things from ), and (rocks produced by alteration of other rocks by heat and pressure.

3.1 Chemistry of Minerals

Rocks are of that have a specific chemical . To understand chemistry, it is essential to examine the fundamental unit of all matter, the atom.

3.1.1 The Atom


Video 3.1: Atomic orbitals.

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Matter is made of atoms. Atoms consists of subatomic particles—protons, neutrons, and electrons. A simple model of the atom has a central nucleus composed of protons, which have positive charges, and neutrons which have no charge. A cloud of negatively charged electrons surrounds the nucleus, the number of electrons equaling the number of protons thus balancing the positive charge of the protons for a neutral atom. Protons and neutrons each have a mass number of 1. The mass of an electron is less than 1/1000th that of a proton or neutron, meaning most of the atom’s mass is in the nucleus.

3.1.2 Periodic Table of the Elements

Matter is composed of which are atoms that have a specific number of protons in the nucleus. This number of protons is called the Atomic Number for the . For example, an oxygen atom has 8 protons and an iron atom has 26 protons. An cannot be broken down chemically into a simpler form and retains unique chemical and physical properties. Each behaves in a unique manner in nature. This uniqueness led scientists to develop a periodic table of the , a tabular arrangement of all known listed in order of their atomic number.

The periodic table of the elements.
Figure 3.1: The periodic table of the elements.

The first arrangement of into a periodic table was done by Dmitri Mendeleev in 1869 using the known at the time. In the periodic table, each has a chemical symbol, name, atomic number, and atomic mass. The chemical symbol is an abbreviation for the , often derived from a Latin or Greek name for the substance. The atomic number is the number of protons in the nucleus. The atomic mass is the number of protons and neutrons in the nucleus, each with a mass number of one. Since the mass of electrons is so much less than the protons and neutrons, the atomic mass is effectively the number of protons plus neutrons.

Formation of carbon-14 from nitrogen-14.
Figure 3.2: Formation of carbon-14 from nitrogen-14.

The atomic mass of natural represents an average mass of the atoms comprising that substance in nature and is usually not a whole number as seen on the periodic table, meaning that an exists in nature with atoms having different numbers of neutrons. The differing number of neutrons affects the mass of an in nature and the atomic mass number represents this average. This gives rise to the concept of  are forms of an with the same number of protons but different numbers of neutrons. There are usually several for a particular . For example, 98.9% of carbon atoms have 6 protons and 6 neutrons. This of carbon is called carbon-12 (12C). A few carbon atoms, carbon-13 (13C), have 6 protons and 7 neutrons. A trace amount of carbon atoms, carbon-14 (14C), has 6 protons and 8 neutrons.

Oxygen and silicon make up 3/4ths of the chart.
Figure 3.3: Element abundance pie chart for Earth’s crust.

Among the 118 known , the heaviest are fleeting human creations known only in high energy particle accelerators, and they decay rapidly. The heaviest naturally occurring is uranium, atomic number 92. The eight most abundant elements in Earth’s are shown in Table 1. These are found in the most common rock forming .

Element Symbol Abundance %
Oxygen O 47%
Silicon Si 28%
Aluminum Al 8%
Iron Fe 5%
Calcium Ca 4%
Sodium Na 3%
Potassium K 3%
Magnesium Mg 2%

Table 3.1: Eight most abundant elements in the Earth’s continental crust (% by weight). All other elements are less than 1%. (Source: USGS).

3.1.3 Chemical Bonding

The hydrogen atoms are on one side, about 105° apart.
Figure 3.4: A model of a water molecule, showing the bonds between the hydrogen and oxygen.

Most substances on Earth are compounds containing multiple . Chemical describes how these atoms attach with each other to form compounds, such as sodium and chlorine combining to form NaCl, common table salt. Compounds that are held together by chemical are called molecules. Water is a compound of hydrogen and oxygen in which two hydrogen atoms are covalently with one oxygen making the water molecule. The oxygen we breathe is formed when one oxygen atom covalently with another oxygen atom to make the molecule O2. The subscript 2 in the chemical formula indicates the molecule contains two atoms of oxygen.

Most are also compounds of more than one . The common has the chemical formula CaCO3 indicating the molecule consists of one calcium, one carbon, and three oxygen atoms. In , one carbon and three oxygen atoms are held together by covalent to form a molecular , called , which has a negative charge. Calcium as an has a positive charge of plus two. The two oppositely charged ions attract each other and combine to form the , CaCO3. The name of the chemical compound is calcium , where calcium is Ca and refers to the molecular CO3-2.

The has the chemical formula (Mg,Fe)2SiO4, in which one silicon and four oxygen atoms are with two atoms of either magnesium or iron. The comma between iron (Fe) and magnesium (Mg) indicates the two can occupy the same location in the crystal structure and substitute for one another.

Valence and Charge

The electrons around the atom’s nucleus are located in shells representing different energy levels. The outermost shell is called the valence shell. Electrons in the valence shell are involved in chemical . In 1913, Niels Bohr proposed a simple model of the atom that states atoms are more stable when their outermost shell is full. Atoms of most thus tend to gain or lose electrons so the outermost or valence shell is full. In Bohr’s model, the innermost shell can have a maximum of two electrons and the second and third shells can have a maximum of eight electrons. When the innermost shell is the valence shell, as in the case of hydrogen and helium, it obeys the when it is full with two electrons. For in higher rows, the of eight electrons in the valence shell applies.

Carbon dioxide molecule with a carbon ion in the center and two oxygen ions on either side, each sharing two electrons with the carbon.
Figure 3.5: The carbon dioxide molecule. Since oxygen is -2 and carbon is +4, the two oxygens bond to the carbon to form a neutral molecule.

The rows in the periodic table present the in order of atomic number and the columns organize with similar characteristics, such as the same number of electrons in their valence shells. Columns are often labeled from left to right with Roman numerals I to VIII, and Arabic numerals 1 through 18. The in columns I and II have 1 and 2 electrons in their respective valence shells and the in columns VI and VII have 6 and 7 electrons in their respective valence shells.

In row 3 and column I, sodium (Na) has 11 protons in the nucleus and 11 electrons in three shells—2 electrons in the inner shell, 8 electrons in the second shell, and 1 electron in the valence shell. To maintain a full outer shell of 8 electrons per the , sodium readily gives up that 1 electron so there are 10 total electrons. With 11 positively charged protons in the nucleus and 10 negatively charged electrons in two shells, sodium when forming chemical is an with an overall net charge of +1.

All in column I have a single electron in their valence shell and a valence of 1. These other column I also readily give up this single valence electron and thus become ions with a +1 charge. in column II readily give up 2 electrons and end up as ions with a charge of +2. Note that elements in columns I and II which readily give up their valence electrons, often form bonds with in columns VI and VII which readily take up these electrons. in columns 3 through 15 are usually involved in covalent . The last column 18 (VIII) contains the noble gases. These are chemically inert because the valence shell is already full with 8 electrons, so they do not gain or lose electrons. An example is the noble gas helium which has 2 valence electrons in the first shell. Its valence shell is therefore full. All in column VIII possess full valence shells and do not form with other .

As seen above, an atom with a net positive or negative charge as a result of gaining or losing electrons is called an . In general the on the left side of the table lose electrons and become positive ions, called because they are attracted to the cathode in an electrical device. The on the right side tend to gain electrons. These are called because they are attracted to the anode in an electrical device. The in the center of the periodic table, columns 3 through 15, do not consistently follow the . These are called transition . A common example is iron, which has a +2 or +3 charge depending on the state of the . Oxidized Fe+3 carries a +3 charge and reduced Fe+2 is +2. These two different states of iron often impart dramatic colors to rocks containing their —the oxidized form producing red colors and the reduced form producing green.

Ionic Bonding

Image of crystal model of halite with ions of sodium and chlorine arranged in a cubic structure.
Figure 3.6: Cubic arrangement of Na and Cl in halite.

Ionic , also called electron-transfer , are formed by the electrostatic attraction between atoms having opposite charges. Atoms of two opposite charges attract each other electrostatically and form an ionic in which the positive transfers its electron (or electrons) to the negative which takes them up. Through this transfer both atoms thus achieve a full valence shell. For example one atom of sodium (Na+1) and one atom of chlorine (Cl-1) form an ionic to make the compound sodium chloride (NaCl). This is also known as the or common table salt. Another example is calcium (Ca+2) and chlorine (Cl-1) combining to make the compound calcium chloride (CaCl2). The subscript 2 indicates two atoms of chlorine are ionically to one atom of calcium.

Covalent Bonding

Each atom is sharing electrons.
Figure 3.7: Methane molecule.

Ionic are usually formed between a metal and a nonmetal. Another type, called a covalent or electron-sharing , commonly occurs between nonmetals. Covalent share electrons between ions to complete their valence shells. For example, oxygen (atomic number 8) has 8 electrons—2 in the inner shell and 6 in the valence shell. Gases like oxygen often form diatomic molecules by sharing valence electrons. In the case of oxygen, two atoms attach to each other and share 2 electrons to fill their valence shells to become the common oxygen molecule we breathe (O2). Methane (CH4) is another covalently bonded gas. The carbon atom needs 4 electrons and each hydrogen needs 1. Each hydrogen shares its electron with the carbon to form a molecule as shown in the figure.


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3.2 Formation of Minerals

form when atoms together in a crystalline arrangement. Three main ways this occurs in nature are: 1) directly from an aqueous (water) with a change, 2) from a with a change, and 3) biological by the action of organisms.

3.2.1 Precipitation From Aqueous Solution

Encrusted calcium carbonate (lime) deposits on faucent
Figure 3.8: Calcium carbonate deposits from hard water.

consist of ions or molecules, known as solutes, in a medium or solvent. In nature this solvent is usually water. Many can be dissolved in water, such as or table salt, which has the sodium chloride, NaCl. The Na+1 and Cl-1 ions separate and disperse into the .

is the reverse process, in which ions in come together to form solid . is dependent on the concentration of ions in and other factors such as and pressure. The point at which a solvent cannot hold any more solute is called . can occur when the of the falls, when the solute evaporates, or with changing chemical conditions in the . An example of in our homes is when water evaporates and leaves behind a rind of on faucets, shower heads, and drinking glasses.

In nature, changes in environmental conditions may cause the in water to form and grow into crystals or cement grains of together. In Utah, deposits of formed from -rich springs that emerged into the Lake Bonneville. Now exposed in dry valleys, this porous was a natural insulation used by pioneers to build their homes with a natural protection against summer heat and winter cold. The at Mammoth Hot Springs in Yellowstone Park are another example formed by at the edges of the shallow -fed ponds.

The Bonneville Salt Flats of Utah
Figure 3.9: The Bonneville Salt Flats of Utah.

Another example of occurs in the Great Salt Lake, Utah, where the concentration of sodium chloride and other salts is nearly eight times greater than in the world’s oceans carry salt ions into the lake from the surrounding mountains. With no other outlet, the water in the lake evaporates and the concentration of salt increases until is reached and the out as . Similar salt deposits include and other precipitates, and occur in other lakes like Mono Lake in California and the Dead Sea.

3.2.2 Crystallization From Magma

Red hot lava flowing next to black solid volcanic ash.
Figure 3.10: Lava, magma at the Earth’s surface.

Heat is energy that causes atoms in substances to vibrate. is a measure of the intensity of the vibration. If the vibrations are violent enough, chemical are broken and the crystals melt releasing the ions into the melt. is molten rock with freely moving ions. When is emplaced at depth or extruded onto the surface (then called ), it starts to cool and crystals can form.

3.2.3 Precipitation by Organisms

Shell of an ammonite, an extinct cephalopod, with a spiral shell in a plane.
Figure 3.11: Ammonite shell made of calcium carbonate.

Many organisms build bones, shells, and body coverings by extracting ions from water and precipitating biologically. The most common by organisms is , or calcium (CaCO3). is often by organisms as a called aragonite. are crystals with the same chemical formula but different crystal structures. invertebrates such as corals and clams aragonite or for their shells and structures. Upon death, their hard parts accumulate on the as , and eventually may become the . Though can form inorganically, the vast majority is formed by this biological process. Another example is organisms called radiolaria, which are zooplankton that silica for their microscopic external shells. When the organisms die, the shells accumulate on the and can form the . An example of biologic from the world is bone, which is mostly of a type of apatite, a in the group. The apatite found in bones contains calcium and water in its structure and is called hydroxycarbonate apatite, Ca5(PO4)3(OH). As mentioned above, such substances are not technically until the organism dies and these hard parts become .


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3.3 Silicate Minerals

It is a pyramid shape with a triangular base
Figure 3.12: Rotating animation of a tetrahedra.

are categorized based on their and structure. are built around a molecular called the . A tetrahedron has a pyramid-like shape with four sides and four corners.  form the largest group of on Earth, comprising the vast majority of the Earth’s and . Of the nearly four thousand known on Earth, most are rare. There are only a few that make up most of the rocks likely to be encountered by surface dwelling creatures like us. These are generally called the rock-forming .

Molecular structure of silicate. 4 red balls labelled O 2 negative stacked in a triangle with one Si 4 positive in the center.
Figure 3.13: Silicate tetrahedron.

The (SiO4) consists of a single silicon atom at the center and four oxygen atoms located at the four corners of the tetrahedron. Each oxygen has a -2 charge and the silicon has a +4 charge. The silicon shares one of its four valence electrons with each of the four oxygen ions in a covalent to create a symmetrical geometric four-sided pyramid figure. Only half of the oxygen’s valence electrons are shared, giving the an ionic charge of -4. This forms bonds with many other combinations of ions to form the large group of .

The silicon is much smaller than the oxygen ions (see the figures) and fits into a small space in the center of the four large oxygen ions, seen if the top ball is removed (as shown in the figure to the right). Because only one of the valence electrons of the corner oxygens is shared, the has chemically active corners available to form with other or other positively charged ions such as Al+3, Fe+2,+3, Mg+2, K+1, Na+1, and Ca+2. Depending on many factors, such as the original chemistry, can combine with other tetrahedra in several different configurations. For example, tetrahedra can be isolated, attached in chains, sheets, or three dimensional structures. These combinations and others create the chemical structure in which positively charged ions can be inserted for unique chemical compositions forming groups.

3.3.1 The Dark Ferromagnesian Silicates

Many small crystall of the green mineral olivine in a mass of basalt
Figure 3.14: Olivine crystals in basalt.

Olivine Family

is the primary component in rock such as and . It is characteristically green when not weathered. The chemical formula is (Fe,Mg)2SiO4. As previously described, the comma between iron (Fe) and magnesium (Mg) indicates these two occur in a Not to be confused with a liquid solution, a occurs when two or more have similar properties and can freely substitute for each other in the same location in the crystal structure.

Tetrahedral structure of olivine showing the independent tetrahedra connected together by anions of iron and/or magnesium.
Figure 3.15: Tetrahedral structure of olivine.

is referred to as a family because of the ability of iron and magnesium to substitute for each other. Iron and magnesium in the family indicates a forming a compositional series within the group which can form crystals of all iron as one end member and all mixtures of iron and magnesium in between to all magnesium at the other end member. Different names are applied to compositions between these end members. In the series of , the iron and magnesium ions in the are about the same size and charge, so either atom can fit into the same location in the growing crystals. Within the cooling , the crystals continue to grow until they solidify into . The relative amounts of iron and magnesium in the parent determine which in the series form. Other rarer with similar properties to iron or magnesium, like manganese (Mn), can substitute into the crystalline structure in small amounts. Such ionic substitutions in crystals give rise to the great variety of and are often responsible for differences in color and other properties within a group or family of . has a pure iron end-member (called fayalite) and a pure magnesium end-member (called forsterite). Chemically, is mostly silica, iron, and magnesium and therefore is grouped among the dark-colored ferromagnesian (iron=ferro, magnesium=magnesian) or , a contraction of their chemical symbols Ma and Fe. are also referred to as dark-colored ferromagnesian . Ferro means iron and magnesian refers to magnesium. Ferromagnesian tend to be more dense than non-ferromagnesian . This difference in density ends up being important in controlling the behavior of the rocks that are built from these : whether a or not is largely governed by the density of its rocks, which are in turn controlled by the density of the that comprise them.

The crystal structure of is built from independent . with independent tetrahedral structures are called neosilicates (or orthosilicates). In addition to , other common neosilicate include garnet, topaz, kyanite, and .

Two other similar arrangements of tetrahedra are close in structure to the neosilicates and toward the next group of , the pyroxenes. In a variation on independent tetrahedra called sorosilicates, there are that share one oxygen between two tetrahedra, and include like pistachio-green epidote, a gemstone. Another variation are the cyclosilicates, which as the name suggests, consist of tetrahedral rings, and include gemstones such as beryl, emerald, aquamarine, and tourmaline

Pyroxene Family

Dark green crystals of diopside, a member of the pyroxene family
Figure 3.16: Crystals of diopside, a member of the pyroxene family.
Single chain of tetrahedra in pyroxene, alternating with adjacent corner oxygens bonded. The outer corners are active to bond with other anions.
Figure 3.17: Single chain tetrahedral structure in pyroxene.

is another family of dark ferromagnesian , typically black or dark green in color. Members of the family have a complex chemical that includes iron, magnesium, aluminum, and other to polymerized . Polymers are chains, sheets, or three-dimensional structures, and are formed by multiple tetrahedra covalently via their corner oxygen atoms. Pyroxenes are commonly found in rocks such as , , and , as well as rocks like eclogite and blue .

Pyroxenes are built from long, single chains of polymerized in which tetrahedra share two corner oxygens. The silica chains are together into the crystal structures by metal cations. A common member of the family is augite, itself containing several series with a complex chemical formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6 that gives rise to a number of individual names.

This single-chain crystalline structure with many , which can also freely substitute for each other. The generalized chemical for is XZ(Al,Si)2O6. X represents the ions Na, Ca, Mg, or Fe, and Z represents Mg, Fe, or Al. These ions have similar ionic sizes, which allows many possible substitutions among them. Although the may freely substitute for each other in the crystal, they carry different ionic charges that must be balanced out in the final crystalline structure. For example Na has a charge of +1, but Ca has charge of +2. If a Na+ substitutes for a Ca+2 , it creates an unequal charge that must be balanced by other ionic substitutions elsewhere in the crystal. Note that ionic size is more important than ionic charge for substitutions to occur in series in crystals.

Amphibole Family

A crystal of orthoclase (potassium feldspar) wth elongated dark crystals of hornblende
Figure 3.18: Elongated crystals of hornblende in orthoclase.
Black crystals of hornblende
Figure 3.19: Hornblende crystals.

are built from polymerized double silica chains and they are also referred to as inosilicates. Imagine two chains that connect together by sharing a third oxygen on each tetrahedra. Amphiboles are usually found in and rocks and typically have a long-bladed . The most common , hornblende, is usually black; however, they come in a variety of colors depending on their chemical . The , amphibolite, is primarily of .

Double chain structure of amphibole; two single chains laying together with the inner corners of each tetrahedron bonded and the outer cornera active to bond with anions
Figure 3.20: Double chain structure.

Amphiboles are of iron, magnesium, aluminum, and other with . These dark ferromagnesian are commonly found in , baslt, , and often form the black specks in . Their chemical formula is very complex and generally written as (RSi4O11)2, where R represents many different . For example, it can also be written more exactly as AX2Z5((Si,Al,Ti)8O22)(OH,F,Cl,O)2. In this formula A may be Ca, Na, K, Pb, or blank; X equals Li, Na, Mg, Fe, Mn, or Ca; and Z is Li, Na, Mg, Fe, Mn, Zn, Co, Ni, Al, Cr, Mn, V, Ti, or Zr. The substitutions create a wide variety of colors such as green, black, colorless, white, yellow, blue, or brown. crystals can also include hydroxide ions (OH), which occurs from an interaction between the growing and water in .

3.3.2 Sheet Silicates

Dark brown crystals of biotite mica showing sheet-like habit
Figure 3.21: Sheet crystals of biotite mica.
Crystal of muscovite mica showing sheet structure of the mineral
Figure 3.22: Crystal of muscovite mica.

Sheet are built from tetrahedra which share all three of their bottom corner oxygens thus forming sheets of tetrahedra with their top corners available for with other atoms. Micas and clays are common types of sheet , also known as phyllosilicates. are usually found in and rocks, while clay are more often found in sedimentary rocks. Two frequently found micas are dark-colored , frequently found in , and light-colored , found in the called .

Continuous sheets of tetradedra with all three base corners bonded to each other; the top corner active to bond with anions
Figure 3.23: Sheet structure of mica.

Chemically, sheet usually contain silicon and oxygen in a 2:5 ratio (Si4O10). Micas contain mostly silica, aluminum, and potassium. has more iron and magnesium and is considered a ferromagnesian . micas belong to the . is a contraction formed from , the dominant in rocks.

Diagram of mica crystal structure with the sheets of tetrahedra inverted onto each other into sandwiches with the active corners bonded with anions and the sandwiches connected together with large potassium ions that form weak bonds easily separated so the crystal comes apart into sheets.
Figure 3.24: Crystal structure of a mica.
Silica sheets layered in mica like bread and hjam in a stack of sandwiches
Figure 3.25: Mica “silica sandwich” structure. In this analogy, you may start with one “sandwich”: the top bun is a silica sheet, with a “jam” of anions filling the sandwich. The bottom bun is another silica sheet. Then if you place this sandwich on top of an existing sandwich, you may use butter to hold the two sandwiches together—this “butter” would be the large potassium ions forming Van der Waals bonds that hold the two sandwiches’ bottom and top buns (silica sheets) together.

The illustration of the crystalline structure of shows the corner O atoms with K, Al, Mg, Fe, and Si atoms, forming polymerized sheets of linked tetrahedra, with an octahedral layer of Fe, Mg, or Al, between them. The yellow potassium ions form Van der Waals (attraction and repulsion between atoms, molecules, and surfaces) and hold the sheets together. Van der Waals differ from covalent and ionic , and exist here between the sandwiches, holding them together into a of sandwiches. The Van der Waals are weak compared to the within the sheets, allowing the sandwiches to be separated along the potassium layers. This gives its characteristic property of easily cleaving into sheets.

Crystal structure of kaolinite, a clay mineral with sheet structure like mica except that the
Figure 3.26: Structure of kaolinite.

Clays occur in formed by the of rocks and are another family of with a tetrahedral sheet structure. Clay form a complex family, and are an important component of many sedimentary rocks. Other sheet include serpentine and chlorite, found in rocks.

Clay are of hydrous aluminum . One type of clay, kaolinite, has a structure like an open-faced sandwich, with the bread being a single layer of and a layer of aluminum as the spread in an octahedral configuration with the top oxygens of the sheets.

3.3.3 Framework Silicates

Freely grown quartz crystals showing crysatl faces
Figure 3.27: Freely growing quartz crystals showing crystal faces.

and are the two most abundant in the . In fact, itself is the single most abundant in the Earth’s . There are two types of , one containing potassium and abundant in rocks of the , and the other with sodium and calcium abundant in the rocks of . Together with , these are classified as framework . They are built with a three-dimensional framework of in which all four corner oxygens are shared with adjacent tetrahedra. Within these frameworks in are holes and spaces into which other ions like aluminum, potassium, sodium, and calcium can fit giving rise to a variety of compositions and names.

Feldspar is 51% of the chart.
Figure 3.28: Mineral abundance pie chart in Earth’s crust.

are usually found in rocks, such as , , and as well as rocks and sedimentary rocks. sedimentary rocks are of mechanically weathered rock particles, like sand and gravel. is especially abundant in sedimentary rocks because it is very resistant to disintegration by . While is the most abundant on the Earth’s surface, due to its durability, the are the most abundant in the Earth’s , comprising roughly 50% of the total that make up the .

A group of crystals of pink potassium feldspar
Figure 3.29: Pink orthoclase crystals.

is of pure silica, SiO2, with the tetrahedra arranged in a three dimensional framework. Impurities consisting of atoms within this framework give rise to many varieties of among which are gemstones like amethyst, rose , and citrine. are mostly silica with aluminum, potassium, sodium, and calcium. Orthoclase (KAlSi3O8), also called potassium or , is made of silica, aluminum, and potassium. and orthoclase are . is the compositional term applied to and rocks that contain an abundance of silica. Another is with the formula (Ca,Na)AlSi3O8, the (Ca,Na) indicating a series of , one end of the series with calcium CaAl2Si2O8, called anorthite, and the other end with sodium NaAlSi3O8, called albite. Note how the accommodates the substitution of Ca++ and Na+. in this solid solution series have different names.

Framework structure of feldspar with all corners of tetrahedra shared with adjacent tetrahedra; there are holes in the structure in which large anions like potassium and sodium/calcium fit
Figure 3.30: Crystal structure of feldspar.

Note that aluminum, which has a similar ionic size to silicon, can substitute for silicon inside the tetrahedra. Because potassium ions are so much larger than sodium and calcium ions, which are very similar in size, the inability of the crystal lattice to accommodate both potassium and sodium/calcium gives rise to the two families of , orthoclase and respectively. Framework are called tectosilicates and include the alkali metal-rich feldspathoids and zeolites.


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3.4 Non-silicate Minerals

The mineral is hexagonal and clear.
Figure 3.31: Hanksite, Na22K(SO4)9(CO3)2Cl, one of the few minerals that is considered a carbonate and a sulfate.

The crystal structure of non- (see table) does not contain . Many non- are economically important and provide resources such as copper, lead, and iron. They also include valuable non- products such as salt, construction materials, and fertilizer.

Mineral group Examples Formula Uses
Native elements Gold, silver, copper Au, Ag, Cu Jewelry, coins, industry
Carbonates Calcite, dolomite CaCO3, CaMg(CO3)2 Lime, Portland cement
Oxides Hematite, magnetite, bauxite Fe2O3, Fe3O4, a mixture of aluminum oxides Ores of iron & aluminum, pigments
Halides Halite, sylvite NaCl, KCl Table salt, fertilizer
Sulfides Galena, chalcopyrite, cinnabar PbS, CuFeS2, HgS Ores of lead, copper, mercury
Sulphates Gypsum, epsom salts CaSo4 · 2H2O, MgSO4 · 7H2O Sheetrock, therapeutic soak
Phosphates Apatite Ca5(PO4)3(F,Cl,OH) Fertilizer, teeth, bones

Table 3.2: Common non-silicatemineral groups.

3.4.1 Carbonates

Calcite crystal in a shape called a rhomb like a cube squahed over toward one corner
Figure 3.32: Calcite crystal in shape of rhomb. Note the double-refracted word “calcite” in the center of the figure due to birefringence.
Piece of limestone rock full of small fossils
Figure 3.33: Limestone with small fossils.

 (CaCO3) and dolomite (CaMg(CO3)2) are the two most frequently occurring , and usually occur in sedimentary rocks, such as and dolostone rocks, respectively. Some rocks, such and dolomite, are formed via evaporation and . However, most -rich rocks, such as , are created by the of fossilized organisms. These organisms, including those we can see and many microscopic organisms, have shells or exoskeletons consisting of calcium (CaCO3). When these organisms die, their remains accumulate on the floor of the water body in which they live and the soft body parts decompose and away. The calcium hard parts become included in the , eventually becoming the called . While may contain large, easy to see , most contain the remains of microscopic creatures and thus originate from biological processes.

Calcite crystal polarize light into two waves that vibrate at right angles to each other and pass through the crystal in different paths.
Figure 3.34: Bifringence in calcite crystals.

crystals show an interesting property called birefringence, meaning they polarize light into two wave components vibrating at right angles to each other. As the two light waves pass through the crystal, they travel at different velocities and are separated by into two different travel paths. In other words, the crystal produces a double image of objects viewed through it. Because they polarize light, crystals are used in special petrographic microscopes for studying and rocks.

Many non- are referred to as salts. The term salts used here refers to compounds made by replacing the hydrogen in natural acids. The most abundant natural acid is that forms by the of carbon dioxide in water. are salts built around the (CO3-2) where calcium and/or magnesium replace the hydrogen in (H2CO3). and a closely related aragonite are secreted by organisms to form shells and physical structures like corals. Many such creatures draw both calcium and from bicarbonate ions (HCO3) in ocean water. As seen in the identification section below, is easily in acid and thus effervesces in dilute hydrochloric acid (HCl). Small dropper bottles of dilute hydrochloric acid are often carried by geologists in the field as well as used in identification labs.

Crystal structure of calcite showing the carbonate units of carbon surrounded by three oxygen ions and bonded to calcium ions.
Figure 3.35: Crystal structure of calcite.

Other salts include (NaCl) in which sodium replaces the hydrogen in hydrochloric acid and (Ca[SO4] • 2 H2O) in which calcium replaces the hydrogen in sulfuric acid. Note that some water molecules are also included in the crystal. Salts are often formed by evaporation and are called .

The figure shows the crystal structure of (CaCO3). Like silicon, carbon has four valence electrons. The unit consists of carbon atoms (tiny white dots) covalently to three oxygen atoms (red), one oxygen sharing two valence electrons with the carbon and the other two sharing one valence electron each with the carbon, thus creating triangular units with a charge of -2. The negatively charged unit forms an ionic with the Ca (blue), which as a charge of +2.

3.4.2 Oxides, Halides, and Sulfides

Image of limonite, a hydrated oxide of iron
Figure 3.36: Limonite, a hydrated oxide of iron.

After , the next most common non- are the , , and .

consist of metal ions covalently with oxygen. The most familiar is rust, which is a combination of iron (Fe2O3) and hydrated . Hydrated form when iron is exposed to oxygen and water. Iron are important for producing iron. When iron or is smelted, it produces carbon dioxide (CO2) and iron.

The red color in rocks is usually due to the presence of iron . For example, the red cliffs in Zion National Park and throughout Southern Utah consist of white or colorless grains of coated with iron which serve as cementing agents holding the grains together.

A red form of hematite called oolitic showing a mass of small round nodules
Figure 3.37: Oolitic hematite.

Other iron include limonite, magnetite, and hematite. Hematite occurs in many different crystal forms. The form shows no external structure. Botryoidal hematite shows large concentric blobs. Specular hematite looks like a mass of shiny crystals. Oolitic hematite looks like a mass of dull red fish eggs. These different forms of hematite are and all have the same formula, Fe2O3.

Other common include:

  • ice (H2O), an of hydrogen
  • (Al2H2O4), hydrated of aluminum, an for producing aluminum
  • corundum (Al2O3), which includes ruby and sapphire gemstones.
Crystals of halite showing cubic crystal habit
Figure 3.38: Halite crystal showing cubic habit.

The consist of halogens in column VII, usually fluorine or chlorine, ionically with sodium or other . These include or sodium chloride (NaCl), common table salt; sylvite or potassium chloride (KCl); and fluorite or calcium fluoride (CaF2).

Photo of salt crust at the Bonneville Salt Flats in Utah with mountains in the background.
Figure 3.39: Salt crystals at the Bonneville Salt Flats.
Purplish crystals of fluorite. The second image shows the deep blue fluorescence of fluorite under ultraviolet light.
Figure 3.40: Fluorite. B shows fluorescence of fluorite under UV light.

usually form from the evaporation of sea water or other isolated bodies of water. A well-known example of deposits created by evaporation is the Bonneville Salt Flats, located west of the Great Salt Lake in Utah (see figure 3.39).

Cubic crystals of iron pyrite, called "fools gold"
Figure 3.41: Cubic crystals of pyrite.

Many important metal are , in which metals are to sulfur. Significant examples include: galena (lead ), sphalerite (zinc ), pyrite (iron , sometimes called “fool’s gold”), and chalcopyrite (iron-copper ).  are well known for being important . For example, galena is the main source of lead, sphalerite is the main source of zinc, and chalcopyrite is the main copper in porphyry deposits like the Bingham (see chapter 16). The largest sources of nickel, antimony, molybdenum, arsenic, and mercury are also .

3.4.3 Sulfates

A clear crystal of gypsum
Figure 3.42: Gypsum crystal.

contain a metal , such as calcium, to a . The is a combination of sulfur and oxygen (SO42). The (CaSO4 • 2H2O) is used in construction materials such as plaster and drywall. is often formed from evaporating water and usually contains water molecules in its crystalline structure. The 2H2O in the formula indicates the water molecules are whole H2O. This is different from like , which contain a hydroxide (OH) that is derived from water, but is missing a hydrogen ion (H+). The calcium without water is a different than called anhydrite (CaSO4).

3.4.4 Phosphates

A crystal of apatite
Figure 3.43: Apatite crystal.

have a tetrahedral unit (PO4-3) combined with various and . In some cases arsenic or vanadium can substitute for phosphorus. are an important ingredient of fertilizers as well as detergents, paint, and other products. The best known is apatite, Ca5(PO4)3(F,Cl,OH), variations of which are found in teeth and bones. The gemstone turquoise [CuAl6(PO4)4(OH)8·4H2O ] is a copper-rich that, like , contains water molecules.

3.4.5 Native Element Minerals

Native sulfur deposited around the vent of a volcanic fumarole
Figure 3.44: Native sulfur deposited around a volcanic fumarole.
Metallic native copper
Figure 3.45: Native copper.

, usually metals, occur in nature in a pure or nearly pure state. Gold is an example of a ; it is not very reactive and rarely with other so it is usually found in an isolated or pure state. The non- and poorly-reactive carbon is often found as a , such as graphite and diamonds. Mildly reactive metals like silver, copper, platinum, mercury, and sulfur sometimes occur as . Reactive metals such as iron, lead, and aluminum almost always to other and are rarely found in a state.


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3.5 Identifying Minerals

The red rocks have a small hole drilled
Figure 3.46: The rover Curiosity drilled a hole in this rock from Mars, and confirmed the mineral hematite, as mapped from satellites.

Geologists identify by their physical properties. In the field, where geologists may have limited access to advanced technology and powerful machines, they can still identify by testing several physical properties: and color, , , , and , and some special properties. Only a few common make up the majority of Earth’s rocks and are usually seen as small grains in rocks. Of the several properties used for identifying , it is good to consider which will be most useful for identifying them in small grains surrounded by other .

3.5.1 Luster and Color

The crystal looks like metal.
Figure 3.47: 15 mm metallic hexagonal molybdenite crystal from Quebec.

The first thing to notice about a is its surface appearance, specifically and color. describes how the looks. looks like a shiny metal such as chrome, steel, silver, or gold. Submetallic has a duller appearance. Pewter, for example, shows submetallic .

Antique pewter plate showing a more dull submetallic luster
Figure 3.48: Submetallic luster shown on an antique pewter plate.

doesn’t look like a metal and may be described as vitreous (glassy), earthy, silky, pearly, and other surface qualities. may be shiny, although their vitreous shine is different from . See the table for descriptions and examples of .

Luster Description Image Image description
Vitreous/glassy Surface is shiny like glass White, transluscent, long, then, glassy crystals poking in many directions Quartz crystals showing vitreous luster
Earthy/dull Dull, like dried mud or clay White, opaque, dusty mineral mimics dried clay Kaolin specimen showing dull or earthy luster
Silky Soft shine like silk fabric Cream, opaque, shiny mineral with many horizontal linear streaks like silk Gypsum specimen showing silky luster
Pearly Like the inside of a clam shell or mother-of-pearl Brown, opaque rock-like mineral with shiny surface that is more cream in color and reflective Mica specimen showing pearly luster
Submetallic Has the appearance of dull metal, like pewter. These minerals would usually still be considered metallic. Submetallic appearance can occur in metallic minerals because of weathering. Dark gray, opaque, dull shine mineral with sharp edges Sphalerite specimen showing submetallic luster

Table 3.3: Nonmetallic luster descriptions and examples.

There are two dark blue disks on white siltstone.
Figure 3.49: Azurite is ALWAYS a dark blue color, and has been used for centuries for blue pigment.

Surface color may be helpful in identifying , although it can be quite variable within the same family. colors are affected by the main as well as impurities in the crystals. These impurities may be rare —like manganese, titanium, chromium, or lithium—even other molecules that are not normally part of the formula. For example, the incorporation of water molecules gives , which is normally clear, a milky color.

Some predominantly show a single color. Malachite and azurite are green and blue, respectively, because of their copper content. Other have a predictable range of colors due to elemental substitutions, usually via a . , the most abundant in the earth’s , are complex, have series, and present several colors including pink, white, green, gray and others. Other also come in several colors, influenced by trace amounts of several . The same may show up as different colors, in different . With notable exceptions, color is usually not a definitive property of . For identifying many . a more reliable indicator is , which is the color of the powdered .

3.5.2 Streak

Pyrite showing a black streak on a white streak plate and rhodochrosite with a white streak on a black streak plate
Figure 3.50: Different minerals may have different streaks.

examines the color of a powdered , and can be seen when a sample is scratched or scraped on an unglazed porcelain . A paper page in a field notebook may also be used for the of some . that are harder than the will not show , but will scratch the porcelain. For these , a test can be obtained by powdering the with a hammer and smearing the powder across a or notebook paper.

While surface colors and appearances may vary, their colors can be diagnostically useful. An example of this property is seen in the iron- hematite. Hematite occurs in a variety of forms, colors and lusters, from shiny silver to earthy red-brown, and different physical appearances. A hematite is consistently reddish brown, no matter what the original specimen looks like. Iron or pyrite, is a brassy yellow. Commonly named fool’s gold, pyrite has a characteristic black to greenish-black .

3.5.3 Hardness

Chart of Mohs Hardness Scale with minerals arranged in hardness from 1 to 10, also showing common items that correlate with the scale.
Figure 3.51: Mohs hardness scale.

measures the ability of a to scratch other substances. The Mohs Hardness Scale gives a number showing the relative scratch-resistance of when compared to a standardized set of of increasing hardness. The Mohs scale was developed by German geologist Fredrick Mohs in the early 20th century, although the idea of identifying by goes back thousands of years. Mohs values are determined by the strength of a ’s atomic .

The figure shows the associated with specific values, together with some common items readily available for use in field testing and identification. The values run from 1 to 10, with 10 being the hardest; however, the scale is not linear. Diamond defines a of 10 and is actually about four times harder than corundum, which is 9. A steel pocketknife blade, which has a value of 5.5, separates between hard and soft on many identification keys.

3.5.4 Crystal Habit

can be identified by , how their crystals grow and appear in rocks. Crystal shapes are determined by the arrangement of the atoms within the crystal structure. For example, a cubic arrangement of atoms gives rise to a cubic-shaped crystal. refers to typically observed shapes and characteristics; however, they can be affected by other crystallizing in the same rock. When are constrained so they do not develop their typical , they are called . crystals are partially formed shapes. For some characteristic is to grow crystal faces even when surrounded by other crystals in rock. An example is garnet. grown freely where the crystals are unconstrained and can take characteristic shapes often form crystal faces. A crystal has a perfectly formed, unconstrained shape. Some crystallize in such tiny crystals, they do not show a specific to the naked eye. Other , like pyrite, can have an array of different crystal habits, including cubic, dodecahedral, octahedral, and . The table lists typical crystal habits of various .

Habit Image Examples
Bladed
Long and flat crystals
The crystals are long and rectangular
Kyanite
Kyanite, amphibole, gypsum
Botryoidal/mammillary
Blobby, circular crystals
The mineral is bulbous
Malchite
Hematite, malachite, smithsonite
Coating/laminae/druse
Crystals that are small and coat surfaces
The rock is hollowed and filled with purple minerals
Quartz (var. amethyst) geode
Quartz, calcite, malachite, azurite
Cubic
Cube-shaped crystals
Dark silver shiny cubes morphed together
Calcite, Galena
Pyrite, galena, halite
Dodecahedral
12-sided polygon shapes
Brown and black colored mineral with 12 sides
Pyrite
Garnet, pyrite
Dendritic
Branching crystals
Black fern-looking mineral on tan background. Zoomed in picture shows branching crystals growing out from what mimics a stem
Manganese dendrites, scale in mm
Mn-oxides, copper, gold
Equant
Crystals that do not have a long direction
Light green chunk that reflects light and has no distinguishable crystals inside
Olivine
Olivine, garnet, pyroxene
Fibrous
Thin, very long crystals
Light gray mineral with long, skinny, distinguishable crystals in every direction
Tremolite, a type of amphibole
Serpentine, amphibole, zeolite
Layered, sheets
Stacked, very thin, flat crystals
Cream colored, rounded, reflective, thin sheets morphed together randomly
Muscovite
Mica (biotite, muscovite, etc.)
Lenticular/platy
Crystals that are plate-like
Orange stringy mineral on white block
Orange wulfenite on calcite
Selenite roses, wulfenite, calcite
Hexagonal
Crystals with six sides
Cream colored translucent crystal with 6 sides
Hanksite
Quartz, hanksite, corundum
Massive/granular
Crystals with no obvious shape, microscopic crystals
Brown, dusty, opaque crystal with no obvious shape or crystals
Limonite, a hydrated oxide of iron
Limonite, pyrite, azurite, bornite
Octahedral
4-sided double pyramid crystals
Two crystals side by side. Left: pink, transluscent, smaller, shape mimics 2 pyramids glued together at their bottoms. Right: same as left but bigger and yellow
Fluorite
Diamond, fluorite, magnetite, pyrite
Prismatic/columnar
Very long, cylindrical crystals
Long green and red stick crystals poking out of white crystals
Tourmaline var. Elbaite with Quartz & Lepidolite on Cleavelandite
Tourmaline, beryl, barite
Radiating
Crystals that grow from a point and fan out
Many bronze colored, reflective, short crystals that have a center and fan out. All start and stop randomly to form a rock shape
Pyrophyllite
Pyrite “suns”, pyrophyllite
Rhombohedral
Crystals shaped like slanted cubes
Rhombus shaped, transluscent, light pink block with no obvious crystal pattern
Calcite
Calcite, dolomite
Tabular/blocky/stubby
Sharp-sided crystals with no long direction
Dark green cylindrical crystals poking from many directions
Diopside, a member of the pyroxene family
Feldspar, pyroxene, calcite
Tetrahedral
3-sided, pyramid-shaped crystals
Dark brown, shiny, opaque crystals with 3 sides morphed together to form one mineral
Tetrahedrite
Magnetite, spinel, tetrahedrite

Table 3.4: Typical crystal habits of various minerals.

The mineral has many horizontal lines on it
Figure 3.52: Gypsum with striations.
The brown minerals are replicated in different directions and connected in the middle
Figure 3.53: Twinned staurolite found at Fairy Stone State Park, located in Patrick County, Virginia.

Another that may be used to identify is striations, which are dark and light parallel lines on a crystal face. Twinning is another, which occurs when the crystal structure replicates in mirror images along certain directions in the crystal.

Striations or parallel dark lines on one cleavage surface on plagioclase feldspar
Figure 3.54: Striations on plagioclase.

Striations and twinning are related properties in some including . Striations are optical lines on a surface. Because of twinning in the crystal, striations show up on one of the two cleavage faces of the crystal.

3.5.5 Cleavage and Fracture

A specimen of a variety of quartz showing conchoidal fracture
Figure 3.55: Citrine, a variety of quartz showing conchoidal fracture.

often show characteristic patterns of breaking along specific cleavage planes or show characteristic patterns. planes are smooth, flat, parallel planes within the crystal. The cleavage planes may show as reflective surfaces on the crystal, as parallel cracks that penetrate into the crystal, or show on the edge or side of the crystal as a series of steps like rice . arises in crystals where the atomic between atomic layers are weaker along some directions than others, meaning they will break preferentially along these planes. Because they develop on atomic surfaces in the crystal, cleavage planes are optically smooth and reflect light, although the actual break on the crystal may appear jagged or uneven. In such cleavages, the cleavage surface may appear like rice on a mountainside that all reflect sunlight from a particular sun angle. Some have a strong cleavage, some only have weak cleavage or do not typically demonstrate cleavage.

Structure of graphite, showing single carbon layers with weak bonds holding them together. Diamonds and coal also shown
Figure 3.56: Graphite showing layers of carbon atoms separated by a gap with weak bonds holding the layers together.

For example, and rarely show cleavage and typically break into patterns.

Graphite has its carbon atoms arranged into layers with relatively strong within the layer and very weak between the layers. Thus graphite cleaves readily between the layers and the layers slide easily over one another giving graphite its lubricating quality.

Specimen of galena showing cubic cleavage
Figure 3.57: Cubic cleavage of galena; note how the cleavage surfaces show up as different but parallel layers in the crystal.

surfaces may be rough and uneven or they may be show . Uneven patterns are described as irregular, splintery, fibrous. A conchoidal fracture has a smooth, curved surface like a shallow bowl or conch shell, often with curved ridges. Natural glass, called , breaks with this characteristic pattern.

To work with , it is important to remember that cleavage is a result of separating along planes of atoms in the crystal structure. On some , cleavage planes may be confused with crystal faces. This will usually not be an issue for crystals of that grew together within rocks. The act of breaking the rock to expose a fresh face will most likely break the crystals along cleavage planes. Some cleavage planes are parallel with crystal faces but many are not. Cleavage planes are smooth, flat, parallel planes within the crystal. The cleavage planes may show as parallel cracks that penetrate into the crystal (see below), or show on the edge or side of the crystal as a series of steps like rice . For some characteristic is to grow crystal faces even when surrounded by other crystals in rock. An example is garnet. grown freely where the crystals are unconstrained and can take characteristic shapes often form crystal faces (see below).

Freely grown quartz crystals showing crysatl faces
Figure 3.58: Freely growing quartz crystals showing crystal faces.

In some , distinguishing cleavage planes from crystal faces may be challenging for the student. Understanding the nature of and referring to the number of cleavage planes and cleavage angles on identification keys should provide the student with enough information to distinguish cleavages from crystal faces. planes may show as multiple parallel cracks or flat surfaces on the crystal. planes may be expressed as a series of steps like terraced rice paddies. See the cleavage surfaces on galena above or below. planes arise from the tendency of crystals to break along specific planes of weakness within the crystal favored by atomic arrangements. The number of cleavage planes, the quality of the cleavage surfaces, and the angles between them are diagnostic for many and cleavage is one of the most useful properties for identifying . Learning to recognize cleavage is an especially important and useful skill in studying .

Image of wollastonite, a crystal showing step-like cleavage on one side. All steps are along the same direction of cleavage.
Figure 3.59: Steps of cleavage along the same cleavage direction.

As an identification property of , is usually given in terms of the quality of the cleavage (perfect, imperfect, or none), the number of cleavage surfaces, and the angles between the surfaces. The most common number of cleavage plane directions in the common rock-forming are: one perfect cleavage (as in ), two cleavage planes (as in , , and ), and three cleavage planes (as in , , and galena). One perfect cleavage (as in ) develops on the top and bottom of the specimen with many parallel cracks showing on the sides but no angle of intersection. Two cleavage planes intersect at an angle. Common cleavage angles are 60°, 75°, 90°, and 120°. has two cleavage planes at 60° and 120°. Galena and have three cleavage planes at 90° (cubic cleavage). cleaves readily in three directions producing a cleavage figure called a rhomb that looks like a cube squashed over toward one corner giving rise to the approximately 75° cleavage angles. has an imperfect cleavage with two planes at 90°.

Photomicrograph showing 120/60 degree cleavage in amphibole
Figure 3.60: Photomicrograph showing 120/60 degree cleavage within a grain of amphibole.

on common rock-forming minerals:

  • —none ( )
  • —none ( )
  • —1 perfect
  • —2 perfect at 90°
  • —2 imperfect at 90°
  • —2 perfect at 60°/120°
  • —3 perfect at approximately 75°
  • , galena, pyrite—3 perfect at 90°

3.5.6 Special Properties

The words on the page are projected upwards onto the mineral
Figure 3.61: A demonstration of ulexite’s image projection.

Special properties are unique and identifiable characteristics used to identify or that allow some to be used for special purposes. Ulexite has a fiber-optic property that can project images through the crystal like a high-definition television screen (see figure 3.61). A simple identifying special property is taste, such as the salty flavor of or common table salt (NaCl). Sylvite is potassium chloride (KCl) and has a more bitter taste.

The nugget is gold
Figure 3.62: Native gold has one of the highest specific gravities.

Another property geologists may use to identify is a property related to density called . measures the weight of a specimen relative to the weight of an equal volume of water. The value is expressed as a ratio between the and water weights. To measure , a specimen is first weighed in grams then submerged in a graduated cylinder filled with pure water at room . The rise in water level is noted using the cylinder’s graduated scale. Since the weight of water at room is 1 gram per cubic centimeter, the ratio of the two weight numbers gives the . is easy to measure in the laboratory but is less useful for identification in the field than other more easily observed properties, except in a few rare cases such as the very dense galena or gold. The high density of these gives rise to a property called “heft.” Experienced geologists can roughly assess by heft, a quality of how heavy the specimen feels in one’s hand relative to its size.

A simple test for identifying and dolomite is to drop a bit of dilute hydrochloric acid (10-15% HCl) on the specimen. If the acid drop effervesces or fizzes on the surface of the rock, the specimen is . If it does not, the specimen is scratched to produce a small amount of powder and test with acid again. If the acid drop fizzes slowly on the powdered , the specimen is dolomite. The difference between these two can be seen in the video. Geologists who work with rocks carry a small dropper bottle of dilute HCl in their field kit. Vinegar, which contains acetic acid, can be used for this test and is used to distinguish non- from . While acidic, vinegar produces less of a fizzing reaction because acetic acid is a weaker acid.


Video 3.2: Calcite and dolomite reacting with hydrochloric acid.

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The paperclip is sticking up into the air.
Figure 3.63: Paperclips attach to lodestone (magnetite).

Some iron- are magnetic and are attracted to magnets. A common name for a naturally magnetic iron is lodestone. Others include magnetite (Fe3O4) and ilmenite (FeTiO3). Magnetite is strongly attracted to magnets and can be magnetized. Ilmenite and some types of hematite are weakly magnetic.

Striations or parallel dark lines on one cleavage surface on plagioclase feldspar
Figure 3.64: Iridescence on plagioclase, also showing striations on the cleavage surface.

Some and mineraloids scatter light via a phenomenon called iridescence. This property occurs in labradorite (a variety of ) and opal. It is also seen in biologically created substances like pearls and seashells. Cut diamonds show iridescence and the jeweler’s diamond cut is designed to maximize this property.

Image showing exsolution lamellae in potassium feldspar. These are separations of sodium feldspar from potassium feldspar within the crystal, not striations.
Figure 3.65: Exsolution lamellae within potassium feldspar.

Striations on faces are an optical property that can be used to separate from potassium (). A process called twinning creates parallel zones in the crystal that are repeating mirror images. The actual cleavage angle in is slightly different than 90o and the alternating mirror images in these twinned zones produce a series of parallel lines on one of ’s two cleavage faces. Light reflects off these twinned lines at slightly different angles which then appear as light and dark lines called striations on the cleavage surface. Potassium does not exhibit twinning or striations but may show linear features called exsolution lamellae, also known as perthitic or simply perthite. Because sodium and potassium do not fit into the same crystal structure, the lines are created by small amounts of sodium (albite) separating from the dominant potassium () within the crystal structure. The two different crystallize out into roughly parallel zones within the crystal, which are seen as these linear markings.

Purplish crystals of fluorite. The second image shows the deep blue fluorescence of fluorite under ultraviolet light.
Figure 3.66: Fluorite. B shows fluorescence of fluorite under UV light.

One of the most interesting special properties is fluorescence. Certain , or trace within them, give off visible light when exposed to ultraviolet radiation or black light. Many exhibits have a fluorescence room equipped with black lights so this property can be observed. An even rarer optical property is phosphorescence. Phosphorescent absorb light and then slowly release it, much like a glow-in-the-dark sticker.


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Summary

are the building blocks of rocks and essential to understanding geology. properties are determined by their atomic . Most begin in a fluid, and either crystallize out of cooling or as ions and molecules out of a . The are largest group of on Earth, by number of varieties and relative quantity, making up a large portion of the and . Based on the , the crystal structure of reflects the fact that silicon and oxygen are the top two of Earth’s most abundant . Non- are also economically important, and providing many types of construction and manufacturing materials. are identified by their unique physical properties, including , color, , , , , , and special properties.


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Text References

  1. Clarke, F.W.H.S.W., 1927, The Composition of the Earth’s Crust: Professional Paper, United States Geological Survey, Professional Paper.
  2. Gordon, L.M., and Joester, D., 2011, Nanoscale chemical tomography of buried organic-inorganic interfaces in the chiton tooth: Nature, v. 469, no. 7329, p. 194–197.
  3. Hans Wedepohl, K., 1995, The composition of the continental crust: Geochim. Cosmochim. Acta, v. 59, no. 7, p. 1217–1232.
  4. Lambeck, K., 1986, Planetary evolution: banded iron formations: v. 320, no. 6063, p. 574–574.
  5. Scerri, E.R., 2007, The Periodic Table: Its Story and Its Significance: Oxford University Press, USA.
  6. Thomson, J.J., 1897, XL. Cathode Rays: Philosophical Magazine Series 5, v. 44, no. 269, p. 293–316.
  7. Trenn, T.J., Geiger, H., Marsden, E., and Rutherford, E., 1974, The Geiger-Marsden Scattering Results and Rutherford’s Atom, July 1912 to July 1913: The Shifting Significance of Scientific Evidence: Isis, v. 65, no. 1, p. 74–82.

Figure References

Figure 3.1: The periodic table of the elements. R.A. Dragoset, A. Musgrove, C.W. Clark, and W.C. Martin — NIST, Physical Measurement Laboratory. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Periodic_Table_-_Atomic_Properties_of_the_Elements.png

Figure 3.2: Formation of carbon-14 from nitrogen-14. Sgbeer; adapted by NikNaks. 2011. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Carbon_14_formation_and_decay.svg

Figure 3.3: Element abundance pie chart for Earth’s crust. Kindred Grey. 2022. CC BY 4.0.

Figure 3.4: A model of a water molecule, showing the bonds between the hydrogen and oxygen. Dan Craggs. 2009. Public domain. https://commons.wikimedia.org/wiki/File:H2O_2D_labelled.svg

Figure 3.5: The carbon dioxide molecule. Jynto. 2011. Public domain. https://commons.wikimedia.org/wiki/File:Carbon_dioxide_3D_ball.png

Figure 3.6: Cubic arrangement of Na and Cl in halite. Benjah-bmm27. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Sodium-chloride-3D-ionic.png

Figure 3.7: Methane molecule. DynaBlast. 2006. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:Covalent.svg

Figure 3.8: Calcium carbonate deposits from hard water. Bbypnda. 2014. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Hard_Water_Calcification.jpg

Figure 3.9: The Bonneville Salt Flats of Utah. Bureau of Land Management. 2015. CC BY 2.0. https://commons.wikimedia.org/wiki/File:Bonneville_Salt_Flats_(17423041595).jpg

Figure 3.10: Lava, magma at the Earth’s surface. Hawaii Volcano Observatory (DAS). 2003. Public domain. https://commons.wikimedia.org/wiki/File:Pahoehoe_toe.jpg

Figure 3.11: Ammonite shell made of calcium carbonate. Dlloyd. 2006. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Ammonite_Asteroceras.jpg

Figure 3.12: Rotating animation of a tetrahedra. Kjell André. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Tetrahedron.gif

Figure 3.13: Silicate tetrahedron. Helgi. 2013. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Silicate_tetrahedron_%2B.svg

Figure 3.14: Olivine crystals in basalt. Vsmith. 2005. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Peridot_in_basalt.jpg

Figure 3.15: Tetrahedral structure of olivine. Matanya (usurped). 2005. Public domain. https://commons.wikimedia.org/wiki/File:Atomic_structure_of_olivine_1.png

Figure 3.16: Crystals of diopside, a member of the pyroxene family. Robert M. Lavinsky. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Diopside-172005.jpg

Figure 3.17: Single chain tetrahedral structure in pyroxene. Bubenik. 2012. CC BY-SA 3.0. https://en.wikipedia.org/wiki/File:Pyroxferroite-chain.png

Figure 3.18: Elongated crystals of hornblende in orthoclase. Dave Dyet. 2007. Public domain. https://commons.wikimedia.org/wiki/File:Orthoclase_Hornblende.jpg

Figure 3.19: Hornblende crystals. Saperaud~commonswiki. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Amphibole.jpg

Figure 3.20: Double chain structure. Bubenik. 2012. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Jimthompsonite-chain.png

Figure 3.21: Sheet crystals of biotite mica. Fred Kruijen. 2005. CC BY-SA 3.0 NL. https://commons.wikimedia.org/wiki/File:Biotite_aggregate_-_Ochtendung,_Eifel,_Germany.jpg

Figure 3.22: Crystal of muscovite mica. Saperaud~commonswiki. 2005. Public domain. https://commons.wikimedia.org/wiki/File:MicaSheetUSGOV.jpg

Figure 3.23: Sheet structure of mica. Benjah-bmm27. 2007. Public domain. https://en.wikipedia.org/wiki/File:Silicate-sheet-3D-polyhedra.png

Figure 3.24: Crystal structure of a mica. Rosarinagazo. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Illstruc.jpg

Figure 3.25: Mica “silica sandwich” structure. Kindred Grey. 2022. CC BY 4.0. Includes Sandwich by Alex Muravev from Noun Project (Noun Project license).

Figure 3.26: Structure of kaolinite. USGS. Public domain. https://pubs.usgs.gov/of/2001/of01-041/htmldocs/clays/kaogr.htm

Figure 3.27: Freely growing quartz crystals showing crystal faces. JJ Harrison. 2009. CC BY-SA 2.5. https://en.wikipedia.org/wiki/File:Quartz,_Tibet.jpg

Figure 3.28: Mineral abundance pie chart in Earth’s crust. Kindred Grey. 2022. CC BY 4.0.

Figure 3.29: Pink orthoclase crystals. Didier Descouens. 2009. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:OrthoclaseBresil.jpg

Figure 3.30: Crystal structure of feldspar. Taisiya Skorina and Antoine Allanore (DOI:10.1039/C4GC02084G). 2015. CC BY 3.0. https://www.researchgate.net/figure/fig1_273641498

Figure 3.31: Hanksite, Na22K(SO4)9(CO3)2Cl, one of the few minerals that is considered a carbonate and a sulfate. Matt Affolter(QFL247). 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Hanksite.JPG

Figure 3.32: Calcite crystal in shape of rhomb. Note the double-refracted word “calcite” in the center of the figure due to birefringence. Alkivar. 2005. Public domain. https://commons.wikimedia.org/wiki/File:Calcite-HUGE.jpg

Figure 3.33: Limestone with small fossils. Jim Stuby. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Limestone_etched_section_KopeFm_new.jpg

Figure 3.34: Bifringence in calcite crystals. Mikael Häggström. 2010. Public domain. https://commons.wikimedia.org/wiki/File:Positively_birefringent_material.svg

Figure 3.35: Crystal structure of calcite. Materialscientist. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Calcite.png

Figure 3.36: Limonite, a hydrated oxide of iron. USGS. 2005. Public domain. https://commons.wikimedia.org/wiki/File:LimoniteUSGOV.jpg

Figure 3.37: Oolitic hematite. Dave Dyet. 2007. Public domain. https://en.wikipedia.org/wiki/File:Hematite_-_oolitic_with_shale_Iron_Oxide_Clinton,_Oneida_County,_New_York.jpg

Figure 3.38: Halite crystal showing cubic habit. Saperaud~commonswiki. 2005. Public domain. https://en.wikipedia.org/wiki/File:ImgSalt.jpg

Figure 3.39: Salt crystals at the Bonneville Salt Flats. Michael. 2008. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Bonneville_salt_flats_pilot_peak.jpg

Figure 3.40: Fluorite. B shows fluorescence of fluorite under UV light. Didier Descouens. 2009. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:FluoriteUV.jpg

Figure 3.41: Cubic crystals of pyrite. CarlesMillan. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:2780M-pyrite1.jpg

Figure 3.42: Gypsum crystal. USGS. 2004. Public domain. https://commons.wikimedia.org/wiki/File:SeleniteGypsumUSGOV.jpg

Figure 3.43: Apatite crystal. Didier Descouens. 2010. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Apatite_Canada.jpg

Figure 3.44: Native sulfur deposited around a volcanic fumarole. Brisk g. 2006. Public domain. https://commons.wikimedia.org/wiki/File:Fumarola_Vulcano.jpg

Figure 3.45: Native copper. Jonathan Zander (Digon3); adapted by Materialscientist. 2009. CC BY-SA 2.5. https://commons.wikimedia.org/wiki/File:NatCopper.jpg

Figure 3.46: The rover Curiosity drilled a hole in this rock from Mars, and confirmed the mineral hematite, as mapped from satellites. NASA/JPL-Caltech/MSSS. Public domain. https://www.nasa.gov/jpl/msl/pia19036/

Figure 3.47: 15 mm metallic hexagonal molybdenite crystal from Quebec. John Chapman. 2008. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Molly_Hill_molybdenite.JPG

Figure 3.48: Submetallic luster shown on an antique pewter plate. Unknown author. ca. 1770 and 1810. Public domain. https://commons.wikimedia.org/wiki/File:Pewter_Plate.jpg

Table 3.3: Nonmetallic luster descriptions and examples. Quartz Brésil by Didier Descouens, 2010 (CC BY-SA 4.0, https://commons.wikimedia.org/wiki/File:Quartz_Br%C3%A9sil.jpg). KaolinUSGOV by Saperaud~commonswiki, 2005 (Public domain, https://commons.wikimedia.org/wiki/File:KaolinUSGOV.jpg). Selenite Gips Marienglas by Ra’ike, 2006 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Selenite_Gips_Marienglas.jpg). Mineral Mica GDFL006 by Luis Miguel Bugallo Sánchez, 2005 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Mineral_Mica_GDFL006.JPG). Sphalerite4 by Andreas Früh, 2005 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Sphalerite4.jpg).

Figure 3.49: Azurite is ALWAYS a dark blue color, and has been used for centuries for blue pigment. Graeme Churchard. 2013. CC BY 2.0. https://en.wikipedia.org/wiki/File:Azurite_in_siltstone,_Malbunka_mine_NT.jpg

Figure 3.50: Different minerals may have different streaks. Ra’ike. 2010. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Streak_plate_with_Pyrite_and_Rhodochrosite.jpg

Figure 3.51: Mohs hardness scale. NPS. Public domain (full license here). https://www.nps.gov/articles/mohs-hardness-scale.htm

Table 3.4: Typical crystal habits of various minerals. Elbaite-Lepidolite-Quartz-gem7-x1a by Robert M. Lavinsky, 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Elbaite-Lepidolite-Quartz-gem7-x1a.jpg). Pyrophyllite-290575 by Robert M. Lavinsky, 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Pyrophyllite-290575.jpg). Kyanite crystals by Aelwyn, 2006 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Kyanite_crystals.jpg). Malachite Kolwezi Katanga Congo by Didier Descouens, 2012 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Malachite_Kolwezi_Katanga_Congo.jpg). Ametyst-geode by Juppi66, 2009 (Public domain, https://commons.wikimedia.org/wiki/File:Ametyst-geode.jpg). Calcite-Galena-elm56c by Robert M. Lavinsky, before March 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Calcite-Galena-elm56c.jpg). Pyrite elbe by Didier Descouens, 2011 (CC BY-SA 4.0, https://commons.wikimedia.org/wiki/File:Pyrite_elbe.jpg). Dendrites01 by Wilson44691, 2008 (Public domain, https://commons.wikimedia.org/wiki/File:Dendrites01.jpg). Peridot2 by S kitahashi, 2006 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Peridot2.jpg). Tremolite Campolungo by Didier Descouens, 2009 (CC BY-SA 4.0, https://en.wikipedia.org/wiki/File:Tremolite_Campolungo.jpg). Muscovite-Albite-122887 by Robert M. Lavinsky, 2010 (CC BY-SA 3.0, https://en.wikipedia.org/wiki/File:Muscovite-Albite-122887.jpg). Calcite-Wulfenite-tcw15a by Robert M. Lavinsky, 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Calcite-Wulfenite-tcw15a.jpg). Hanksite by Matt Affolter(QFL247), 2009 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Hanksite.JPG). LimoniteUSGOV by USGS, unknown date (Public domain, https://en.wikipedia.org/wiki/File:LimoniteUSGOV.jpg). Fluorite crystals 270×444 by Ryan Salsbury, 2004 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Fluorite_crystals_270x444.jpg). Calcite-HUGE by Alkivar, 2005 (Public domain, https://commons.wikimedia.org/wiki/File:Calcite-HUGE.jpg). Diopside-172005 by Robert M. Lavinsky, before March 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Diopside-172005.jpg). Tetrahedrite-Chalcopyrite-Sphalerite-251531 by Robert M. Lavinsky, before March 2010 (CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Tetrahedrite-Chalcopyrite-Sphalerite-251531.jpg).

Figure 3.52: Gypsum with striations. Didier Descouens. 2009. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:Gypse_Caresse.jpg

Figure 3.53: Twinned staurolite. Virginia State Parks. 2013. CC BY 2.0. https://flic.kr/p/WxNWqi

Figure 3.54: Striations on plagioclase. Mike Beauregard. 2011. CC BY 2.0. https://flic.kr/p/9xh4MS

Figure 3.55: Citrine, a variety of quartz showing conchoidal fracture. James St. John. 2021. CC BY 2.0. https://flic.kr/p/2ky61rb

Figure 3.56: Graphite showing layers of carbon atoms separated by a gap with weak bonds holding the layers together. Itub; adapted by Materialscientist. 2009. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Diamond_and_graphite2.jpg

Figure 3.57: Cubic cleavage of galena; note how the cleavage surfaces show up as different but parallel layers in the crystal. Modris Baum. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Argentiferous_Galena-458851.jpg

Figure 3.58: Freely growing quartz crystals showing crystal faces. JJ Harrison. 2009. CC BY-SA 2.5. https://en.wikipedia.org/wiki/File:Quartz,_Tibet.jpg

Figure 3.59: Steps of cleavage along the same cleavage direction. USGS; adapted by David Remahl. 2004. Public domain. https://commons.wikimedia.org/wiki/File:WollastoniteUSGOV.jpg

Figure 3.60: Photomicrograph showing 120/60 degree cleavage within a grain of amphibole. Eurico Zimbres. 1990. CC BY-SA 2.5. https://en.wikipedia.org/wiki/File:Amphibol.jpg

Figure 3.61: A demonstration of ulexite’s image projection. Dave Merrill. 2005. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/File:Ulexite_on_flickr_%2821734610%29.jpg

Figure 3.62: Native gold has one of the highest specific gravities. Gump Stump. 2008. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Latrobe_gold_nugget_Natural_History_Museum.jpg

Figure 3.63: Paperclips attach to lodestone (magnetite). Ryan Somma. 1980. CC BY-SA 2.0. https://commons.wikimedia.org/wiki/File:Magnetite_Lodestone.jpg

Figure 3.64: Iridescence on plagioclase, also showing striations on the cleavage surface. Mike Beauregard. 2011. CC BY 2.0. https://flic.kr/p/9xh4MS

Figure 3.65: Exsolution lamellae within potassium feldspar. Jstuby. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Perthitic_feldspar_Dan_Patch_SD.jpg

Figure 3.66: Fluorite. B shows fluorescence of fluorite under UV light. Didier Descouens. 2009. CC BY-SA 4.0. https://commons.wikimedia.org/wiki/File:FluoriteUV.jpg

 

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