3 Minerals

KEY CONCEPTS

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

  • Define mineral.
  • Describe the basic structure of the atom.
  • Derive basic atomic information from the Periodic Table of Elements.
  • Describe chemical bonding related to minerals.
  • Describe the main ways minerals form.
  • Describe the silicon-oxygen tetrahedron and how it forms common silicate minerals.
  • List common non-silicate minerals in oxide, sulfide, sulfate, and carbonate groups.
  • Identify minerals using physical properties and identification tables.

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

A rock is a substance that contains one or more minerals or mineraloids. There are three types of rocks composed of minerals: igneous (rocks crystallizing from molten material), sedimentary (rocks composed of products of mechanical weathering (sand, gravel, etc.) and chemical weathering (things precipitated from solution), and metamorphic(rocks produced by alteration of other rocks by heat and pressure.

3.1 Chemistry of Minerals

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

Image of atom with defined nucleus and electrons surrounding it in a cloud with concentrations of electrons in energy shells
Electron cloud model of the atom

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. Protons and neutrons each have a mass 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.

3.1.1 Periodic Table of the Elements

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

The Periodic Table of the Elements showing all elements with their chemical symbols, atomic weight, and atomic number.
The Periodic Table of the Elements
The first arrangement of elements into a periodic table was done by Dmitri Mendeleev in 1869 using the elements known at the time . In the periodic table, each element has a chemical symbol, name, atomic number, and atomic mass. The chemical symbol is an abbreviation for the element, 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, each with a mass number of one, in the nucleus. 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

The atomic mass of natural elements 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 element exists in nature with atoms having different numbers of neutrons. The differing number of neutrons affects the mass of an element in nature and the atomic mass number represents this average. This gives rise to the concept of isotopes. Isotopes are forms of an element with the same number of protons but different numbers of neutrons. There are usually several isotopes for a particular element. For example, 98.9% of carbon atoms have 6 protons and 6 neutrons. This isotope 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), have 6 protons and 8 neutrons. Among the 118 known elements, the heaviest are fleeting human creations known only in high energy particle accelerators, and they decay rapidly. The heaviest naturally occurring element is uranium, atomic number 92. The eight most abundant elements in Earth’s continental crust are shown in Table 1 . These elements are found in the most common rock forming minerals.

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 1. Eight Most Abundant Elements in the Earth’s Continental Crust % by weight (source: USGS). All other elements are less than 1%.

3.1.3 Chemical Bonding

The hydrogen atoms are on one side, about 105° apart.
A model of a water molecule, showing the bonds between the hydrogen and oxygen.
Most substances on Earth are compounds containing many elements. Chemical bonding describes how these atoms attach with each other to form compounds, such as sodium and chlorine combining to form NaCl, or common table salt. Compounds that are held together by covalent bonds are called molecules. Water is a compound of hydrogen and oxygen in which two hydrogen atoms are covalently bonded with one oxygen making the water molecule. The oxygen we breathe is formed when one oxygen atom covalently bonds 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 minerals are also compounds of more than one element. The common mineral calcite has the chemical formula of CaCO3 indicating the molecule consists of one calcium, one carbon, and three oxygen atoms. In calcite, one carbon and three oxygen atoms are held together by covalent bonds to form a molecular ion, called carbonate, which has a negative charge. Calcium as an ion has a positive charge. The two oppositely charged ions attract each other and combine to form the mineral calcite, CaCO3. The name of the chemical compound is calcium carbonate, where calcium is Ca and carbonate refers to the molecular ion CO3-2.

The mineral olivine has the chemical formula (Mg,Fe)2SiO4, in which one silicon and four oxygen atoms are bonded with two atoms of either magnesium or iron. The comma between iron (Fe) and magnesium (Mg) indicates the two elements 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. In 1913, Niels Bohr proposed a simple model that states atoms are more stable when their outermost shell is full . Atoms of most elements tend to gain or lose electrons so the outermost or valence shell is full. This tendency of atoms to have a full valence shell is known as the octet rule. In Bohr’s model, the innermost shell can have a maximum of two electrons and the second and third shells can have up to eight electrons. 

Carbon dioxide molecule with a carbon ion in the center and two oxygen ions on either side, each sharing two electrons with the carbon.
The carbon dioxide molecule. Since Oxygen is -2 and Carbon is +4, the two oxygens
The rows in the periodic table present the elements in order of atomic number and the columns organize elements 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 elements in columns I and II have 1 and 2 electrons in their respective valence shells and the elements in columns VI and VII have 6 and 7 electrons in their respective valence shells.

Sodium (Na), in the second row and column 1, has 11 protons and two electrons in its inner electron shell, eight electrons in the second shell, and one electron in the valence shell. Since the outermost shell has one electron, all elements in this column have a valence of 1. To maintain a full outer shell (of eight), sodium will readily give up that one electron, so that there are 10 total electrons. With 11 positively charged protons in the nucleus and 10 negatively charged electrons in its now two shells, the overall net charge would be +1. When stripped of that valence electron, elements in column I thus have a +1 charge and elements in column II are +2. Note that the primary columns are designated with Roman numerals. Columns numbered 3 through 15 are usually involved with covalent bonding. 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. Since the noble gas helium only has two valence electrons in the first shell, helium only needs two valence electrons for its outer shell to be full. In column 17 (VII), chlorine has seven electrons in its valence shell, so it gains an electron to be full. Thus chlorine with 17 protons and now 18 electrons has a net charge of -1.

An atom that has gained or lost electrons resulting in a net charge is called an ion. Although there are many groups of elements on the Periodic Table with different bonding properties, in general the elements on the left side lose electrons, become positive ions, and are called cations because they are attracted to the cathode in an electrical device. The elements on the right side tend to gain electrons. These are called anions because they are attracted to the anode in an electrical device. In the center of the periodic table (columns 3 through 15), the octet rule is less consistent in its application. Called transition elements, their charge is not straight forward. A common example of this is iron, which typically has a +2 or +3 charge depending on the oxidation state of the element. Fe+3 is referred to as “oxidized.”  Fe+2 is referred to as “reduced.” These two different oxidation states of iron often impart dramatic colors to rocks containing their minerals, the oxidized form producing red colors, the reduced form producing green.

Ionic (ELECTRON TRANSFER) Bonding

Image of crystal model of halite with ions of sodium and chlorine arranged in a cubic structure.
Cubic arrangement of Na and Cl ions in Halite
Atoms of two opposite charges attract each other electrostatically to form an ionic bond. One atom of sodium, Na+1, and one atom of chlorine, Cl-1, combine in an ionic bond to make the compound sodium chloride (NaCl). This is also known as the mineral halite or common table salt. Another example is one atom of calcium, Ca+2, and two atoms of chlorine, Cl-1, combine in an ionic bond to make the compound calcium chloride (CaCl2) where the subscript of two indicates two atoms of chlorine in the bond.

Covalent (ELECTRON SHARING) Bonding

Each atom is sharing electrons.
Methane molecule
Ionic compounds are usually formed between a metal  and a nonmetal, however another bonding type commonly occurs between combinations of nonmetals called covalent bonding. Covalent bonds share electrons between ions to complete their valence shells. For example, consider oxygen, atomic number 8. It has 8 electrons, 2 in the inner shell and 6 in its valence shell. Gasses like oxygen often form diatomic molecules by sharing some valence electrons; in the case of oxygen two atoms attach together and share 2 electrons so that each fills its valence shell. Methane (CH4) is covalently bonded since carbon needs four more electrons and the hydrogens each need one.  Each hydrogen shares an electron with the carbon forming a CH4 molecule as shown in the figure.

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3.2 Formation of Minerals

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

3.2.1 Precipitation from aqueous solution

Encrusted calcium carbonate (lime) deposits on faucent
Calcium carbonate deposits from hard water on a faucet
Solutions have ions or molecules (known as solutes) dissolved in a medium (known as the solvent).  In nature this is usually water. The amount of solute that can be dissolved in a solvent depends on the temperature and pressure. Minerals can be dissolved in water such as the mineral halite (or table salt) with the composition sodium chloride, NaCl. The Na+1 and Cl-1 ions separate and disperse into the solution. Precipitation is the reverse process, in which ions in solution turn into solid minerals. Precipitation is dependent on the concentration of the ions in the solution and other factors such as temperature and pressure. The point at which a solvent cannot hold any more solute is called saturation. It can occur when temperature falls, when water evaporates, or even with changing chemical conditions in the solution. An example of precipitation in our homes is when water evaporates and leaves behind a rind of minerals on faucets, shower heads, and glasses. In nature, when water flows through rocks and conditions change, ions come out of solution to form bonds and grow crystals in caves or between grains of sediment. When groundwater enriched with dissolved carbon dioxide (carbonic acid) releases it as gas after it emerges from springs, calcite is precipitated from solution as tufa or travertine. Deposits of tufa built up when springs emerged underwater in former Lake Bonneville in Utah.  Now exposed in the dry valleys, this porous tufa was used by the pioneers to build their homes; it proved a natural insulation against summer heat and winter cold.  The travertine terraces at Mammoth Hot Springs in Yellowstone Park are being formed by calcite precipitation at the edges of the shallow spring-fed ponds. Most iron deposits on Earth were formed over 2.5 billion years ago when oxygen entered the oceans via photosynthesis, the iron was oxidized in solution, and precipitated in banded iron formations.

The Bonneville Salt Flats of Utah
The Bonneville Salt Flats of Utah

Another example occurs in the Great Salt Lake, Utah, where the concentration of salt ions (including sodium and chlorine and other salts) in the Great Salt Lake is nearly eight times greater than in the world’s oceans . These ions are carried into the Great Salt Lake by streams draining the surrounding mountains and carrying weathered ions. The water in the lake evaporates and concentrates the dissolved salts. The concentration of salt increases until saturation is reached and salt precipitates out in the sediments of the lake. Such salt deposits, including halite and other precipitates, occur in other salt lakes like the Mono Lake, California and the Dead Sea.

3.2.2 Crystallization from magma

A lava flow
Lava, magma at the earth’s surface
Magma is molten rock with freely moving ions. As the magma cools, the atoms move slower and atomic vibrations become slow enough to allow ions to form bonds and crystallize as minerals. In magma, ions such as silicon, oxygen, iron and magnesium bond together in a mix of ionic and covalent bonds to create minerals like olivine (Mg,Fe)2SiO4. The two elements in parentheses, magnesium (Mg) and iron (Fe), are in what scientists call a solid solution. Not to be confused with a liquid solution, this occurs when two or more elements have similar properties and can freely substitute for each other. These minerals grow within a liquid magma until the magma has completely solidified to form an igneous rock. As the magma cools, both iron and magnesium (whose ions are about the same size and charge) can fit into the newly growing crystal. The new mineral ends up having a mixture of the two as different elements come out of the magma and into the solid mineral. Even rare elements with similar properties, like manganese (Mn), substitute into the structure in small amounts. Such ionic substitutions give rise to the great variety of minerals on and in the earth and are often responsible for differences in color and other properties within a group or family of minerals.

3.2.3 Precipitation by organisms

Shell of an ammonite, an extinct cephalopod, with a spiral shell in a plane.
Ammonite shell made of calcium carbonate
Many organisms build bones, shells, and body coverings by extracting ions from water and precipitating minerals biologically. The most common mineral precipitated by organisms is calcite (calcium carbonate, CaCO3) and polymorphs like aragoniteMarine invertebrates such as corals and clams precipitate calcium carbonate in the form of aragonite or calcite for their living structures. Upon death, their hard parts disintegrate into small fragments on the ocean floor and make the common sedimentary rock limestone. Though limestone can form inorganically, the vast majority are formed in this way. Another example is marine radiolaria, which are zooplankton that precipitate microscopic silica for their small shells. When the organism dies, the shell accumulates on the ocean floor to form the sedimentary rock chert. An example from the vertebrate world includes human bone that is mostly a type of apatite mineral in the phosphate mineral group that contains water in its structure called hydroxyapatite, Ca5(PO4)3(OH).

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3.3 Silicate Minerals

It is a pyramid shape with a triangular base
Rotating animation of a tetrahedron

There are nearly four thousand known minerals on Earth, and 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 minerals. Minerals are grouped based on their composition and structure, specifically based on the anion that forms similar structures within the group. The largest of these groups, comprising the vast majority of the Earth’s mantle and crust is the silicate minerals. Silicate minerals are built around a molecular ion called the silicon-oxygen tetrahedron. A tetrahedron has a pyramid-like shape with four sides and corners.

Model of silicon-oxygen tetrahedron of ping pong balls with a tiny silicon ion in the space in the middle of the four large balls
Ping pong ball model of tetrahedron: balls are oxygen, small space in center is silicon

The silicon-oxygen tetrahedron consists of a single silicon atom at the center and an oxygen atom located at each of the four corners of the tetrahedron. The oxygen ion has a -2 charge and silicon ion has a +4 charge. The silicon ion shares one of its four electrons with each of the four oxygen ions in a covalent bond to create a symmetrical geometric four-sided pyramid figure. Only half of the oxygen’s valence electrons are shared, this thus makes the silicon-oxygen tetrahedron itself a molecular ion with an ionic charge (SiO4)-4. The silicon-oxygen is thus capable of forming bonds with other ions to form the large group of silicate minerals.

Top ball removed showing the tiny silicon ion in the center
The silicon atom in the center of the tetrahedron (with the top oxygen removed) represented by a metal ball

 The silicon ion is much smaller than the oxygens (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 silicon-oxygen tetrahedron has chemically active corners available to form bonds with other silica tetrahedra 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 magma chemistry, silica-oxygen tetrahedra 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 silicate mineral groups.   

3.3.1 The dark ferromagnesian silicates

Many small crystall of the green mineral olivine in a mass of basalt
Green olivine in basalt
The Olivine Family

Olivine is characteristically green (when not weathered, see Chapter 5). This mineral is common in mantle rock (peridotite) and basalt. The chemical formula is (Fe,Mg)2SiO4  and shows iron (Fe) and magnesium (Mg) separated by a comma. This indicates that iron and magnesium can occupy the same location in the crystal structure and substitute for one another, creating what is called a solid-solution between the iron and magnesium end members. While in this case iron and magnesium both have a  +2 charge, it turns out that the size of ions is actually more important than charge in determining how they fit into crystal structures.

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

Olivine is referred to as a mineral family because of the ability for iron and magnesium to substitute, and has a pure iron end-member (called fayalite) and a pure magnesium end-member (called forsterite). Chemically, olivine is mostly silica, iron, and magnesium and therefore is grouped among the dark-colored ferromagnesian (iron=ferro, magnesium=magnesian) or mafic minerals. The olivine crystal structure is based on independent silica tetrahedra, a type of silicate structure that contains isolated silica tetrahedra ions that have not polymerized (connected together by sharing corner oxygens; see below). The independent silica tetrahedra are held together by bonds between their corner oxygens and metallic ions. Other important silicate minerals with independent tetrahedra (called nesosilicates or orthosilicates) include garnet, topaz, kyanite, and zircon. Two other similar arrangements of tetrahedra are close in structure to the neosilicates and grade toward the next group of minerals, the pyroxenes. Sorosilicates share one oxygen between two tetrahedra, and include minerals like pistachio-green epidote. Cyclosilicates, as the name suggests, have rings of tetrahedra, and include valuable minerals like beryl (with varieties emerald and aquamarine) and tourmaline.

3.3.2 Pyroxene Family

Dark green crystals of diopside, a member of the pyroxene family
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.
Single chain
Pyroxene is another family of minerals, also ferromagnesian, typically with black or dark green color. Pyroxenes have a complex chemical composition with iron, magnesium, aluminum and other elements bonded with polymerized silica tetrahedra. They are common in the upper mantle rock peridotite, the igneous rocks basalt and gabbro (see Chapter 4), as well as the metamorphic equivalents of these rocks, eclogite and blueschist, and silica-rich carbonate metamorphic rocks (see Chapter 6).

Pyroxenes have a single-chain structure in which polymerized silica tetrahedra share two corners, forming long strands of silica tetrahedraMinerals constructed on this single-chain arrangement bond with many elements substituting for each other in the crystal structure. This complex composition can be written as XZ(Al,Si)2O6, in which X typically equals Na, Ca, Mg, or Fe and Z typically equals Mg, Fe, or Al. Similarities in ionic size between Na/Ca, Mg/Fe/Al, and Al/Si lead to many possible substitutions. Since the charges of all of the substituting elements are not equal (e.g. Na+1 and Ca+2), the net charge must still balance overall and substitutions of unequal change in one location in the crystal are balanced by accompanying unequal charge substitutions in another. All minerals which have a chain of tetrahedra (called inosilicates) are within the pyroxene family, or are very similar in their shape to the pyroxenes.

3.3.3 Amphibole Family

Black crystals of hornblende
Hornblende crystals

A crystal of orthoclase (potassium feldspar) wth elongated dark crystals of hornblende
Elongated crystals of hornblende in orthoclase
The amphibole family of minerals are common in many igneous rocks, have a long bladed crystal habit, and a variety of colors depending on composition but the most common amphibole, hornblende, is usually black. 

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
Double chain structure
Amphiboles are composed of iron, magnesium, aluminum, and other cations bonded with silica tetrahedra. Because of the abundance of iron and magnesium, dark colored amphiboles are ferromagnesian minerals and are common in gabbro, basalt, diorite, and often form the dark specks in granite. They also define the metamorphic rock “amphibolite” which is primarily composed of amphibole minerals (see Chapter 6). Amphiboles have a double chain tetrahedral structure with a complex formula, (RSi4O11)2, where R is a large number of different cations. To show this complexity, it can be written more exactly as AX2Z5((Si,Al,Ti)8O22)(OH,F,Cl,O)2, in which A can be blank, Ca, Na, K, or Pb; X equals Li, Na, Mg, Fe+2, Mn+2, or Ca; and Z is Li, Na, Mg, Fe+2, Mn+2, Zn, Co, Ni, Al, Fe+3, Cr+3, Mn+3, V+3, Ti, or Zr. The substitutions add elements that cause amphiboles to have many colors such as green, black, colorless, white, yellow, blue, or brown. Amphibole minerals can also have hydroxyl ions (OH-1) which indicates an interaction between the growing minerals and water dissolved in the magma from which they precipitate, as discussed in Chapter 4  . All minerals which have a double chain of tetrahedra (also called inosilicates) are within the amphibole family.

3.3.4 Micas and Clays (Sheet Silicates)

Dark brown crystals of biotite mica showing sheet-like habit
Sheet crystals of biotite mica
Crystal of muscovite mica showing sheet structure of the mineral
A stack of sheets of muscovite mica

Mica minerals and clay minerals are the common sheet silicates, and have a formula that has silicon and oxygen in a 2:5 ratio (commonly Si4O10). Micas are common in igneous rocks (Chapter 4) and metamorphic rocks (Chapter 6), while clay minerals are common in sedimentary rocks (Chapter 5). Two common micas are the dark-colored biotite (a ferromagnesian form) and the light-colored muscovite (a felsic form, see Chapter 4).

Continuous sheets of tetradedra with all three base corners bonded to each other; the top corner active to bond with anions
Sheet structure of mica, view perpendicular to the sheets

Chemically, micas contain mostly silica, aluminum, and potassium, however the biotite mica has more iron and magnesium and therefore biotite micas are grouped among the dark-colored ferromagnesian silicate minerals. Muscovite micas are grouped among light-colored silicate minerals. Biotite is common in granite and muscovite is common in metamorphic rocks such as schist. Double chains are connected (polymerized) by their remaining outer third corners into continuous sheets of tetrahedra to make sheet silicates. The remaining top corner is chemically active to bond with anions.

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.
Crystal structure of a mica, view parallel to the sheets
Silica sheets layered in mica like bread and hjam in a stack of sandwiches
Mica “silica sandwich” structure related to layers in illite structure.

 

 

 

 

 

 

 

 

This illustration shows the crystal structure of a mica where the remaining corner oxygens (blue) of the tetrahedra bond with anions into sheets, with an octahedral layer between. In the figure, sheets of tetrahedra are shown with oxygen (blue) and silicon (purple). The two sheets of tetrahedra can be viewed like the bread in a sandwich (see illustration to the right). The “sandwich spread” is the iron, magnesium, and aluminum ions in the octahedral layers between the tetrahedral sheets. Large yellow potassium ions form weak Van der Waals bonds between these “sandwiches,” holding them together into “sandwich stacks.” These weak Van der Waals bonds can be easily separated along these potassium layers giving mica its characteristic property of easily cleaving into sheets. The sheet silicates are called phyllosilicates.

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

Although not mica minerals, other silicate minerals with sheet structure  include the many clay minerals that form by weathering. Clay minerals are hydrous aluminum silicates. One such clay mineral, kaolinite, has a structure like an “open-faced sandwich” with a single layer of tetrahedra (with silicon/oxygen) and a single layer of octahedra (with aluminum). The clay minerals form a complex family and are an important component of many sedimentary rocks. Other sheet silicates include serpentine and chlorite in metamorphic rocks.

3.3.5 Quartz and Feldspar (Framework Silicates)

A mass of quartz crystals showing typical six sided habit with points
Quartz crystals
Quartz and the feldspars are the two most common minerals in the continental crust. Quartz and the feldspars are light-colored silicates that are often transparent, white, gray, or pink. Their color comes from abundant silica and aluminum content. These minerals are very common in the igneous rocks granite, and rhyolite as well as detrital sedimentary rocks (composed of mechanically weathered rock particles like sand and gravel) and some metamorphic rocks. Feldspar is the most abundant mineral in the Earth’s crust . Quartz is one of the most common minerals in detrital sedimentary rocks because it is especially resistant to disintegration by weathering.  

A group of crystals of pink potassium feldspar
Pink orthoclase crystals
Compositionally, quartz is pure silica, SiO2, and feldspars are mostly silica with some aluminum, potassium, sodium, and calcium. Two common feldspars are potassium feldspar (k-spar) and plagioclase. Potassium feldspar is silica, aluminum, and potassium (KAlSi3O8) and continental igneous rocks are called felsic because of the abundance of k-spar in them). Plagioclase is best described here as a series of minerals – one end of the series with calcium (CaAl2Si2O8, called anorthite) and the other end with sodium (NaAlSi3O8, called albite). Minerals in this series have many mineral 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
Crystal structure of potassium feldspar

Quartz and the feldspars are framework silicates, meaning the silica tetrahedra are arranged such that all four of their corners are shared in three dimensions. Thus, quartz has a formula of SiO2, since the tetrahedra are connected together by sharing all four corners. Tetrahedra in the feldspar group also share all corners but the crystal frameworks have holes and spaces that accommodate other positively charged ions like potassium (shown), sodium, and calcium.  Note that aluminum, close in ionic size to silicon, can substitute for it in the tetrahedra. These are unequal charge substitutions of sodium and calcium with the accompanying substitution of aluminum and silicon and will be discussed in the igneous rocks chapter. Because potassium ions are so much larger than sodium and calcium ions, which are very similar in size, also note that the inability of the crystal lattice to accommodate both potassium and sodium/calcium gives rise to the two families of feldspar, orthoclase (k-spar) and plagioclaseOther framework silicates (tectosilicates) include the alkali metal-rich feldspathoids and zeolites.

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3.4 Non-Silicate Minerals

The mineral is hexagonal and clear.
Hanksite, Na22K(SO4)9(CO3)2Cl, one of the few minerals that is considered a member of two groups: carbonate and sulfate
Non-silicate minerals (see table below) include all minerals that do not use the silica-oxygen tetrahedra in their crystal structure. Many of these minerals are important economically and provide many of our natural metallic resources such as copper, lead, iron, as well as many other non-metallic products of value such as salt, construction materials, and fertilizer. See chapter 16 for more details on the mining, processing, and uses for these minerals.

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

Common non-silicate mineral groups.

3.4.1 Carbonates

Calcite crystal in a shape called a rhomb like a cube squahed over toward one corner
Calcite crystal in shape of rhomb. Note the double-refracted word “calcite” in the center of the figure due to birefringence.

The two most common carbonate minerals are calcite (CaCO3) and dolomite (CaMg(CO3)2). These minerals make up the common rocks limestone and dolostone, respectively. Crystals of calcite show an interesting property called birefringence due to the fact that they polarize light into two wave directions.  The two light waves pass through the crystal at different velocities giving rise to seeing a double image of objects seen through the crystal. Because they polarize light, calcite crystals are used in special petrographic microscopes for studying minerals and rocks.

Piece of limestone rock full of small fossils
Limestone full of small fossils

Many of these carbonate rocks contain fossils, and comprise many common rocks on Earth’s surface, including limestone. Though some calcite and dolomite can form via evaporation, the most common origin of the carbonate minerals in limestone and related rocks come directly from the fossil organisms. Over time, the calcite comprising shells and other hard parts of marine organisms may break down and blend together to form the limestone rock, possibly surrounding visible fossils. So, while limestones can have large, easy to see fossils, most limestones, even those that seem fossil-free, are actually the product of former life. The term salts used here refers to compounds made by replacing the hydrogen in natural acids. The most abundant natural acid is carbonic acid that forms by the solution of carbon dioxide in water. Carbonate minerals are salts built around the carbonate ion (CO3-2) where calcium and/or magnesium replace the hydrogen in carbonic acid (H2CO3). Calcite and a closely related form aragonite are secreted by organisms to form shells and physical structures like corals. Many such creatures draw calcium and carbonate from dissolved bicarbonate ions (HCO3-1) in ocean. As seen in the mineral identification section below, calcite is easily dissolved in acid and thus effervesces in dilute hydrochloric acid (HCl) as shown in this video. Small dropper bottles of dilute hydrochloric acid are often carried by geologists in the field as well as used in mineral identification labs.

Crystal structure of calcite showing the carbonate units of carbon surrounded by three oxygen ions and bonded above and below to two calcium ions.
Crystal structure of calcite

This figure shows the crystal structure of calcite with carbonate units consisting of carbon ions (tiny white dots) surrounded by three oxygen ions (red dots) that are bonded to Ca+2 ions (blue). Recall bonding types from Section 3.1.3 in which ionic and covalent bonding were discussed. The carbon and oxygen in the carbonate  CO3-2 unit bond covalently into the red triangular shapes in the diagram leaving a negative 2 charge on the molecular ion.  That charged unit bonds ionically with a Ca+2 ion (blue in the diagram) to form the mineral calcite CaCO3.

3.4.2 Oxides, Halides, and Sulfides

After carbonates, the next most common non-silicates are the oxide, halide, and sulfide groups. Oxides are minerals in which positive metal ions bond with oxygen.

Image of limonite, a hydrated oxide of iron
Limonite, a hydrated oxide of iron

The most familiar oxide is iron rust, which is a combination of iron oxides and hydrated iron oxides (contain water molecules in their structure). Iron oxide minerals include limonite, magnetite, and hematite. These minerals form when iron is exposed to oxygen in the presence of water. Most red colors in rocks are due to iron oxide which imparts color even in small quantities. For example, the cliffs of red sandstone in Zion National Park and Southern Utah consist of white or colorless grains of quartz coated by iron oxide between the grains. Iron oxides are important ores of iron from which metallic iron can be smelted. The result of that smelting process is the production of an oxide of carbon, carbon dioxide (CO2), and metallic iron.

A red form of hematite called oolitic showing a mass of small round nodules
Oolitic hematite
The same mineral hematite is often seen in many different forms, including massive, botryoidal, oolitic, and specular. Massive is without internal structure. Botryoidal is hematite in large concentric blobs. Specular hematite looks like a mass of shiny metallic crystals. Oolitic hematite looks like a mass of dull red fish eggs.  All are of the same chemistry, Fe2O3.

Other common oxide minerals include ice, an oxide of hydrogen (H2O), bauxite, a rock consisting of hydrated oxides of aluminum (a source of that important metal), and corundum (Al2O3). Corundum, when colored and translucent, make the gems ruby and sapphire.

Crystals of halite showing cubic crystal habit
Halite crystal showing cubic habit

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

The halides are combinations of fluorine or chlorine (or other column 17 elements, the halogens) with sodium or other cations. Common halides are halite, which is sodium chloride (NaCl) or common table salt, sylvite, which is potassium chloride (KCl), and fluorite, which is calcium fluoride (CaF2).

Photo of salt crust at the Bonneville Salt Flats in Utah with mountains in the background.
Salt crystals at the Bonneville Salt Flats
These halide minerals are often formed by evaporation of sea water or other isolated bodies of water. A well known example of halide accumulation by evaporation is the Salt Flats west of the Great Salt Lake in Utah. The image shows the crust of salt crystals at the Salt Flats. See chapter 5.3.3 for more details of the formation of these evaporite minerals.

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

3.4.3 Sulfates

A clear crystal of gypsum
Gypsum crystal

Sulfates, unlike the sulfides above, are a combination of sulfur and oxygen, in the anion SO4-2. Sulfate minerals contain a positively charged metal ion, such as calcium, bonded to the negative sulfate ion. A common sulfate mineral is gypsum (CaSO4ᐧ2H20) which is often formed from evaporating water. The “ᐧ2H20″ in the mineral structure means that whole water molecules are structurally captured into the mineral’s crystalline structure. This is different than minerals like Amphibole which have hydroxide ion (OH-1) derived from water, but not the whole molecule. Gypsum is used for plaster and drywall in the construction industry. Gypsum without the water is a different mineral called anhydrite (CaSO4).

3.4.4 Phosphates

A crystal of apatite
Apatite crystal

The phosphates are a group of minerals built with the tetrahedral PO4-3 unit (with some As and V substituting for P) in combination with various anions. The most well-known phosphate mineral is apatite, Ca5(PO4)3(F,Cl,OH). An important use of the phosphates is in the production of fertilizers. Variations of the mineral apatite are found in animal teeth and bones. A copper-rich phosphate with water is turquoise, CuAl6(PO4)4(OH)8·4H2O, often used as a gem stone.

3.4.5 Native Element Minerals

Metallic native copper
Native copper
Native sulfur deposited around the vent of a volcanic fumarole
Native sulfur deposited around a volcanic fumarole

Some important elements occur as native element minerals in nature; gold is among the best known. Not all elements occur naturally in their native state. Some metals found in native form like gold are not very reactive, and are almost never found in nature bonded with other elements. Other metals like iron, lead, and aluminum are more reactive, are almost always bonded to other elements, and are rarely found isolated (or native) in nature. Metals like silver, copper, platinum, mercury, and sulfur are mildly reactive, and can be found in minerals or in a native state. Carbon (C) can be found as a native element, in both graphite (used for writing, known as “pencil lead” in pencils) and diamond.

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3.5 Identifying Minerals

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

Identifying minerals requires the recognition of some physical properties. Only a few common minerals make up the majority of Earth’s rocks and are usually seen as small grains in rocks. The following physical properties are the most useful in identifying the common minerals in the field. Even with new advances in technology that can identify minerals with powerful machines, these techniques are still useful in the sometimes harsh and isolated locations that geologists work.

3.5.1 Luster and Color

The crystal looks like metal.
15 mm metallic hexagonal molybdenite crystal from Quebec.
The first thing seen on a mineral grain is the appearance of its surface. How light is reflected from its surface is called luster. Luster describes how the mineral looks, and the two main categories of luster are metallic luster and nonmetallic luster. Metallic luster looks shiny, like a metal such as chrome, steel, silver, or gold.

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

A dull, not-shiny, yet metallic appearance like pewter is called submetallic. Nonmetallic luster does not have an appearance like metal, but rather can be glassy, earthy, or other lusters. Nonmetallic minerals can still have a shiny appearance, just not the same shine as metal. Some nonmetallic luster terms are illustrated in the table below.

Luster Image Description
Vitreous/glassy
A mass of quartz crystals showing typical six sided habit with points
Quartz crystals
Surface is shiny like glass
Earthy/dull

Specimen of kaolin, a clay oineral, showing dull or earthy luster
Kaolin specimen showing dull or earthy luster

Dull, like dried mud or clay
Silky
Specimen showing silky luster
Specimen showing silky luster
Soft shine like silk fabric
Pearly
Specimen showing pearly luster like the inside of a clam shell
Specimen showing pearly luster
Like the inside of a clam shell or mother-of-pearl
Submetallic
Photo of mineral exhibiting submetallic luster
Submetallic luster on sphalerite
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.

There are two dark blue disks on white siltstone.
Azurite is ALWAYS a dark blue color, and has been used for centuries for blue pigment.
In addition to luster, color may be helpful in identification. Color may also be quite variable even within the same mineral family and is a result of the elements present within the mineral, both the main elements and sometimes impurities in the crystals. These impurities are either rare elements (like Mn, Ti, Cr, Li) or elements/molecules that are not part of the mineral formula (e.g. water molecules in quartz make it milky). Some minerals (e.g. pyrite, olivine, epidote, bornite, galena, magnetite, etc.) have a predictable color. For example, copper minerals like malachite and azurite are green and blue because of their copper content. Other minerals (e.g. muscovite, biotite, pyroxene, limonite, feldspar, talc, etc.) have a predictable range of colors. These colors can change as different elements substitute via solid solution, as in feldspar. Typically, the K-feldspar is more colorful than the plagioclase feldspar (with Na and Ca). Other minerals (e.g. quartz, tourmaline, calcite, corundum, topaz, etc.) are so influenced by their trace element inclusions that they can be any color imaginable. Note that the same element can have different effects on different minerals. Therefore, it is important to use color with caution and with knowledge of the variability associated with specific minerals; this comes with experience. A more reliable indicator is the color of the mineral powdered, called streak (described below).

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
Some minerals have different streaks than their visual color

Many minerals show a property called streak which is the color of the mineral’s powder when scratched on an unglazed porcelain streak plate (a paper page in a field notebook may also give a useful streak). While the color and appearance of a mineral may vary, the streak color may be diagnostic. An example is the iron oxide mineral hematite. Specimens of hematite show different colors and lusters ranging from shiny metallic silver to earthy red/brown,  even different physical appearances. But hematite shows a reddish brown streak, no matter its appearance in specimens. Another example is the iron sulfide, pyrite, which has a yellowish brassy color (hence the common name fool’s gold) but has a characteristic black to greenish black streak. Minerals which are harder than the streak plate will not show streak, but will instead leave a scratch in the streak plate. A streak can still technically be obtained via powdering the mineral with a hammer.

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.
Mohs Hardness Scale

Mineral hardness is the ability of one mineral to scratch another. The idea of a hardness scale comes from the ability of one mineral or substance to scratch another softer mineral or substance. The Mohs Hardness Scale scale is a relative scale that gives the resistance of minerals to scratching by a set of specific defining minerals and objects. It was developed by German geologist Fredrick Mohs in the early 20th century, though the idea of minerals with varying hardness goes back thousands of years. The hardness values on the Mohs Hardness Scale vary from 1 to 10 and are determined by the relative strengths of the atomic bonds within a mineral. The minerals that define the hardness values are shown in the figure together with some common substances that are often available in the field. The hardness values from 1 to 9 are not linear, but appear to vary in a fairly consistent way. Diamond, which defines a hardness of 10, is actually about four times harder than corundum which defines 9 and would appear as about 40 on a linear scale. As indicated above, the chart shows common items that can be used for hardness testing in the field. Most geologists use their fingernail, a copper penny, and a pocket knife or fingernail file for a personal hardness set. In fact, the steel blade of a pocketknife will distinguish between “hard” and “soft” for use with many mineral identification keys.

3.5.4 Crystal Habit

Crystal habit refers to how mineral crystals typically grow and appear in rocks. These shapes derive from the arrangement of the atoms within the crystal structure (e.g. a cubic arrangement of atoms in the crystal can result in a cubic shape for the mineral crystal). The shapes produced refer to the typical shapes and character that are observed in minerals. However, it is important to realize that all minerals can be constrained in their growth by other minerals, for example when crystallizing together in a rock. When they are so constrained and don’t demonstrate their typical crystal habit, they are called anhedral. Subhedral is a crystal with a partially formed shape, and euhedral is a perfectly-formed crystal. Some minerals are so finely-crystallized, that even when euhedral, they do not show a specific crystal habit to the naked eye. Other minerals, like pyrite, can have an array of different crystal habits: from cubic, dodecahedral, octahedral, to massive. Below are some of the many examples of crystal habit.

Habit Image Examples
Bladed

long and flat crystals

The crystals are long and rectangular
Bladed kyanite
kyanite, amphibole, gypsum
Botryoidal/mammillary

blobby, circular crystals

The mineral is bulbous
Malachite from the Congo
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) in a geode
quartz, calcite, malachite, azurite
Cubic

cube-shaped crystals

Cubic crystals of galena, a sulfide of lead
Cubic crystals of galena

pyrite, galena, halite
Dodecahedral

12-sided polygon shapes

Crystals of pyrite showing dodecahedral habit
Pyrite crystals with dodecahedral habit
garnet, pyrite
Dendritic

branching crystals

The mineral look like a fern. They are black and branching.
Manganese dendrites, scale in mm.
Mn-oxides, copper, gold
Equant

crystals that do not have a long direction

The crystal is light green.
Olivine crystal
olivine, garnet, pyroxene
Fibrous

thin, very long crystals

It is white and fiberous
Tremolite, a type of amphibole
serpentine, amphibole, zeolite
Layered, sheets

stacked, very thin, flat crystals

Sheets of muscovite mica in crystal mass
Sheet crystals of muscovite

mica (biotite, muscovite, etc.)
Lenticular/platy

crystals that are plate-like

 
The orange wulfenite is platey
Orange wulfenite on calcite
selenite roses, wulfenite, calcite
Hexagonal

crystals with six sides

The mineral is hexagonal and clear.
Hexagonal hanksite

quartz, hanksite, corundum
Massive/granular

Crystals with no obvious shape, microscopic crystals

Image of limonite, a hydrated oxide of iron
Limonite, a hydrated oxide of iron

limonite, pyrite, azurite, bornite
Octahedral

4-sided double pyramid crystals

Perfedt octahedral cleavage in fluorite generates octagon-shaped cleavage flakes.
Octahedral fluorite
diamond, fluorite, magnetite, pyrite
Prismatic/columnar

very long, cylindrical crystals

The mineral is a long cylinder.
Columnar tourmaline
tourmaline, beryl, barite
Radiating

crystals that grow from a point and fan out

The mineral is orange
Pyrophyllite
pyrite “suns”, pyrophyllite
Rhombohedral

crystals shaped like slanted cubes

Calcite crystal in a shape called a rhomb like a cube squahed over toward one corner
Calcite crystal in shape of rhomb

calcite, dolomite
Tabular/blocky/stubby

sharp-sided crystals with no long direction

Dark green crystals of diopside, a member of the pyroxene family
Crystals of diopside, a member of the pyroxene family
feldspar, pyroxene, calcite
Tetrahedral

three-sided, pyramid-shaped crystals

The dark brown mineral is triangular
Tetrahedrite
magnetite, spinel, tetrahedrite

The brown minerals are replicated in different directions
Twinned staurolite
The mineral has many parallel lines on it
Gypsum with striations
Other crystal habit properties that may be useful include striations, which are lines on a crystal face, and twinning, which occurs when the crystal structure replicates in mirror images along certain directions in the crystal.

3.5.5 Cleavage and Fracture

A specimen of a variety of quartz showing conchoidal fracture
Citrine, a variety of quartz, showing conchoidal fracture

In most rock specimens mineral crystals have broken surfaces. Mineral grains often show characteristic patterns of breaking with specific fracture patterns or cleavage planes. Minerals that break with fracture patterns have a rough uneven surface or show conchoidal fracture. Uneven fracture patterns are irregular, splintery, and or fibrous. Conchoidal fracture has a curved appearance like a shallow bowl or conch shell, often with curved ridges. Large pieces of glass or natural volcanic glass (obsidian) break with this characteristic conchoidal pattern. Quartz and olivine, both without a strong cleavage, break in conchoidal fracture patterns. 

The animation rotates, showing the gap.
Rotating image of atomic arrangement in graphite. Note the large gap, which is a weakness. This weakness is the cleavage plane in graphite.

Cleavage arises in crystals where the bonds between ions are weaker along some directions than others. These are planes in the crystal structure which are repetitive, meaning it will break preferentially along these planes. Cleavage results in shiny planar surfaces. Some minerals have a strong cleavage, some minerals only have weak cleavage and do not typically demonstrate cleavage in samples.

Specimen of galena showing cubic cleavage
Cubic cleavage of galena; note how the cleavage surfaces show up as different but parallel layers in the crystal.
Cleavage planes are a series of smooth, flat, parallel planes in a crystal. The cleavage planes may show as parallel cracks that penetrate into the crystal or show on the edge or side of a mineral as a series of steps like stairs. To work with cleavage, it is important to remember that cleavage is a result of bonds separating along planes of atoms in the crystal. Cleavage planes may be confused with crystal faces. This will usually not be an issue for crystals  of minerals 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.

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

To distinguish cleavage planes from crystal faces, cleavage planes may have multiple parallel cracks or flat surfacesCleavage planes may be expressed as a series of steps like terraced rice paddies. See the cleavage surfaces on galena above. Cleavage planes arise from the tendency of mineral crystals to break along specific planes of weakness within the crystal favored by atomic arrangementsThe number of cleavage planes, the quality of the cleavage surfaces, and the angles between them are diagnostic for many minerals and cleavage is one of the most useful properties for identifying minerals. Learning to recognize cleavage is an especially important and useful skill in studying minerals.

Photomicrograph showing 120/60 degree cleavage in amphibole
Photomicrograph showing 120/60 degree cleavage within a grain of amphibole
The most common number of cleavage plane directions in the common rock-forming minerals are: one perfect cleavage (as in mica), two cleavage planes (as in feldspar, pyroxene, and amphibole), and three cleavage planes (as in halite, calcite, and galena).  One perfect cleavage (as in mica) develops on the top and bottom of the mineral specimen with many parallel cracks on the sides but no angle of intersection. Two cleavage planes intersect at an angle. Common cleavage angles are 60°, 75°, 90°, and 120°, such as in galena and halite with three cleavage planes at 90° (cubic cleavage) or amphibole minerals with two cleavage planes at around 60° or 120°Calcite 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.

3.5.6 Special Properties

The words on the page are projected upwards onto the mineral
A demonstration of ulexite’s image projection
Special properties used in mineral identification include anything that is unique and identifiable about that mineral. This can range from complex properties, like the natural fiber-optic nature of ulexite, to simple things like taste. Halite is common table salt, and thus, tastes salty. Sylvite is a potassium salt and has more of a bitter taste.

The nugget is gold
Native gold has one of the highest specific gravities.
Mineralogists use a property related to density called specific gravitywhich is the ratio of the weight of a mineral specimen to the weight of an equal volume of water. Since the volume of the specimen can be easily measured by submerging it in a graduated cylinder in the lab and noting the rise in water level on the graduated scale, and the weight of the specimen and the weight of an equal volume of water is also easily measured in the lab, this property avoids the issues associated with determining mass and density. While specific gravity may be distinctive for each mineral, most students will find it less useful for mineral identification in the field than other more easily observed properties, except in a few rare cases, like with the very dense galena or native gold. For handy identification, most geologists judge specific gravity by the subjective quality of “heft,” which is how heavy the specimen feels in one’s hand relative to its size.

The simplest test for calcite and dolomite is to drop a bit of dilute hydrochloric acid (HCl) on the specimen. If it effervesces (fizzes), it is calcite. If it does not, scratch the specimen into a powder and try the acid again. I fit now fizzes, it is dolomite. The difference can be seen in the video below. Geologists who work with carbonate rocks carry a small dropper bottle of dilute (10-15%) HCl in their field kit for quick determination. Even vinegar can be used for this test, though it is less acidic and therefore, the fizzing reaction is weaker. Acetic acid (the weak acid in vinegar) is often used to separate certain non-calcite fossils from limestone.

The paperclip is sticking up into the air.
Paperclips attracted to lodestone (magnetite).
Some iron oxide minerals are attracted to a magnet or are magnetized themselves and act as a magnet. These minerals are considered magnetic, and include magnetite (Fe3O4) and ilmenite (FeTiO3). While magnetite is always attracted to a magnet and can be magnetized, ilmenite is weakly magnetic, and even some hematite is weakly magnetic. a common name for naturally magnetic is lodestone.

Striations or parallel dark lines on one cleavage surface on plagioclase feldspar
Iridescence on plagioclase
Some minerals and mineraloids have small atomic units within them that scatter light, a phenomenon called iridescence. This occurs in labradorite (a variety of plagioclase), opal, and biologic/geologic substances like natural pearl, and even certain shells. Even diamond, when cut, shows this, and is the basis of the design of most diamond cuts.

Image showing exsolution lamellae in potassium feldspar. These are separations of sodium feldspar from potassium feldspar within the crystal, not striations.
Exsolution lamellae or perthitic lineations within potassium feldspar

Striations on certain cleavage faces are a optical phenomenon and can be used to separate plagioclase feldspar from K-spar. The cleavage angle of feldspar is not exactly 90 degrees, differing by a tiny amount.  Striations are produced by a process called twinning in which parallel zones in the crystal replicate in mirror images so that the tiny differences in cleavage angle on one cleavage surface appear in reflected light as light and dark lines. Unlike plagioclase feldspar, potassium feldspar does not have this twinning property and does not show striations, but instead may show linear features called exsolution lamellae, also known as perthite.  These are not striations, but represent separation of small amounts of sodium feldspar from the dominant potassium feldspar in a crystal because K and Na are different in ionic size and cannot fit into the same places in the crystal lattice.  This causes two different minerals (potassium feldspar and sodium feldspar/albite) to crystallize out into roughly parallel zones in the same crystal, which can be seen as these perthitic lineations.

Purplish crystals of fluorite. The second image shows the deep blue fluorescence of fluorite under ultraviolet light.
Fluorite. Lower image shows fluorescence of fluorite under UV light

One of the most interesting special properties of minerals is florescence. Certain minerals, or inclusions within them, give off light in visible wavelengths when exposed to ultraviolet radiation. Many mineral exhibits have a florescence room with “black lights” where this property can be observed. Even more rare are minerals that absorb the light, then slowly release the light, much like a glow-in-the-dark sticker. This is known as phosphorescence.

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Summary

Minerals are the building blocks of rocks, and thus, are essential to understanding geology. Atomic bonds within minerals determine mineral properties. Most minerals begin in a fluid. This is either by crystallization as magma cools, or by precipitation as ions and molecules come out of solution. Some minerals form by changes that occur in the solid state (see Chapter 6). The largest group of minerals, both by size and relative quantity on Earth are silicates. Based on the SiO4-4 tetrahedra, they make up a large portion of the crust and mantle of Earth, and reflect the fact that silicon and oxygen are the two most common elements. Non-silicate minerals are also important, and provide materials that have economic value. Geologists use many properties of minerals to identify them, including luster, cleavage, and crystal habit.

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References