At the end of this chapter, students should be able to:
- Define a 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: a naturally occurring, inorganic solid (at room temperature), with an internal crystal structure, and defined chemical composition. There are some natural substances that technically should not be considered minerals, but they are exceptions to the rule. For example, both water (ice) and mercury are liquid at room temperature, but both have been considered minerals because they were classifed before the definition that excludes them was accepted. Calcite is quite often formed by organic processes, but is still considered a mineral. Because of these discrepancies, the International Mineralogical Association in 1985 amended the 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.
3.1 Chemistry of Minerals
Matter is any physical material which has volume (takes up space) and has mass. Mass is a measure of the amount of matter present. Weight is the effect of gravity on the mass, but on Earth, mass is numerically similar to weight. Matter exists on Earth in three main states (or phases): solid, liquid, and gas. Many materials can exist as all three on earth such as when water occurs as ice, liquid water, and water vapor. A solid is rigid and possesses a definite shape. A liquid flows and takes the shape of its container, except that its upper surface is flat or slightly curved depending on the properties of the particular liquid. Both liquid and solid samples have volumes that are mostly independent of pressure. A gas takes both the shape and volume of its container, however its volume depends on pressure. Minerals are solid but they can crystallize from liquids such as lava (liquid rock).
3.1.1 The Atom
Matter is made of atoms. An atom is the smallest unit of an element that has the properties of that element. Atoms consists of subatomic particles—protons, neutrons, and electrons. A simple model of the atom has a nucleus at the center composed of a specific number of protons with positive charge and neutrons with no charge (they are neutral). Protons are the positively charged particle in the nucleus with a mass of 1. Neutrons are the particle in the nucleus with no charge and have the same mass as a proton. A cloud of negatively charged electrons surrounds the nucleus to balance the charge of the protons. The mass of an electron is less than 1/1000th of that of a proton or neutron, so most of the atom’s mass is in the nucleus.
3.1.2 Periodic Table of the Elements
Elements are composed of atoms with a specific number of protons in the nucleus with the same chemical and physical properties. For example, an atom with 8 protons in the nucleus is oxygen, and 26 protons in the nucleus is iron. An element is a form of matter that is unable to be broken down by chemical means into a simpler form and still has unique chemical and physical properties. Each element behaves in a unique manner in nature. The uniqueness of each element lead to the development of the Periodic Table of Elements, a tabular arrangement of all known elements in order of their atomic number.
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 of the element, which often derives 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 electrons have so much less mass than protons and neutrons, the atomic mass is effectively the number of protons plus neutrons. The atomic mass represents an average mass of the atoms comprising that substance in nature and is usually not a whole number, meaning that an element exists in nature with atoms having different numbers of neutrons. This affects the mass of an element and the atomic mass number represents this average. This gives rise to the concept of the isotope.
Isotopes are forms of an element that have its specific number of protons (atomic number) but different number of neutrons. There are usually several isotopes for a particular element. For example, most carbon (98.9%) has 6 protons and 6 neutrons in the nucleus, and is called carbon-12 (12C). A few carbon atoms (1.1%) have 6 protons and 7 neutrons, and are called carbon-13 (13C). A trace amount of carbon atoms have 6 protons and 8 neutrons, and are called carbon-14 (14C). Among the 118 known elements, the heaviest are mostly 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 and their abundance are shown in Table 1 . These elements are found in the most common rock forming minerals.
3.1.3 Chemical Bonding
Most substances on Earth are compounds containing many elements. Chemical bonding describes how these atoms attach with each other to form compounds. One or more elements bonded together form a compound, such as NaCl, which is common table salt. Some compounds are held together by covalent bonds, and they are called molecules, such as when one oxygen atom bonded with another oxygen atom make the oxygen gas, O2. This is the oxygen we breathe. The subscript 2 indicates two atoms of oxygen are present in the molecule. Most minerals are compounds with more than one element. The common mineral calcite has the chemical formula of CaCO3 indicating one calcium, one carbon, and three oxygen atoms. Between the carbon and oxygen, there are covalent bonds, which means that CO3-2 can be considered a molecule. It ionically bonds with calcium (Ca+2) to form the compound calcite. The name of the chemical compound is calcium carbonate where calcium is Ca and carbonate refers to the molecule CO3-2. Another common mineral is olivine with a chemical formula of (Mg,Fe)2SiO4, in which one silicon and four oxygen atoms are bonded with two atoms of either magnesium or iron. Iron (Fe) and magnesium (Mg) separated by a comma indicates that iron and magnesium can occupy the same location in the crystal structure and substitute for one another.
Valence and Charge
Around the atom’s nucleus, the electrons are located in shells representing different energy levels. The outermost shell is called the valence shell. In 1913 Niels Bohr proposed a simple model for an atom, in which atoms are more stable when their outermost shell is full . In this model, the first shell can have a maximum of two electrons, the second and third shell can have up to eight electrons in their valence shell. Atoms of most elements tend to gain or lose electrons so the outermost shell is full. This is known as the octet rule.
On the Periodic Table, the rows are the elements in order of atomic number and the columns organize elements with some similar characteristics including that they have the same number of electrons in their valence shells. Columns are often labeled with Roman numerals from left to right, I to VIII, along with Arabic numerals above the columns shown with numbers, 1 through 18. 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 and that several columns have been added after II. The last column 18 (VIII) are 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, the octet rule is less applicable. Called transition elements, their charge is not a straight forward. The most common example of this is iron, which typically has a +2 or +3 charge depending on the oxidation state of the element.
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.
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 6 valence 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 molecule as shown in the figure.
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
Solutions have ions or molecules (known as solutes) dissolved in any medium (known as the solvent), which 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. Most iron deposits on Earth formed over 2.5 billion years ago when oxygen entered the oceans via photosynthesis, and iron became unstable in solution, precipitating in banded iron formations.
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
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
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 aragonite. Marine 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 called hydroxyapatite, Ca5(PO4)3(OH).
3.3 Silicate Minerals
There are nearly four thousand known minerals on Earth, and most are rare. The remainder of this chapter focuses on the few minerals that make up the most common rocks, called rock-forming minerals. Minerals are grouped based on their composition and structure, specifically based on the anion that forms similar structures in the group. The largest of these groups is the silicate minerals. The silicate group makes up a vast majority of the Earth’s crust. 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.
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, thus this makes the silicon-oxygen tetrahedron itself a compound with an ionic charge (SiO4)-4.
The silicon ion is much smaller than the oxygens and fits into a small space in the center of the four large ions as seen in the figure if the top ball is removed. 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.
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.
Olivine is referred to as a mineral family because of the ability for iron and magnesium to substitute, and has a pure iron version (called fayalite) and a pure magnesium version (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 lone silica tetrahedra ions that have not polymerized (connected together; 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
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 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 silica tetrahedra share two corners, forming long strands of silica tetrahedra. Minerals constructed on this single-chain arrangement bond with many elements substituting. 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 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
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.
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 more throughly, it can be written 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). That indicates an interaction between the growing minerals and water dissolved in the magma, 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)
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 light-colored muscovite and the dark-colored biotite.
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 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.
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. The “sandwich spread” is the iron, magnesium, and aluminum ions in the octahedral layers. Large yellow potassium ions form the weak Van der Waals bonds between these “sandwiches”. These sandwiches can be easily separated giving mica its characteristic property of easily peeling into sheets.
Although not mica minerals, other silicate minerals with sheet structure (called phyllosilicates) 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) 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.
3.3.5 Quartz and Feldspar (Framework Silicates)
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 and some metamorphic rocks. Feldspar is the most abundant mineral in the Earth’s crust. Quartz is one of the most common minerals in sedimentary rocks because it is especially resistant to disintegration by weathering.
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). 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). This series has many mineral names.
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. Other framework silicates (tectosilicates) include the alkali metal-rich feldsathoids and zeolites.
3.4 Non-Silicate Minerals
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.
|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|
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.
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 carbonate minerals in limestone and related rocks come directly from the fossil organisms. Over time, the calcite in shells, claws, antennae or other marine organism hard parts break down and blend together to form the limestone rock. 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. 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 its related form aragonite are secreted by organisms to form shells and physical structures like corals. Many such creatures draw calcium and carbonate from ocean water where dissolved bicarbonate ions (HCO3-1) are present. 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. This dilute acid is often carried by geologists in the field as well as used in mineral identification labs.
3.4.2 Oxides, Halides, and Sulfides
The most familiar oxide is rust, which is a combination of iron oxides and hydrated iron oxides. 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. For example, the cliffs of red sandstone in Zion National Park 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.
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.
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).
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.
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.
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 used and 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 has some water molecules added and without the water is a different mineral called anhydrite (CaSO4).
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.
3.4.5 Native Element Minerals
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 tend to be less reactive, and are almost never found within a bonded mineral. Other metals like iron, lead, and aluminum are more reactive, and tend to bond to other elements present. These are almost never 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.
3.5 Identifying Minerals
Identifying minerals requires the recognition of some physical properties. Only a few common minerals make up the majority of 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 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. Nonmetallic luster does not have an appearance like metal, but rather can be glassy, earthy, or other lusters. Nonmetallic minerals can still have a shine, just not the same shine as metal. Some nonmetallic luster terms are illustrated in the table below.
|Vitreous/glassy||Surface is shiny like glass|
|Earthy/dull||Dull, like dried mud or clay|
|Silky||Soft shine like silk fabric|
|Pearly||Like the inside of a clam shell|
|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.|
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 (with Na and Ca) feldspar. 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).
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 of a mineral can 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 steak can still technically be obtained via powdering the mineral with a hammer.
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 scale is based on the ability of one mineral or substance to scratch another softer mineral or substance. 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. 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 about four times harder than corundum which defines 9. 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, 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.
long and flat crystals
|kyanite, amphibole, gypsum|
blobby, circular crystals
|hematite, malachite, smithsonite|
crystals that are small and coat surfaces
|quartz, calcite, malachite, azurite|
|pyrite, galena, halite|
12-sided polygon shapes
|Mn-oxides, copper, gold|
crystals that do not have a long direction
|olivine, garnet, pyroxene|
thin, very long crystals
|serpentine, amphibole, zeolite|
stacked, very thin, flat crystals
|mica (biotite, muscovite, etc.)|
crystals that are plate-like
|selenite roses, wulfenite, calcite|
crystals with six sides
Crystals with no obvious shape, microscopic crystals
|limonite, pyrite, azurite, bornite|
4-sided double pyramid crystals
|diamond, fluorite, magnetite, pyrite|
very long, cylindrical crystals
|tourmaline, beryl, barite|
crystals that grow from a point and fan out
|pyrite “suns”, pyrophyllite|
crystals shaped like slanted cubes
sharp-sided crystals with no long direction
|feldspar, pyroxene, calcite|
three-sided, pyramid-shaped crystals
|magnetite, spinel, tetrahedrite|
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
In most rock specimens mineral crystals have broken surfaces. Mineral grains often show characteristic patterns of breaking – 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.
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 surfaces. Some minerals have a strong cleavage, some minerals only have weak cleavage and do not typically demonstrate cleavage in samples.
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.
To distinguished cleavage planes from crystal faces, cleavage planes will have multiple parallel cracks or flat surfaces. Cleavage planes may be expressed as a series of steps like terraced rice paddies. Cleavage planes arise from the tendency of mineral 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 minerals and cleavage is one of the most useful properties for identifying minerals.
The most common number of cleavage plane directions 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 angles are 60°, 75°, 90°, and 120°, such as in galena 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.
3.5.6 Special Properties
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.
Mineralogists use a property related to density called specific gravity, which 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 than other more easily observed properties in the field, except in a few rare cases, like with the very dense galena or native gold. In such cases, 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. If now it 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.
Some iron oxide minerals are attracted to a magnet or can be magnetized themselves to 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. When magnetite is naturally magnetic, it is called lodestone.
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 items like pearl, and even certain shells. Even diamond, when cut, shows this, and is the basis of the design of most diamond cuts.
Striations on certain cleavage faces are a optical phenomenon 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 as light and dark lines. Unlike plagioclase feldspar, potassium feldspar 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.
One of the most interesting special properties is florescence. Certain minerals, or inclusions within them, give off light in another wavelength 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.
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.