Minerals in Geology: What Are They?
by Owen Borville
January 30, 2021
Learning, Geology, Chemistry, Science
A mineral is defined as a naturally-occurring, inorganic, crystalline solid substance with a defined and definite chemical composition and atomic structure. A rock, however, is a naturally occurring solid material that can be composed of one or more minerals.
Minerals form inside the earth as hot, mineral saturated waters rise upward through crevices toward the surface and react with the surrounding rock, cool off, and crystallize.
Mineral Properties That Help Identify Particular Mineral:
mineral crystal habit, hardness, luster, transparency, color, streak, tenacity, cleavage, fracture, specific gravity, magnetism, fluorescence, radioactivity, taste, smell, and acid reaction.
More Mineral Distinguishing Properties:
Polymorphism
Twinning: The Intergrowth of two or more crystals of a single mineral species
Hardness: Moh's Hardness Scale
Luster: How light is reflected from the mineral's surface
Transparency: Ability of light to pass through the mineral
Color: Mineral species can have different colors with the same atomic formula
Play of Colors
Asterism
Chatoyancy (cat's eye) is the wavy banding of color as the sample is rotated
Iridescence is the variety of colors as light is scattered off the mineral
Tarnish
Pleochroism
Streak mineral color in powdered form as the mineral sample is scratched against a harder substance.
Cleavage is related to crystallography as a sample is broken along a plane and can be in one to six directions
Parting is "false cleavage" caused by structural defects in the mineral when stress is applied.
Fracture is when a mineral is broken not along a plane of cleavage or uneven fracture.
Tenacity is related to cleavage and fracture
Specific gravity is the density of a mineral.
Acid reaction
Magnetism magnetite iron
Taste or smell test
Radioactivity test
Mineral Definition
Minerals are naturally occurring, inorganic crystalline solids with a defined chemical composition and physical properties. Crystalline solids consist of atoms arranged in a three dimensional framework. The chemical composition of a mineral is shown by the chemical formula, which indicates the elements included in the mineral and the number of atoms of each element in the mineral unit structure. The most common elements inside the Earth’s crust are Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, and Magnesium.
Mineral Identification
Color is the most noticeable property of a mineral. However, color is not always the best identification tool because a mineral can have a variety of colors. The variety of colors is caused by impurities inside the mineral which do not affect the chemical composition. Iron and magnesium rich minerals are commonly darker in color than other minerals and silica rich minerals are commonly lighter in color, however.
Luster is the appearance of the reflection of light from the surface of a mineral and is described as metallic, non-metallic, greasy, transparent, translucent, opaque, or dull. Adamantine describes a mineral luster found in transparent or translucent minerals that reflects light brilliantly, as in diamond. Vitreous is a glassy luster.
Crystal forms are the geometric shapes formed by mineral crystals. Examples include:
cubic: 6 sides or faces at right angles
hexagonal prism: 6 sides plus base and top
hexagonal pyramid: 6 side pyramid plus the base
tabular: flat plates
rhombohedron: 6 sides or faces not at right angles
tetrahedron: 4 faces
dodecahedron: 12 faces
scalenohedron: 8 or 12 faces
octahedron: 8 faces
pyritohedron: 12 faces
Crystal Structure, System
Minerals can also be classified into six crystal systems based on crystal geometry. Hexagonal mineral crystals have four axes of symmetry, three of which are of equal length. The fourth axis is of a different length and is at right angles to the other three axes. Isometric mineral crystals have three axes of symmetry, all of which are equal length and are at right angles to each other. Monoclinic mineral crystals have three unequal axes with two at right angles to each other in a plane. The third axis is at an angle to the other two. Orthorhombic mineral crystals have three unequal axes in which all are at right angles to each other. Tetragonal mineral crystals have three axes of symmetry, two of which are equal length. The third axis is at a right angle to the other two. Triclinic mineral crystals have three axes of symmetry, each of which are different lengths and none of which are perpendicular to each other.
Streak is the color of a mineral in fine powder form. Scratching the mineral against a hard surface such as concrete or a porcelain streak plate can reveal the powder form of a mineral. Streak color is a good mineral indicator as samples of the same mineral usually produce the same color streak.
Cleavage in Minerals
Cleavage is the tendency of a mineral or rock to break along flat surfaces or planes of weakness. Cleavage is associated with the strength of the atomic bonds inside the mineral. A mineral can have cleavage in one direction, two directions, three directions, four directions, or six directions. Some minerals do not have any cleavage.
Possible cleavage configurations include the following.
A mineral with one cleavage direction is called basal cleavage. Mica minerals have one cleavage direction. A mineral can have two cleavage directions at right angles to each other. Feldspar minerals have cleavage in two directions at right angles. A mineral with three cleavage directions at right angles is called cubic cleavage. The minerals halite and galena have cleavage in three directions at right angles. A mineral with three cleavage directions not at right angles is called rhombohedral cleavage. The minerals calcite and dolomite have three cleavage directions not at right angles. A mineral with four cleavage directions is called octahedral cleavage. The mineral fluorite has cleavage in four directions. Six cleavage directions are found in the mineral sphalerite.
Cleavage can be described based on the flatness of the surface of cleavage. Cleavage can range from perfect or excellent to good, poor, or non-existent. The mineral quartz does not have any cleavage.
Fracture occurs when a mineral is broken along irregular or curved surfaces. Some minerals have both cleavage and fracture, such as feldspar. Smooth curved fracture is called conchoidal fracture.
The hardness of a mineral is defined as the resistance to scratch and is based on the atomic structure of the mineral. A hardness scale was designed by geologist Friedrich Mohs. The scale contains whole numbers from one to ten with ten being the hardest. Ten minerals are designated a number on the scale based on the hardness of the mineral. All minerals are given a hardness number based on the scale: a whole number or a decimal number in between.
Mohs Hardness Scale:
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Quartz
8 Topaz
9 Corundum
10 Diamond
A harder mineral or material can scratch a softer mineral. However, a softer mineral or material cannot scratch a harder mineral. Common objects are given hardness numbers to help with mineral identification, such as a fingernail (2.5), a copper penny (3), glass (5.5), and a knife (5.5). Minerals below (5.5) on the hardness scale are considered soft while minerals above (5.5) are considered hard.
Specific gravity is the ratio of the weight of a mineral to the weight of an equal volume of water. Metallic minerals are generally heavier than non-metallic minerals.
Minerals can feature a variety of crystal habits including:
Acicular- needle-like crystals (long and slender)
Bladed- as a knife
Dendritic- branching pattern
Prismatic-prism
Striated- parallel grooves
Tabular- flat plates
Twinned- two crystals joined together
Botryoidal- resembles an accumulation of grapes
Fibrous- resembles fibers
Mammillary- accumulation of rounded masses
Massive- no crystal structure
Reniform- kidney shaped
More Terminology:
Other properties and terminology related to minerals include anhedral, which refers to mineral or rock grains that do not have well-defined crystal faces or shape. Euhedral describes a mineral crystal with well-defined faces. Birefringence describes the optical phenomenon of double refraction as a single beam of light travels through a transparent, molecularly ordered material or mineral and is split into two separate beams of light. Double refraction can be observed in the mineral calcite. Fluorescence is a property of some minerals that absorb light at a certain wavelength and emit the light at a different (visible) wavelength. Polymorphism refers to the existence of two or more minerals with identical chemical composition but with different crystal structure. A spectrometer is an instrument designed to measure properties of light over the electromagnetic spectrum which can help identify minerals and other materials.
Chromophore elements are chemical elements whose structure allows them to absorb visible light energy and produce visible colors in minerals. Certain elements are more effective in producing colors than others and minerals containing chromophore elements will likely produce color.
The Origin of Minerals
Minerals originate from several sources:
Many mineral likely formed near the surface during the events of the Global Flood as hot-mineral saturated water and magma rose toward the surface, cooled, and crystallized.
Magma is rock in melted liquid form which exists inside the Earth’s crust and is a main source of minerals. Magma which reaches the surface is called lava. This lava cools and solidifies into rock containing a variety of minerals.
Hydrothermal veins are cracks in rock in which hot mineral saturated water passes upward through and toward the surface. This water originates from magma. When the water cools, minerals are deposited (or precipitated) in solid form. Precipitation is the deposition of minerals from a liquid solution in solid form, which in the process separates the mineral from the solution.
Minerals also form when seawater evaporates and the minerals inside the water deposit in solid form. These minerals are called evaporites. Many sea organisms have shells made of calcite, which is deposited on the seafloor when the organisms die. Some minerals are formed by the chemical alteration of other minerals, such as feldspar to clay minerals. Also, some minerals are formed by metamorphism, or the combination of heat, pressure, and fluid activity on existing rock.
Many mineral resources exist inside the Earth. A resource is the total amount available of a mineral. A reserve is the amount of the mineral which can be mined economically. Some mineral resources cannot be mined economically and therefore are not mined. Most minerals have a variety of economic and industrial uses. These range from metallic ore minerals to oil and natural gas.
Minerals are divided into groups including silicates, carbonates, halides, oxides, sulfates, and sulfides. One or more minerals can exist inside a rock. Accessory minerals are found in small amounts in rocks but are not considered a major mineral in a particular rock.
Ionic Substitution in minerals occurs when one ion or element is substituted for another ion in a mineral atomic structure. In order for substitution to occur, the ions must be of similar size and charge neutrality must be maintained. An example is the potassium feldspar mineral group, where sodium (Na) and potassium (K) can substitute for each other. The atomic formula is written (Na,K)AlSi3O8 where the substituting ions appear in parentheses and are separated by commas. Anions (negatively charged ions) can also substitute for each other. Sometimes a higher charged ion can replace two lower charged ions, but the charge balance must be maintained. For example, an ion of +2 charge can replace two ions of +1 charge. This results in one vacant position in the atomic structure and is called omission substitution. Coupled substitution occurs when a lower charged ion replaces a higher charged ion and a second substitution must occur to maintain charge balance. In general, higher temperature can help initiate substitution in minerals.
Mineral Groups:
Ice (frozen water, snowflakes) H20
Silicate Minerals comprise the largest group of minerals, about 90 percent of the Earth's crust. Underneath the crust is a lower silica percentage and higher iron and magnesium percentage. Feldspar and quartz are the most common silicate minerals in the crust. Pyroxenes, amphiboles, micas, and clay minerals are also common silicate minerals in the Earth's crust. Other mineral groups in the crust include: Oxides, Sulfates, Sulfides, Carbonates (limestone), Native Elements: (gold, silver, copper), Halides, Phosphates, Clay Minerals. The most common elements in Earth's crust: Oxygen, silicon, aluminum, iron, calcium, sodium, magnesium, potassium.
Silica Tetrahedron: The fundamental feature of silicate minerals is the silica tetrahedron (plural tetrahedra), which contains one silicon atom surrounded by four oxygen atoms. The structure of the tetrahedron has a triangular base and three triangular sides. The four oxygen atoms represent the corners of the tetrahedron and the silicon atom is located inside the center of the tetrahedron. The silica tetrahedron has a total electric charge of -4 and requires positive charged ions with a total charge of +4 to form a neutral compound. The silica tetrahedron formula is written as (SiO4)-4.
Orthosilicate Minerals
The silica tetrahedron can be bonded with positive charged ions of magnesium or iron to form an electrically neutral silica tetrahedron. The mineral olivine is an example of a neutral silica tetrahedron or orthosilicate. The formula of olivine is (Mg,Fe)2SiO4. Olivine has a one to four silicon to oxygen ratio in which there are four oxygen atoms for every one silicon atom. Magnesium and iron (Fe) each have a charge of +2, so two atoms of either of these are needed to balance the tetrahedron. Olivine is found most commonly in igneous rocks but also in metamorphic rocks.
The orthosilicate minerals also include garnet, which is an aluminum silicate mineral with a molecular formula ending with Al2(SiO4)3 and is bonded with cations such as iron, calcium, or magnesium. Almandine and grossular are varieties of garnet and both form in metamorphic rocks. Almandine is rich in iron and grossular is rich in calcium. Some varieties of garnet can form in igneous rocks, such as pyrope, which is rich in magnesium. Varieties of garnet produce popular gemstones.
Other orthosilicate minerals include:
Zircon ZrSiO4
Topaz Al2SiO4 (F,OH)2
Andalusite Al2SiO5
Silimanite Al2SiO5
Kyanite Al2SiO5
Titanite (sphene) CaTiSiO5
Willemite ZnSiO4
Chain Silicate Minerals (Pyroxenes)
Chains of silica tetrahedra occur where the tetrahedra form a single strand or line and connect at two corners of each tetrahedron. These chain silicates form when silica tetrahedra share two oxygen atoms with adjacent tetrahedra. Single chain silicates have a ratio of one silicon atom for every three oxygen atoms. The pyroxene group of minerals form single chains of silicates. The formula for single chain silicates consists of the (SiO3)-2 ion bonded with positively charged ions. These ions make the formula ending Si2O6. Pyroxene minerals are formed in igneous rocks particularly volcanic rocks. Pyroxene minerals include:
Augite (Ca, Na)(Mg, Fe, Al, Ti)(Si, Al)2O6
Diopside CaMgSi2O6
Spodumene LiAlSi2O6
Enstatite Mg2Si2O6
Hypersthene (Mg,Fe)2Si2O6
Hedenbergite CaFeSi2O6
Aegirine NaFeSi2O6
Jadeite Na(Al,Fe)Si2O6
Double Chain Silicate Minerals (Amphiboles) form two parallel chains of silica tetrahedra and occurs when tetrahedra share two and three oxygen atoms respectively with adjacent tetrahedra. Double chain silicates have a ratio of four silicon atoms for every 11 oxygen atoms. The amphibole group of minerals forms double chains of silica tetrahedra. Double chain silicates contain the (Si4O11)-6 ion bonded with positively charged ions and these minerals commonly end with Si8O22(OH)2 in the molecular formula. Amphibole minerals form prism or needle-like crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts along with silicon, oxygen, and water.
Hornblende is a common dark green to black variety of amphibole and is a component in many igneous and metamorphic rocks. Amphibolite is a metamorphic rock consisting primarily of amphibole minerals such as hornblende and plagioclase feldspar. Actinolite is a mineral of the amphibole group, is gray-green, commonly has a bladed or fibrous crystal habit, and is a characteristic mineral of the greenschist facies of metamorphism. Tremolite is also an amphibole mineral which forms in metamorphic rocks. Amphibiole minerals include:
Actinolite Ca2(Mg,Fe)5Si8O22(OH)2
Glaucophane Na2(Mg,Fe)3Al2Si8O22(OH)2
Hornblende (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2
Tremolite Ca2(Mg,Fe)5Si8O22(OH)2
Sheet Silicate Minerals (or Phyllosilicate minerals) form sheets or planes of silica tetrahedra in which three oxygen atoms are shared by adjacent tetrahedra. Sheet silicates have a ratio of two silicon atoms for every five oxygen atoms. The mica group of minerals and clay minerals form sheet silicates. Sheet silicates contain the (Si4O10)-4 ion bonded with positively charged ions. Biotite is a dark colored mineral of the mica group while muscovite is a light colored mineral of the mica group. Muscovite has a lighter color because it contains more silica than biotite. Other sheet silicates include talc, serpentine, kaolinite, phlogopite, and chlorite. The sheet silicate minerals include:
Serpentine (Mg,Fe)3Si2O5(OH)4
Clay mineral group:
Kaolinite Al2Si2O5(OH)4
Vermiculite (Mg,Fe,Al)3(Al,Si)4O10(OH)2·4H2O
Talc Mg3Si4O10(OH)2
Pyrophyllite Al2Si4O10(OH)2
Mica group:
Biotite K(Mg,Fe)3(AlSi3)O10(OH,F)2
Muscovite KAl2(AlSi3)O10(OH,F)2
Phlogopite KMg3(AlSi3)O10(OH,F)2
Lepidolite K(Li,Al)3(Si,Al)4O10(F,OH)2
Glauconite (K,Na)(Al,Mg,Fe)2(Si,Al)4O10(OH)2
Chlorite (Mg,Fe)6(AlSi3)O10(OH)8
Phyllosilicate Minerals contain two types of sheet layers in their atomic structure: tetrahedral and octahedral layers. Tetrahedral layers (or sheets) are atomic layers found in phyllosilicate minerals consisting of silicon and oxygen tetrahedra (SiO4)4- sharing three of the four oxygen atoms with adjacent tetrahedra and forming a hexagon or six side structure and continuing laterally. The tetrahedral layers connect with octahedral sheets to form alternating stacked sheets.
Octahedral layers (or sheets) are layers of connected octahedron shaped structures composed of hydroxide (OH)- anions with cations commonly including aluminum, magnesium, or iron. These sheets along with tetrahedral sheets make up the structure of clay or phyllosilicate minerals. Phyllosilicate minerals can be dioctahedral if two of the three positions on the hydroxide ion in the octahedral layer are filled with a cation or trioctahedral if all three positions are filled with a cation.
Clay Minerals
The primary structural unit of the kaolinite group of minerals is a repeating layer composed of one octahedral sheet and one tetrahedral sheet. All members of the kaolinite group form primarily during hydrothermal alteration or weathering of feldspar minerals under acidic conditions.
Shrink-swell refers to the property of many clay soils and sediments that swell or expand when saturated with water and shrink when dried. Water molecules enter between the sheet-like layers and cause the clay to expand. Shrink-swell occurs in the smectite group of clay minerals. Clay soils which have the shrink swell property can cause damage to roads and buildings. Shrink-swell soils are common in the southeast and western United States.
Smectite is a group of clay minerals in which the basic structural unit is a layer consisting of two inward pointing tetrahedral sheets with a central aluminum octahedral sheet, or TOT structure. Smectite can be dioctahedral or trioctahedral. The layers are continuous in two directions, but the bonds between layers are weak and have excellent cleavage, allowing water and other molecules to enter between the layers. This causes expansion in a third direction.
Smectites commonly result from the weathering of basic rocks. (Basic rocks contain a large percentage of calcium rich feldspar minerals and no quartz.) Smectite formation is favored by level to gently sloping terrains that are poorly drained, mildly alkaline (such as in marine environments), and have high silicon and magnesium potentials. Other factors that favor the formation of smectite include the availability of calcium and the lack of potassium. Poor drainage is necessary because otherwise water can leach away ions freed in the alteration reactions. Smectites are used in industry as fillers, carriers, absorbents, and a component in drilling fluids.
Illite is a family of non-expanding, clay-sized, dioctahedral, micaceous minerals. Illite is a three layer clay mineral composed of the TOT structure, or two inward-pointing silica tetrahedral sheets with a central octahedral sheet. Two inter-layer cations fill between each TOT structure. Illites, which are the dominant clay minerals in argillaceous rocks, form by the weathering of silicate minerals (primarily feldspar) through the alteration of other clay minerals and during the degradation of muscovite.
The atomic structure of the chlorite group of minerals contains an octahedral layer in between the two tetrahedral layers, or TOT structure. An additional octahedral layer with a positive charge is bonded with the negatively charged TOT structure. Chlorite occurs in volcanic rocks, metamorphic rocks, and in hydrothermal deposits.
Framework Silicate Minerals (or Tectosilicates) occur when all four oxygen atoms are shared by adjacent tetrahedra, forming a three dimensional framework. The common mineral quartz has the formula SiO2 and is a framework silicate. Quartz is commonly transparent or colorless but can have a variety of colors. Quartz can have a crystalline texture or can be microcrystalline.
Agate is a type of chalcedony, which is a type of microcrystalline quartz gemstone. Chert, flint, and jasper are other microcrystalline quartz varieties. Amethyst is a purple quartz mineral in which the purple color comes from impurities in the mineral. Other quartz varieties include smoky quartz, rose quartz, milky quartz, and citrine. A geode is a cavity in a rock which is partially filled with a mineral (commonly quartz crystals) which grows on the cavity walls. Opal is a type of non-crystalline silica which solidifies or precipitates from silica-rich waters. Opal is a popular gemstone.
Potassium feldspar (also called alkali feldspar) (KAlSi3O8) is a silicate mineral group rich in potassium (some also contain sodium) and common in igneous rocks such as granite, metamorphic rocks, and sedimentary rocks such as sandstone. Feldspar crystals are stubby prisms, often pink to white and some have a streaky appearance (referred to as perthitic texture). Potassium feldspar minerals include sanidine, orthoclase, microcline, and anorthoclase. Amazonite is a green variety of microcline and is used as a gemstone.
Plagioclase Feldspar (NaAlSi3O8 -CaAl2Si2O8) is a silicate mineral group and one of the most common minerals in igneous, metamorphic, and sedimentary rocks. Plagioclase feldspars are silicates that can be sodium or calcium rich. Feldspar crystals are stubby prisms, generally white to gray and have a glassy luster. Twinning is a common feature in plagioclase feldspars in which twin mineral grains are formed with different orientations. Plagioclase feldspar mineral varieties include albite, oligoclase, andesine, labradorite, bytownite, and anorthite. Albite is the most sodium rich while anorthite is the most calcium rich variety.
Exsolution occurs as different minerals in a magma solution separate (see section on magma) as the temperature decreases and crystallization occurs, forming lamellae or layers of alternating minerals. Intergrowths of sodium feldspar can occur inside of potassium feldspar where potassium feldspar is the dominant mineral. These intergrowths are termed perthite.
Undersaturated commonly describes a rock or mineral with low silica content. Feldspathoid minerals are structurally similar to feldspar but have low silica content and are found in alkaline igneous rocks. Alkaline rocks have a high percentage of potassium and sodium oxide in relation to silica. Feldspathoid minerals include nepheline, leucite, and sodalite. These minerals are not as common as feldspar but have many industrial uses. Zeolites are hydrated aluminum silicate minerals containing the alkaline and alkaline-earth metals (or elements). Examples of zeolite minerals include analcime and natrolite. Zeolites form naturally in volcanic rocks or can be produced synthetically and have many industrial uses.
Ring Silicates (cyclosilicates) are a less common type of silicate in which the atomic structure contains six tetrahedra connected together as a ring shape. The rings stack on top of each other to form a column. These silicates include the minerals beryl, cordierite, and tourmaline. These minerals are used as gemstones and also have industrial uses.
Another less common structure consisting of only two tetrahedra joined together (sharing one oxygen) can form and is called a disilicate. This structure can be found in the epidote group of minerals. These minerals are used as gemstones and also have industrial uses.
Carbonate Minerals contain the (CO3)-2 ion in the molecular formula, such as calcite, CaCO3. Calcite can change into the mineral dolomite, CaMg(CO3)2 with the addition of magnesium. Rock containing dolomite is called dolostone. Carbonate rocks are rocks that contain mostly carbonate minerals. Aragonite is a carbonate mineral which is found in many invertebrate skeletons and can change into calcite. Aragonite and calcite are polymorphs of calcium carbonate. Limestone rock contains primarily the mineral calcite. Some carbonate minerals contain hydroxide (OH)-. The most common of these are azurite Cu3(CO3)2(OH)2 and malachite Cu2(CO)3(OH)2.
Other carbonate minerals include:
Magnesite MgCO3
Siderite FeCO3
Rhodochrosite MnCO3
Ankerite Ca(Mg,Fe)(CO3)2
Witherite BaCO3
Strontianite SrCO3
Cerussite PbCO3
Smithsonite ZnCO3
Halide Minerals are a group of minerals with a halogen element (commonly fluorine or chlorine) as the major anion. Halide minerals include halite (NaCl), fluorite (CaF2), and sylvite (KCl). Halite is an evaporite mineral that is deposited primarily from the evaporation of salt water. As the salt water evaporates, the mineral is deposited in solid form.
Oxide and Hydroxide Minerals
Oxide minerals consist of oxygen bonded with cations which are commonly metallic and produce important ore deposits such as iron oxide. Oxide minerals include hematite (Fe2O3) and magnetite (Fe3O4). Corundum (Al2O3) is an oxide mineral used as a gemstone along with industrial uses.
Other oxide minerals include:
Cuprite Cu2O
Chromite FeCr2O4
Spinel MgAl2O4
Ilmenite FeTiO3
Rutile TiO2
Pyrolusite MnO2
Uraninite UO2
Hydroxides are a group of minerals containing the (OH)- ion bonded with positive charged ions. These minerals are commonly produced by weathering processes and hydration of other minerals. Hydroxides commonly contain iron or aluminum. Limonite FeO•OH•nH2O is a mineral composed of iron oxides and water. Limonite is very common in many rocks after weathering (oxidation or rust) at the Earth's surface and produces brown or yellow colors in many rocks.
Manganese oxide and hydroxide minerals (also known as wad) are commonly products of the weathering of manganese minerals. Bauxite is a naturally occurring, heterogeneous material composed primarily of one or more aluminum hydroxide minerals, plus various mixtures of silica, iron oxide, titania, aluminosilicates, and other impurities in minor or trace amounts. Brucite is a magnesium hydroxide mineral commonly found in metamorphic rocks associated with limestone, dolomite, and serpentine.
Sulfates are minerals or compounds containing sulfur and oxygen. The sulfate ion formula is written as SO42−. Sulfate minerals include gypsum (CaSO4* 2H2O) and anhydrite (CaSO4) which are also evaporite minerals. Barite (BaSO4) is a sulfate mineral which is unusually heavy, commonly white to gray, and often found in veins, replacements, or residual deposits commonly associated with quartz, fluorite, and calcite. Celestine (SrSO4) is also a sulfate mineral. Anhydrous refers to minerals or other materials which do not have water as a primary constituent.
Sulfide minerals contain the element sulphur bonded with metals including copper, zinc, lead, cobalt, nickel, and silver. Many sulfides are found in hydrothermal deposits or in altered soils and are important ore minerals. Sulfide minerals include:
Chalcocite Cu2S
Galena PbS
Sphalerite ZnS
Cinnabar HgS
Covellite CuS
Realgar AsS
Orpiment As2S3
Stibnite Sb2S3
Pyrite FeS2
Arsenopyrite FeAsS
Molybdenite MoS2
Chalcopyrite CuFeS2
Bornite Cu5FeS4
Phosphates are minerals that contain the (PO4)3- ion. The mineral apatite is the most common phosphate mineral and is found in a variety of igneous and metamorphic rocks. Apatite is used in fertilizer and in other industrial uses. Turquoise is a phosphate mineral found in volcanic rocks and is commonly used in jewelry. Amblygonite is a phosphate mineral containing lithium, sodium, aluminum, fluoride, and hydroxide. Amblygonite occurs in pegmatite deposits and is commonly light colored. The phosphate minerals include:
Apatite Ca5(PO4)3(F,Cl,OH)
Turquoise CuAl6(PO4)4(OH)8*4H2O
Amblygonite (Li,Na)AlPO4(F,OH)
Borates are naturally-occurring minerals containing the element boron and oxygen along with other elements. There are two basic types of borate minerals. The relatively rare anhydrous borates, which have no water incorporated into their structure, are found in metamorphic and igneous rock. Hydrous borates, minerals with water incorporated into their structure, form by the evaporation of mineral-rich water (evaporite minerals) on desert playas. Borax is a common borate evaporite mineral with the formula Na2B4O5(OH)4 8H2O. Borax is a hydrous borate and is a relatively soft, white, colorless, or transparent mineral. Borax is an important industrial mineral.
Single Element Minerals include gold, silver, copper, antimony, sulfur, mercury, diamond, and graphite. Placer deposits are minerals that separate from the surrounding rock because of greater density and are commonly economically valuable such as gold and diamond.
Bowen's Reaction Series Describes Crystallization of Minerals
Bowen’s Reaction Series was introduced by N.L. Bowen, who developed a chart describing the order of crystallization of minerals inside magma as the temperature of the magma decreases (cools). The series also has two branches: the discontinuous branch and the continuous branch. The discontinuous branch begins with the crystallization of the mafic mineral olivine at the highest temperature. Then as temperature decreases pyroxene crystallizes. As temperature decreases further amphibole and finally biotite mica crystallizes. After further temperature decrease, the silica-rich minerals potassium feldspar, muscovite mica, and finally quartz crystallize. As the mafic minerals at the top of the series crystallize, the magma becomes progressively more silica rich. The continuous branch begins with the crystallization of calcium-rich plagioclase at the highest temperature. As temperature decreases sodium rich plagioclase begins to crystallize. After the calcium and sodium rich plagioclase crystallize, the continuous branch merges with the discontinuous branch and potassium feldspar begins to crystallize as temperature decreases. As temperature decreases further, muscovite mica and finally quartz begin to crystallize. The magma becomes progressively richer in silica as minerals on the upper part of the continuous branch crystallize. The magma also becomes initially richer in sodium plagioclase and potassium feldspar as calcium plagioclase crystallizes at the top of the series.
by Owen Borville
January 30, 2021
Learning, Geology, Chemistry, Science
A mineral is defined as a naturally-occurring, inorganic, crystalline solid substance with a defined and definite chemical composition and atomic structure. A rock, however, is a naturally occurring solid material that can be composed of one or more minerals.
Minerals form inside the earth as hot, mineral saturated waters rise upward through crevices toward the surface and react with the surrounding rock, cool off, and crystallize.
Mineral Properties That Help Identify Particular Mineral:
mineral crystal habit, hardness, luster, transparency, color, streak, tenacity, cleavage, fracture, specific gravity, magnetism, fluorescence, radioactivity, taste, smell, and acid reaction.
More Mineral Distinguishing Properties:
Polymorphism
Twinning: The Intergrowth of two or more crystals of a single mineral species
Hardness: Moh's Hardness Scale
Luster: How light is reflected from the mineral's surface
Transparency: Ability of light to pass through the mineral
Color: Mineral species can have different colors with the same atomic formula
Play of Colors
Asterism
Chatoyancy (cat's eye) is the wavy banding of color as the sample is rotated
Iridescence is the variety of colors as light is scattered off the mineral
Tarnish
Pleochroism
Streak mineral color in powdered form as the mineral sample is scratched against a harder substance.
Cleavage is related to crystallography as a sample is broken along a plane and can be in one to six directions
Parting is "false cleavage" caused by structural defects in the mineral when stress is applied.
Fracture is when a mineral is broken not along a plane of cleavage or uneven fracture.
Tenacity is related to cleavage and fracture
Specific gravity is the density of a mineral.
Acid reaction
Magnetism magnetite iron
Taste or smell test
Radioactivity test
Mineral Definition
Minerals are naturally occurring, inorganic crystalline solids with a defined chemical composition and physical properties. Crystalline solids consist of atoms arranged in a three dimensional framework. The chemical composition of a mineral is shown by the chemical formula, which indicates the elements included in the mineral and the number of atoms of each element in the mineral unit structure. The most common elements inside the Earth’s crust are Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, and Magnesium.
Mineral Identification
Color is the most noticeable property of a mineral. However, color is not always the best identification tool because a mineral can have a variety of colors. The variety of colors is caused by impurities inside the mineral which do not affect the chemical composition. Iron and magnesium rich minerals are commonly darker in color than other minerals and silica rich minerals are commonly lighter in color, however.
Luster is the appearance of the reflection of light from the surface of a mineral and is described as metallic, non-metallic, greasy, transparent, translucent, opaque, or dull. Adamantine describes a mineral luster found in transparent or translucent minerals that reflects light brilliantly, as in diamond. Vitreous is a glassy luster.
Crystal forms are the geometric shapes formed by mineral crystals. Examples include:
cubic: 6 sides or faces at right angles
hexagonal prism: 6 sides plus base and top
hexagonal pyramid: 6 side pyramid plus the base
tabular: flat plates
rhombohedron: 6 sides or faces not at right angles
tetrahedron: 4 faces
dodecahedron: 12 faces
scalenohedron: 8 or 12 faces
octahedron: 8 faces
pyritohedron: 12 faces
Crystal Structure, System
Minerals can also be classified into six crystal systems based on crystal geometry. Hexagonal mineral crystals have four axes of symmetry, three of which are of equal length. The fourth axis is of a different length and is at right angles to the other three axes. Isometric mineral crystals have three axes of symmetry, all of which are equal length and are at right angles to each other. Monoclinic mineral crystals have three unequal axes with two at right angles to each other in a plane. The third axis is at an angle to the other two. Orthorhombic mineral crystals have three unequal axes in which all are at right angles to each other. Tetragonal mineral crystals have three axes of symmetry, two of which are equal length. The third axis is at a right angle to the other two. Triclinic mineral crystals have three axes of symmetry, each of which are different lengths and none of which are perpendicular to each other.
Streak is the color of a mineral in fine powder form. Scratching the mineral against a hard surface such as concrete or a porcelain streak plate can reveal the powder form of a mineral. Streak color is a good mineral indicator as samples of the same mineral usually produce the same color streak.
Cleavage in Minerals
Cleavage is the tendency of a mineral or rock to break along flat surfaces or planes of weakness. Cleavage is associated with the strength of the atomic bonds inside the mineral. A mineral can have cleavage in one direction, two directions, three directions, four directions, or six directions. Some minerals do not have any cleavage.
Possible cleavage configurations include the following.
A mineral with one cleavage direction is called basal cleavage. Mica minerals have one cleavage direction. A mineral can have two cleavage directions at right angles to each other. Feldspar minerals have cleavage in two directions at right angles. A mineral with three cleavage directions at right angles is called cubic cleavage. The minerals halite and galena have cleavage in three directions at right angles. A mineral with three cleavage directions not at right angles is called rhombohedral cleavage. The minerals calcite and dolomite have three cleavage directions not at right angles. A mineral with four cleavage directions is called octahedral cleavage. The mineral fluorite has cleavage in four directions. Six cleavage directions are found in the mineral sphalerite.
Cleavage can be described based on the flatness of the surface of cleavage. Cleavage can range from perfect or excellent to good, poor, or non-existent. The mineral quartz does not have any cleavage.
Fracture occurs when a mineral is broken along irregular or curved surfaces. Some minerals have both cleavage and fracture, such as feldspar. Smooth curved fracture is called conchoidal fracture.
The hardness of a mineral is defined as the resistance to scratch and is based on the atomic structure of the mineral. A hardness scale was designed by geologist Friedrich Mohs. The scale contains whole numbers from one to ten with ten being the hardest. Ten minerals are designated a number on the scale based on the hardness of the mineral. All minerals are given a hardness number based on the scale: a whole number or a decimal number in between.
Mohs Hardness Scale:
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Quartz
8 Topaz
9 Corundum
10 Diamond
A harder mineral or material can scratch a softer mineral. However, a softer mineral or material cannot scratch a harder mineral. Common objects are given hardness numbers to help with mineral identification, such as a fingernail (2.5), a copper penny (3), glass (5.5), and a knife (5.5). Minerals below (5.5) on the hardness scale are considered soft while minerals above (5.5) are considered hard.
Specific gravity is the ratio of the weight of a mineral to the weight of an equal volume of water. Metallic minerals are generally heavier than non-metallic minerals.
Minerals can feature a variety of crystal habits including:
Acicular- needle-like crystals (long and slender)
Bladed- as a knife
Dendritic- branching pattern
Prismatic-prism
Striated- parallel grooves
Tabular- flat plates
Twinned- two crystals joined together
Botryoidal- resembles an accumulation of grapes
Fibrous- resembles fibers
Mammillary- accumulation of rounded masses
Massive- no crystal structure
Reniform- kidney shaped
More Terminology:
Other properties and terminology related to minerals include anhedral, which refers to mineral or rock grains that do not have well-defined crystal faces or shape. Euhedral describes a mineral crystal with well-defined faces. Birefringence describes the optical phenomenon of double refraction as a single beam of light travels through a transparent, molecularly ordered material or mineral and is split into two separate beams of light. Double refraction can be observed in the mineral calcite. Fluorescence is a property of some minerals that absorb light at a certain wavelength and emit the light at a different (visible) wavelength. Polymorphism refers to the existence of two or more minerals with identical chemical composition but with different crystal structure. A spectrometer is an instrument designed to measure properties of light over the electromagnetic spectrum which can help identify minerals and other materials.
Chromophore elements are chemical elements whose structure allows them to absorb visible light energy and produce visible colors in minerals. Certain elements are more effective in producing colors than others and minerals containing chromophore elements will likely produce color.
The Origin of Minerals
Minerals originate from several sources:
Many mineral likely formed near the surface during the events of the Global Flood as hot-mineral saturated water and magma rose toward the surface, cooled, and crystallized.
Magma is rock in melted liquid form which exists inside the Earth’s crust and is a main source of minerals. Magma which reaches the surface is called lava. This lava cools and solidifies into rock containing a variety of minerals.
Hydrothermal veins are cracks in rock in which hot mineral saturated water passes upward through and toward the surface. This water originates from magma. When the water cools, minerals are deposited (or precipitated) in solid form. Precipitation is the deposition of minerals from a liquid solution in solid form, which in the process separates the mineral from the solution.
Minerals also form when seawater evaporates and the minerals inside the water deposit in solid form. These minerals are called evaporites. Many sea organisms have shells made of calcite, which is deposited on the seafloor when the organisms die. Some minerals are formed by the chemical alteration of other minerals, such as feldspar to clay minerals. Also, some minerals are formed by metamorphism, or the combination of heat, pressure, and fluid activity on existing rock.
Many mineral resources exist inside the Earth. A resource is the total amount available of a mineral. A reserve is the amount of the mineral which can be mined economically. Some mineral resources cannot be mined economically and therefore are not mined. Most minerals have a variety of economic and industrial uses. These range from metallic ore minerals to oil and natural gas.
Minerals are divided into groups including silicates, carbonates, halides, oxides, sulfates, and sulfides. One or more minerals can exist inside a rock. Accessory minerals are found in small amounts in rocks but are not considered a major mineral in a particular rock.
Ionic Substitution in minerals occurs when one ion or element is substituted for another ion in a mineral atomic structure. In order for substitution to occur, the ions must be of similar size and charge neutrality must be maintained. An example is the potassium feldspar mineral group, where sodium (Na) and potassium (K) can substitute for each other. The atomic formula is written (Na,K)AlSi3O8 where the substituting ions appear in parentheses and are separated by commas. Anions (negatively charged ions) can also substitute for each other. Sometimes a higher charged ion can replace two lower charged ions, but the charge balance must be maintained. For example, an ion of +2 charge can replace two ions of +1 charge. This results in one vacant position in the atomic structure and is called omission substitution. Coupled substitution occurs when a lower charged ion replaces a higher charged ion and a second substitution must occur to maintain charge balance. In general, higher temperature can help initiate substitution in minerals.
Mineral Groups:
Ice (frozen water, snowflakes) H20
Silicate Minerals comprise the largest group of minerals, about 90 percent of the Earth's crust. Underneath the crust is a lower silica percentage and higher iron and magnesium percentage. Feldspar and quartz are the most common silicate minerals in the crust. Pyroxenes, amphiboles, micas, and clay minerals are also common silicate minerals in the Earth's crust. Other mineral groups in the crust include: Oxides, Sulfates, Sulfides, Carbonates (limestone), Native Elements: (gold, silver, copper), Halides, Phosphates, Clay Minerals. The most common elements in Earth's crust: Oxygen, silicon, aluminum, iron, calcium, sodium, magnesium, potassium.
Silica Tetrahedron: The fundamental feature of silicate minerals is the silica tetrahedron (plural tetrahedra), which contains one silicon atom surrounded by four oxygen atoms. The structure of the tetrahedron has a triangular base and three triangular sides. The four oxygen atoms represent the corners of the tetrahedron and the silicon atom is located inside the center of the tetrahedron. The silica tetrahedron has a total electric charge of -4 and requires positive charged ions with a total charge of +4 to form a neutral compound. The silica tetrahedron formula is written as (SiO4)-4.
Orthosilicate Minerals
The silica tetrahedron can be bonded with positive charged ions of magnesium or iron to form an electrically neutral silica tetrahedron. The mineral olivine is an example of a neutral silica tetrahedron or orthosilicate. The formula of olivine is (Mg,Fe)2SiO4. Olivine has a one to four silicon to oxygen ratio in which there are four oxygen atoms for every one silicon atom. Magnesium and iron (Fe) each have a charge of +2, so two atoms of either of these are needed to balance the tetrahedron. Olivine is found most commonly in igneous rocks but also in metamorphic rocks.
The orthosilicate minerals also include garnet, which is an aluminum silicate mineral with a molecular formula ending with Al2(SiO4)3 and is bonded with cations such as iron, calcium, or magnesium. Almandine and grossular are varieties of garnet and both form in metamorphic rocks. Almandine is rich in iron and grossular is rich in calcium. Some varieties of garnet can form in igneous rocks, such as pyrope, which is rich in magnesium. Varieties of garnet produce popular gemstones.
Other orthosilicate minerals include:
Zircon ZrSiO4
Topaz Al2SiO4 (F,OH)2
Andalusite Al2SiO5
Silimanite Al2SiO5
Kyanite Al2SiO5
Titanite (sphene) CaTiSiO5
Willemite ZnSiO4
Chain Silicate Minerals (Pyroxenes)
Chains of silica tetrahedra occur where the tetrahedra form a single strand or line and connect at two corners of each tetrahedron. These chain silicates form when silica tetrahedra share two oxygen atoms with adjacent tetrahedra. Single chain silicates have a ratio of one silicon atom for every three oxygen atoms. The pyroxene group of minerals form single chains of silicates. The formula for single chain silicates consists of the (SiO3)-2 ion bonded with positively charged ions. These ions make the formula ending Si2O6. Pyroxene minerals are formed in igneous rocks particularly volcanic rocks. Pyroxene minerals include:
Augite (Ca, Na)(Mg, Fe, Al, Ti)(Si, Al)2O6
Diopside CaMgSi2O6
Spodumene LiAlSi2O6
Enstatite Mg2Si2O6
Hypersthene (Mg,Fe)2Si2O6
Hedenbergite CaFeSi2O6
Aegirine NaFeSi2O6
Jadeite Na(Al,Fe)Si2O6
Double Chain Silicate Minerals (Amphiboles) form two parallel chains of silica tetrahedra and occurs when tetrahedra share two and three oxygen atoms respectively with adjacent tetrahedra. Double chain silicates have a ratio of four silicon atoms for every 11 oxygen atoms. The amphibole group of minerals forms double chains of silica tetrahedra. Double chain silicates contain the (Si4O11)-6 ion bonded with positively charged ions and these minerals commonly end with Si8O22(OH)2 in the molecular formula. Amphibole minerals form prism or needle-like crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts along with silicon, oxygen, and water.
Hornblende is a common dark green to black variety of amphibole and is a component in many igneous and metamorphic rocks. Amphibolite is a metamorphic rock consisting primarily of amphibole minerals such as hornblende and plagioclase feldspar. Actinolite is a mineral of the amphibole group, is gray-green, commonly has a bladed or fibrous crystal habit, and is a characteristic mineral of the greenschist facies of metamorphism. Tremolite is also an amphibole mineral which forms in metamorphic rocks. Amphibiole minerals include:
Actinolite Ca2(Mg,Fe)5Si8O22(OH)2
Glaucophane Na2(Mg,Fe)3Al2Si8O22(OH)2
Hornblende (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2
Tremolite Ca2(Mg,Fe)5Si8O22(OH)2
Sheet Silicate Minerals (or Phyllosilicate minerals) form sheets or planes of silica tetrahedra in which three oxygen atoms are shared by adjacent tetrahedra. Sheet silicates have a ratio of two silicon atoms for every five oxygen atoms. The mica group of minerals and clay minerals form sheet silicates. Sheet silicates contain the (Si4O10)-4 ion bonded with positively charged ions. Biotite is a dark colored mineral of the mica group while muscovite is a light colored mineral of the mica group. Muscovite has a lighter color because it contains more silica than biotite. Other sheet silicates include talc, serpentine, kaolinite, phlogopite, and chlorite. The sheet silicate minerals include:
Serpentine (Mg,Fe)3Si2O5(OH)4
Clay mineral group:
Kaolinite Al2Si2O5(OH)4
Vermiculite (Mg,Fe,Al)3(Al,Si)4O10(OH)2·4H2O
Talc Mg3Si4O10(OH)2
Pyrophyllite Al2Si4O10(OH)2
Mica group:
Biotite K(Mg,Fe)3(AlSi3)O10(OH,F)2
Muscovite KAl2(AlSi3)O10(OH,F)2
Phlogopite KMg3(AlSi3)O10(OH,F)2
Lepidolite K(Li,Al)3(Si,Al)4O10(F,OH)2
Glauconite (K,Na)(Al,Mg,Fe)2(Si,Al)4O10(OH)2
Chlorite (Mg,Fe)6(AlSi3)O10(OH)8
Phyllosilicate Minerals contain two types of sheet layers in their atomic structure: tetrahedral and octahedral layers. Tetrahedral layers (or sheets) are atomic layers found in phyllosilicate minerals consisting of silicon and oxygen tetrahedra (SiO4)4- sharing three of the four oxygen atoms with adjacent tetrahedra and forming a hexagon or six side structure and continuing laterally. The tetrahedral layers connect with octahedral sheets to form alternating stacked sheets.
Octahedral layers (or sheets) are layers of connected octahedron shaped structures composed of hydroxide (OH)- anions with cations commonly including aluminum, magnesium, or iron. These sheets along with tetrahedral sheets make up the structure of clay or phyllosilicate minerals. Phyllosilicate minerals can be dioctahedral if two of the three positions on the hydroxide ion in the octahedral layer are filled with a cation or trioctahedral if all three positions are filled with a cation.
Clay Minerals
The primary structural unit of the kaolinite group of minerals is a repeating layer composed of one octahedral sheet and one tetrahedral sheet. All members of the kaolinite group form primarily during hydrothermal alteration or weathering of feldspar minerals under acidic conditions.
Shrink-swell refers to the property of many clay soils and sediments that swell or expand when saturated with water and shrink when dried. Water molecules enter between the sheet-like layers and cause the clay to expand. Shrink-swell occurs in the smectite group of clay minerals. Clay soils which have the shrink swell property can cause damage to roads and buildings. Shrink-swell soils are common in the southeast and western United States.
Smectite is a group of clay minerals in which the basic structural unit is a layer consisting of two inward pointing tetrahedral sheets with a central aluminum octahedral sheet, or TOT structure. Smectite can be dioctahedral or trioctahedral. The layers are continuous in two directions, but the bonds between layers are weak and have excellent cleavage, allowing water and other molecules to enter between the layers. This causes expansion in a third direction.
Smectites commonly result from the weathering of basic rocks. (Basic rocks contain a large percentage of calcium rich feldspar minerals and no quartz.) Smectite formation is favored by level to gently sloping terrains that are poorly drained, mildly alkaline (such as in marine environments), and have high silicon and magnesium potentials. Other factors that favor the formation of smectite include the availability of calcium and the lack of potassium. Poor drainage is necessary because otherwise water can leach away ions freed in the alteration reactions. Smectites are used in industry as fillers, carriers, absorbents, and a component in drilling fluids.
Illite is a family of non-expanding, clay-sized, dioctahedral, micaceous minerals. Illite is a three layer clay mineral composed of the TOT structure, or two inward-pointing silica tetrahedral sheets with a central octahedral sheet. Two inter-layer cations fill between each TOT structure. Illites, which are the dominant clay minerals in argillaceous rocks, form by the weathering of silicate minerals (primarily feldspar) through the alteration of other clay minerals and during the degradation of muscovite.
The atomic structure of the chlorite group of minerals contains an octahedral layer in between the two tetrahedral layers, or TOT structure. An additional octahedral layer with a positive charge is bonded with the negatively charged TOT structure. Chlorite occurs in volcanic rocks, metamorphic rocks, and in hydrothermal deposits.
Framework Silicate Minerals (or Tectosilicates) occur when all four oxygen atoms are shared by adjacent tetrahedra, forming a three dimensional framework. The common mineral quartz has the formula SiO2 and is a framework silicate. Quartz is commonly transparent or colorless but can have a variety of colors. Quartz can have a crystalline texture or can be microcrystalline.
Agate is a type of chalcedony, which is a type of microcrystalline quartz gemstone. Chert, flint, and jasper are other microcrystalline quartz varieties. Amethyst is a purple quartz mineral in which the purple color comes from impurities in the mineral. Other quartz varieties include smoky quartz, rose quartz, milky quartz, and citrine. A geode is a cavity in a rock which is partially filled with a mineral (commonly quartz crystals) which grows on the cavity walls. Opal is a type of non-crystalline silica which solidifies or precipitates from silica-rich waters. Opal is a popular gemstone.
Potassium feldspar (also called alkali feldspar) (KAlSi3O8) is a silicate mineral group rich in potassium (some also contain sodium) and common in igneous rocks such as granite, metamorphic rocks, and sedimentary rocks such as sandstone. Feldspar crystals are stubby prisms, often pink to white and some have a streaky appearance (referred to as perthitic texture). Potassium feldspar minerals include sanidine, orthoclase, microcline, and anorthoclase. Amazonite is a green variety of microcline and is used as a gemstone.
Plagioclase Feldspar (NaAlSi3O8 -CaAl2Si2O8) is a silicate mineral group and one of the most common minerals in igneous, metamorphic, and sedimentary rocks. Plagioclase feldspars are silicates that can be sodium or calcium rich. Feldspar crystals are stubby prisms, generally white to gray and have a glassy luster. Twinning is a common feature in plagioclase feldspars in which twin mineral grains are formed with different orientations. Plagioclase feldspar mineral varieties include albite, oligoclase, andesine, labradorite, bytownite, and anorthite. Albite is the most sodium rich while anorthite is the most calcium rich variety.
Exsolution occurs as different minerals in a magma solution separate (see section on magma) as the temperature decreases and crystallization occurs, forming lamellae or layers of alternating minerals. Intergrowths of sodium feldspar can occur inside of potassium feldspar where potassium feldspar is the dominant mineral. These intergrowths are termed perthite.
Undersaturated commonly describes a rock or mineral with low silica content. Feldspathoid minerals are structurally similar to feldspar but have low silica content and are found in alkaline igneous rocks. Alkaline rocks have a high percentage of potassium and sodium oxide in relation to silica. Feldspathoid minerals include nepheline, leucite, and sodalite. These minerals are not as common as feldspar but have many industrial uses. Zeolites are hydrated aluminum silicate minerals containing the alkaline and alkaline-earth metals (or elements). Examples of zeolite minerals include analcime and natrolite. Zeolites form naturally in volcanic rocks or can be produced synthetically and have many industrial uses.
Ring Silicates (cyclosilicates) are a less common type of silicate in which the atomic structure contains six tetrahedra connected together as a ring shape. The rings stack on top of each other to form a column. These silicates include the minerals beryl, cordierite, and tourmaline. These minerals are used as gemstones and also have industrial uses.
Another less common structure consisting of only two tetrahedra joined together (sharing one oxygen) can form and is called a disilicate. This structure can be found in the epidote group of minerals. These minerals are used as gemstones and also have industrial uses.
Carbonate Minerals contain the (CO3)-2 ion in the molecular formula, such as calcite, CaCO3. Calcite can change into the mineral dolomite, CaMg(CO3)2 with the addition of magnesium. Rock containing dolomite is called dolostone. Carbonate rocks are rocks that contain mostly carbonate minerals. Aragonite is a carbonate mineral which is found in many invertebrate skeletons and can change into calcite. Aragonite and calcite are polymorphs of calcium carbonate. Limestone rock contains primarily the mineral calcite. Some carbonate minerals contain hydroxide (OH)-. The most common of these are azurite Cu3(CO3)2(OH)2 and malachite Cu2(CO)3(OH)2.
Other carbonate minerals include:
Magnesite MgCO3
Siderite FeCO3
Rhodochrosite MnCO3
Ankerite Ca(Mg,Fe)(CO3)2
Witherite BaCO3
Strontianite SrCO3
Cerussite PbCO3
Smithsonite ZnCO3
Halide Minerals are a group of minerals with a halogen element (commonly fluorine or chlorine) as the major anion. Halide minerals include halite (NaCl), fluorite (CaF2), and sylvite (KCl). Halite is an evaporite mineral that is deposited primarily from the evaporation of salt water. As the salt water evaporates, the mineral is deposited in solid form.
Oxide and Hydroxide Minerals
Oxide minerals consist of oxygen bonded with cations which are commonly metallic and produce important ore deposits such as iron oxide. Oxide minerals include hematite (Fe2O3) and magnetite (Fe3O4). Corundum (Al2O3) is an oxide mineral used as a gemstone along with industrial uses.
Other oxide minerals include:
Cuprite Cu2O
Chromite FeCr2O4
Spinel MgAl2O4
Ilmenite FeTiO3
Rutile TiO2
Pyrolusite MnO2
Uraninite UO2
Hydroxides are a group of minerals containing the (OH)- ion bonded with positive charged ions. These minerals are commonly produced by weathering processes and hydration of other minerals. Hydroxides commonly contain iron or aluminum. Limonite FeO•OH•nH2O is a mineral composed of iron oxides and water. Limonite is very common in many rocks after weathering (oxidation or rust) at the Earth's surface and produces brown or yellow colors in many rocks.
Manganese oxide and hydroxide minerals (also known as wad) are commonly products of the weathering of manganese minerals. Bauxite is a naturally occurring, heterogeneous material composed primarily of one or more aluminum hydroxide minerals, plus various mixtures of silica, iron oxide, titania, aluminosilicates, and other impurities in minor or trace amounts. Brucite is a magnesium hydroxide mineral commonly found in metamorphic rocks associated with limestone, dolomite, and serpentine.
Sulfates are minerals or compounds containing sulfur and oxygen. The sulfate ion formula is written as SO42−. Sulfate minerals include gypsum (CaSO4* 2H2O) and anhydrite (CaSO4) which are also evaporite minerals. Barite (BaSO4) is a sulfate mineral which is unusually heavy, commonly white to gray, and often found in veins, replacements, or residual deposits commonly associated with quartz, fluorite, and calcite. Celestine (SrSO4) is also a sulfate mineral. Anhydrous refers to minerals or other materials which do not have water as a primary constituent.
Sulfide minerals contain the element sulphur bonded with metals including copper, zinc, lead, cobalt, nickel, and silver. Many sulfides are found in hydrothermal deposits or in altered soils and are important ore minerals. Sulfide minerals include:
Chalcocite Cu2S
Galena PbS
Sphalerite ZnS
Cinnabar HgS
Covellite CuS
Realgar AsS
Orpiment As2S3
Stibnite Sb2S3
Pyrite FeS2
Arsenopyrite FeAsS
Molybdenite MoS2
Chalcopyrite CuFeS2
Bornite Cu5FeS4
Phosphates are minerals that contain the (PO4)3- ion. The mineral apatite is the most common phosphate mineral and is found in a variety of igneous and metamorphic rocks. Apatite is used in fertilizer and in other industrial uses. Turquoise is a phosphate mineral found in volcanic rocks and is commonly used in jewelry. Amblygonite is a phosphate mineral containing lithium, sodium, aluminum, fluoride, and hydroxide. Amblygonite occurs in pegmatite deposits and is commonly light colored. The phosphate minerals include:
Apatite Ca5(PO4)3(F,Cl,OH)
Turquoise CuAl6(PO4)4(OH)8*4H2O
Amblygonite (Li,Na)AlPO4(F,OH)
Borates are naturally-occurring minerals containing the element boron and oxygen along with other elements. There are two basic types of borate minerals. The relatively rare anhydrous borates, which have no water incorporated into their structure, are found in metamorphic and igneous rock. Hydrous borates, minerals with water incorporated into their structure, form by the evaporation of mineral-rich water (evaporite minerals) on desert playas. Borax is a common borate evaporite mineral with the formula Na2B4O5(OH)4 8H2O. Borax is a hydrous borate and is a relatively soft, white, colorless, or transparent mineral. Borax is an important industrial mineral.
Single Element Minerals include gold, silver, copper, antimony, sulfur, mercury, diamond, and graphite. Placer deposits are minerals that separate from the surrounding rock because of greater density and are commonly economically valuable such as gold and diamond.
Bowen's Reaction Series Describes Crystallization of Minerals
Bowen’s Reaction Series was introduced by N.L. Bowen, who developed a chart describing the order of crystallization of minerals inside magma as the temperature of the magma decreases (cools). The series also has two branches: the discontinuous branch and the continuous branch. The discontinuous branch begins with the crystallization of the mafic mineral olivine at the highest temperature. Then as temperature decreases pyroxene crystallizes. As temperature decreases further amphibole and finally biotite mica crystallizes. After further temperature decrease, the silica-rich minerals potassium feldspar, muscovite mica, and finally quartz crystallize. As the mafic minerals at the top of the series crystallize, the magma becomes progressively more silica rich. The continuous branch begins with the crystallization of calcium-rich plagioclase at the highest temperature. As temperature decreases sodium rich plagioclase begins to crystallize. After the calcium and sodium rich plagioclase crystallize, the continuous branch merges with the discontinuous branch and potassium feldspar begins to crystallize as temperature decreases. As temperature decreases further, muscovite mica and finally quartz begin to crystallize. The magma becomes progressively richer in silica as minerals on the upper part of the continuous branch crystallize. The magma also becomes initially richer in sodium plagioclase and potassium feldspar as calcium plagioclase crystallizes at the top of the series.
