Section 4:

The Classification of Mineral Species

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  1. Introduction to Berzelian Classification
  2. Native elements
          Metals, Semimetals, Nonmetals
  3. Sulfides, Antimonides, Arsenides, Selenides, Tellurides and Sulfosalts
  4. Oxides and Hydroxides
          Simple and Multiple Oxides
  5. Halides
  6. Carbonates
          Anhydrous and Hydrous Carbonates
  7. Nitrates
  8. Borates
  9. Sulfates
          Anhydrous and Hydrous Sulfates
  10. Chromates
  11. Molybdates and Tungstates
  12. Phosphates, Arsenates and Vanadates
  13. Silicates
          Nesosilicates, Sorosilicates, Cyclosilicates, Inosilicates, Phyllosilicates, Tectosilicates


1. Introduction to Berzelian Classification

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      The Berzelian mineral classification system was named in honor of the Swedish chemist and mineralogist Jons Jakob Berzelius (1779-1848). The Berzelian system categorizes mineral species according to the main anion group present in their chemical structure. All mineral species of a certain class are therefore chemically similar because they possess the same main anion group. Mineral classes may then be further subdivided according to physical features, which cations are present, the presence or absence of water or the hydroxyl anion, or internal structure.
      The main classes which are recognized under Berzelius' scheme include the native elements; sulfides and sulfosalts; oxides and hydroxides; halides; carbonates, nitrates and borates; sulfates; phosphates; and silicates. The antimonides, arsenides, selenides, and tellurides closely resemble the sulfides in composition, while the chromates, molybdates and tungstates resemble the sulfates. The arsenates and vanadates are closely akin to the phosphates.
      The native elements include all mineral species which are composed entirely of atoms in an uncombined state. Such minerals either contain the atoms of only one element or else are metal alloys. The native elements are divided into metallic, semimetallic, and nonmetallic subgroups. Metals tend to be dense and malleable substances which possess a characteristic metallic luster and conduct electricity well. Semimetals and nonmetals are brittle and conduct poorly compared to metals.
      Minerals of the sulfide class are compounds which contain the nonmetallic element sulfur in combination with atoms of a metal or a semimetal. Compounds in which anions of antimony (Sb), arsenic (As), selenium (Se), or tellurium (Te) replace the sulfur anion and bond with metallic or semimetallic cations are classed respectively as antimonides, arsenides, selenides, and tellurides. If the sulfur anion, a metallic element, and a semimetal are all present then the mineral is categorized as one of the rare sulfosalts. Most sulfides and sulfosalts are soft, dark, heavy, and brittle, possessing a distinct metallic luster and high conductivity.
      The minerals of the oxide class are those which contain oxygen bonded to one or more metallic elements. Hydroxides are compounds of a metallic element and water or the hydroxyl anion (OH)-. The oxide minerals tend to be relatively hard, and some of them may be used as gemstones. Many provide economically important metal ores. Minerals of the hydroxide class tend to be softer and less dense than oxides.
      In members of the halide class an element of the halogen group such as fluorine (F), chlorine (Cl), bromine (Br), or iodine (I) bonds to a metal or semimetal cation such as sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), aluminum (Al), copper (Cu), or silver (Ag). Halides are constructed entirely of ionic bonds. The halide minerals tend to be soft, brittle, easily soluble in water, and possess medium to high melting points. They are poor conductors when in solid state.
      Mineral species which are members of the carbonate class are compounds of a metal or semimetal with the carbonate anion (CO3)2-. The plane triangular carbonate anion groups thus form the basic unit from which carbonate minerals are constructed. The nitrates are structurally very closely akin to the carbonates. Nitrogen bonds to three oxygen atoms to form the nitrate radical, (NO3)-, which forms the basic building block of the minerals of this species. The nitrates tend to be softer and to possess lower melting points than the carbonates. Atoms of the element boron (B) join to three oxygen atoms in order to form the borate radical, (BO3)3-. This anion group closely resembles the carbonate and nitrate radicals in structure. However, the oxygen atoms of the borate radical may, unlike those of the carbonate or nitrate radicals, be shared between anion groups. Borate radicals may therefore be linked into polymerized chains, sheets, or multiple groups. These are the chemical structures which compose the minerals of the borate class.
      The sulfur cation may form very strong bonds with four oxygen atoms, producing the anion group (SO4)2-. This sulfate radical forms the basic structural unit of the minerals of the sulfate class. The sulfate radical does not share oxygen atoms and cannot polymerize. Minerals of the chromate class are compounds of metallic cations with the chromate anion group (CrO4)2-. Just as sulfur and chromium form the anion groups (SO4)2- and (CrO4)2-, the ions of molybdenum (Mo) and tungsten (W) bond with oxygen atoms to create the anion groups (MoO4)2- and (WO4)2-. These anion groups then bond with metal cations to form the minerals of the molybdate and tungstate classes. Molybdenum and tungsten may freely substitute for one another within the ionic groups (MoO4)2- and (WO4)2-, allowing the formation of series of solid solution. They may not, however, substitute for sulfur within the sulfate radical (SO4)2- or form solid solution with minerals of the sulfate or chromate classes. Species of the molybdate and tungstate classes are typically heavy, soft, and brittle. They tend to be dark or vividly colored.
      Like sulfur, the elements phosphorous (P), arsenic (As), and vanadium (V) form tetrahedral anion groups in combination with oxygen. The resulting phosphate radical, (PO4)3-, provides the basic structural unit of the minerals of the phosphate class; the arsenate and vanadate radicals (AsO4)3- and (VaO4)3- form the basic structural units of the arsenate and vanadate classes. The mineral species of these three classes are thus composed of the respective phosphate, arsenate, and vanadate radicals linked by various metal and semimetal cations. Phosphate, arsenic and vanadium ions may substitute for one another within the three anion groups, forming series of solid solution.
      The basic constituent of the minerals of the silicate class is the silicate radical (SiO4)4-. Each oxygen atom within a silicate radical may bond with another silicon ion, becoming part of a second silicate radical and linking the two radicals together. One, two, three, or four of the oxygen atoms in each silicate anion group may bind to other silicate tetrahedra in this way. Many different structures are therefore possible; silicate radicals may remain structurally isolate, join together in pairs, or link into frameworks, sheets, chains, or rings. The various species of the silicate class are grouped according to their structural type. Silicate minerals are usually of relatively great hardness, and single crystals are often translucent.


2. Native Elements

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      The native elements are those minerals formed wholely from elements which occur in an uncombined state. No ionic or covalent bonding may join atoms of one element to atoms of another within the lattice structure of such a mineral. Usually only one type of atom is present in the molecular structure of these species. However, the metal alloys, which contain two or more metals in solid solution, are also classified as native elements because the different species of atom present in their lattices are joined only by metallic bonds, not by ionic or covalent bonds. The native elements are further categorized into subgroups containing metals, semimetals, and nonmetals.

Metals

Photos

      Metallic elements which are found in the native state include gold, silver, copper, lead, iron, nickel, platinum, and the rarer elements palladium, iridium, and osmium. Mercury, tantalum, tin, and zinc have also been found. The uncombined atoms of the metals act as perfect spheres and are relatively inert; they tend to form lattices of face-centered cubic, body-centered cubic or hexagonal close-packed structure. The lattice structures of these native metals are composed of metallic bonds, which are relatively weak and produce soft, malleable, ductile, and sectile substances with rather low melting points. Because many electrons are free to move about within the lattice the native metals are very conductive, and because light cannot propagate inside a good conductor the metals possess the characteristic highly reflective 'metallic' luster.
      The most common native metals are members of the gold group. These include the elements gold (Au), silver (Ag), copper (Cu), and lead (Pb). Mineral species composed of uncombined atoms of the gold group elements possess face-centered cubic lattices, which schema is also known as cubic closest packing. (For a brief discussion of the lattice structure termed 'cubic closest packing' please refer to Section 3.) This cubic closest packing results in a high number of atoms per unit volume and thus in a mineral of relatively high density. In such lattices there exist no atomic planes distinguished by higher or lower density; minerals of the gold group therefore demonstrate hackly fracture and possess no cleavage.
      Gold and silver atoms possess equal 1.44 angstrom atomic radii. These two substances are thus mutually soluble, and occur in mixtures with a wide range of relative compositions. Copper, however, has a radius of 1.28 angstrom. For this reason it is present only in tiny amounts within mixtures of gold and silver and conversely contains only tiny amounts of gold and silver in solution.
      The iron group contains iron (Fe) and nickel (Ni). These metals possess a body-centered cubic structure. Nickel and iron are mutually soluble because their atomic radii are both equal to 1.24 angstrom. This solid solution is frequently found in meteorites and probably constitutes much of the earth's core.
      Platinum (Pt), palladium (Pd), iridium (Ir), and osmium (Os) number among the less common metallic native elements and are classified as the platinum group. The mineral species which form from the uncombined atoms of these elements are platinum, palladium, platiniridium and iridosmine. Platiniridium is a rare alloy of iridium and platinum, while iridosmine is an equally rare alloy of iridium and osmium. Both alloys possess hexagonal close-packed structure, while platinum and palladium are cubic close packed. The metals of the platinum group are harder and possess higher melting points than those of the gold group.

Semimetals

      The native semi-metals include arsenic (As), antimony (Sb), and bismuth (Bi), as well as the less common elements selenium (Se) and tellurium (Te). Arsenic, antimony and bismuth crystallize in the hexagonal-scalenohedral class while selenium and tellurium crystallize in the trigonal-trapezoidal class. Natural crystals of all three species are rare. (For a discussion of crystal classes, please refer to Section 3.) The semimetals are brittle, and conduct heat and electricity poorly compared to the metals. However, like the metals they display a metallic luster.
      The lattices of the semimetals are composed of bonds intermediate in type between metallic and covalent. Such bonds are stronger than metallic bonds but are also more directional. The structure of the hexagonal semimetals is therefore based on a distorted form of cubic closest packing in which sheets of atoms parallel to the base of the crystal separate into pairs. This sheetlike structure results in perfect basal cleavage, or perfect cleavage along the paired planes.
      The distorted form of cubic closest packing found in the semimetal lattice possesses a greater volume than the close packing which composes the liquid form of the substances. The semimetals thus display the unusual property of expansion upon crystallization.

Nonmetals

      The native nonmetals include carbon (C), in the form of diamond and graphite, and sulphur (S).

Polymorphism in Diamond and Graphite

      The mineral species diamond and graphite offer a spectacular example of the trait of polymorphism. Polymorphism occurs when two or more mineral species contain exactly the same elements in exactly the same proportions, and therefore share a chemical formula, yet possess dissimilar lattice structures. Two polymorphic minerals possess identical chemical formulae but different crystal structures; the minerals may therefore exhibit very different physical traits.
      Chemically the two species diamond and graphite are identical. Both are native elements composed wholely of elemental carbon; both may be burned to carbon dioxide at high temperatures; and both contain closely similar carbon-carbon bonds. However, the two substances are structurally very different. Diamond possesses an exceptionally strong lattice in which each carbon atom is bonded by four covalent bonds to four neighboring carbon atoms, which occupy the apices of a regular tetrahedron. All four valence electrons are taken up by covalent bonds, so that none are free to conduct electricity; hence diamond forms a highly successful insulator. Graphite, on the other hand, is composed of sheets of six-atom rings in which each carbon has three neighboring atoms positioned at the corners of an equilateral triangle. Three valence electrons are occupied by covalent bonds; the fourth is free to act as a conductor of electricity. Graphite thus conducts relatively well. The sheets are stacked a distance much greater than one angstrom apart, and the van der Waals forces which bind the stacked sheets together are very weak. The wide separation and weak binding forces between parallel sheets result in perfect basal cleavage.
      The two nonmetallic native elemental species graphite and diamond possess identical chemical formulae but, because of their differing lattice structures, demonstrate widely disseparate physical traits. They thus provide an apt example of polymorphism.


3. Sulfides, Antimonides, Arsenides, Selenides, Tellurides, and Sulfosalts

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Sulfides

      Sulfide minerals are compounds of one or more metal or semimetal elements with the nonmetallic element sulfur (S). In a sulfide, the sulfur anion (S2+) is thus combined with metallic cations such as iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), molybdenum (Mo), silver (Ag), cadmium (Cd), tin (Sn), platinum (Pt), gold (Au), mercury (Hg), tellurium (Tl), lead (Pb), and the semimetals arsenic (As), antimony (Sb), and bismuth (Bi). Compounds of sulfur and one of the semimetals are termed semimetal sulfides. Most sulfides are structured by ionic bonds, although some may contain metallic bonding and display metallic properties.

Antimonides, Arsenides, Selenides, Tellurides

      The nonmetal sulfur (S) and the semimetals selenium (Se) and tellurium (Te) consistently play the electronegative, anionic role in their compounds. However, the semimetals arsenic (As) and antimony (Sb) may either play the anionic role often filled by sulfur or else act as a cation by donating electrons and forming a compound with sulfur.
      When the semimetals antimony (Sb), arsenic (As), selenium (Se), and tellurium (Te) occupy the role of a nonmetal and substitute for the sulfur anion, the minerals formed possess a chemical structure very similar to that of the sulfides. These are classified respectively as antimonides, arsenides, selenides, and tellurides according to which semimetal is present. When the semimetals arsenic (As) and antimony (Sb) form compounds with sulfur, however, they occupy the role of a metal cation and the minerals which contain them are termed semimetal sulfides. If a metal, a semimetal, and nonmetallic sulfur are all present then the mineral is categorized as one of the rare sulfosalts.
      If both a metal and a semimetal such as arsenic or antimony are present in a compound and there is no sulfur anion present, then the semimetal replaces sulfur in order to play the electronegative role. In this case the mineral is classified as an antimonide, arsenide, selenide, or telluride. For example, niccolite (NiAs) contains the metal nickel and the semimetal arsenic, which acts as an nonmetal and provides the anion. Niccolite is therefore an arsenide. The mineral breithauptite (NiSb) contains nickel and the semimetal antimony, which acts as the electronegative, nonmetallic element of the compound. Breithauptite is thus an antimonide. The mineral calaverite (AuTe2), in turn, contains the metallic element gold and the semimetal tellurium, which provides an anion; calaverite is a telluride.
      If only sulfur and either arsenic or antimony are present then the semimetal acts as a cation by donating electrons as it combines with the sulfur. For example, the mineral realgar (AsS) is a sulfide which contains only the semimetal arsenic (As) and the nonmetal sulfur (S). In this case, the arsenic acts as a metal by donating electrons to the electronegative sulfur. This type of compound is considered a semimetal sulfide; other examples are orpiment (As2S3) and bismuthinite (Bi2S3).

Sulfosalts

      The rare minerals which are compounds of sulfur (S), a semimetal such as arsenic (As) or antimony (Sb), and one or more metals are termed sulfosalts. In these species the semimetal plays the role of a metal. The mineral cobaltite, which possesses the chemical formula CoAsS, is a sulfosalt. The metallic element in this compound is cobalt (Co); the semimetal, which also donates electrons, is arsenic (As), and the nonmetal sulfur receives the electrons donated by both the cobalt and the arsenic. Pyrargyrite ("dark ruby silver", chemical formula Ag3SbS3), is also a sulfosalt. Its metallic element is silver (Ag), the semimetal is antimony (Sb), and the nonmetal is sulfur. Proustite (Ag3AsS3; "light ruby silver") is another sulfosalt with a chemical formula and structure very similar to that of pyrargyrite, differing only because the semimetal antimony (Sb) has been replaced by arsenic (As).
      Most of the sulfides contain ionic bonding. Others contain a degree of metallic bonding. Typical sulfides and sulfosalts are soft, dark, heavy, and brittle, possessing a distinct metallic luster and high conductivity. Most are opaque, demonstrating the distinctive colors and colored streak derived from the presence of chromophores (iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu)). Those sulfides which are not opaque (cinnabar, realgar, and orpiment) are transparent only in very thin sections. Some species such as pyrite emit a sulfurous odor when they are struck with a mallet.

Categorization by Ratio of Metal to Nonmetal

      The sulfides, antimonides, arsenides, selenides, tellurides, and sulfosalts may be grouped according to the ratio of metal to nonmetal contained in their chemical formulae. Let the letter A represent a metal or a semimetal acting as a cation, while X represents a nonmetal or a semimetal acting as an anion. Then the formulae may be categorized as AX, AX2, A2X, and so on according to the ratio of metal to nonmetal contained. Galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), covellite (CuS), and cinnabar (HgS) are examples of sulfides whose formulae possess the ratio AX. In all of these the nonmetallic element sulfur bonds to various metals such as lead, zinc, copper, iron, and mercury. In the semimetal sulfide realgar (AsS) arsenic fills the role of a metal and bonds with sulfur. Niccolite (NiAs) offers an example of an AX-type arsenide, in which arsenic plays the nonmetal, electronegative role while nickel donates electrons. Breithauptite (NiSb) in turn is an AX antimonide in which antimony fills the nonmetal role and bonds with nickel. Chalcocite (Cu2S) provides an example of an A2X sulfide, while pyrite (FeS2), and molybdenite (MoS2) are sulfides possessing formulae of the AX2 type. The mineral species cobaltite (CoAsS) is a sulfosalt and calaverite (AuTe2) is a telluride of the AX2 type. Species of the A2X3 type include the semimetal sulfides orpiment (As2S3) and bismuthinite (Bi2S3) while the sulfosalts pyrargyrite (Ag3SbS3) and proustite (Ag3AsS3) are of the form A4X3.

AX
galena
sphalerite
chalcopyrite
covellite
cinnabar
realgar
niccolite
breithauptite   
 
PbS
ZnS
CuFeS2
CuS
HgS
AsS
NiAs
NiSb
 
sulfide
sulfide
sulfide
sulfide
sulfide
semimetal sulfide
arsenide
antimonide
A2X
chalcocite
 
Cu2S
 
sulfide
AX2
pyrite
cobaltite
molybdenite
calaverite
 
FeS2
CoAsS
MoS2
AuTe2
 
sulfide
sulfosalt
sulfide
telluride
AX3
skutterudite
 
CoAs3
 
sulfide
A2X3
orpiment
bismuthinite
 
As2S3
Bi2S3
 
semimetal sulfide
semimetal sulfide
A4S3
cobaltite
pyrargyrite
proustite
 
CoAsS
Ag3SbS3
Ag3AsS3   
 
sulfosalt
sulfosalt
sulfosalt

Secondary Enrichment of Sulfides

      Many of the sulfide minerals are water soluble. Such minerals can be dissolved and transported underground by heated groundwater. This hydrothermal transportation of minerals is the mechanism by which secondary enrichment occurs. (Please refer to the discussion of secondary enrichment in Section 5.) Sulfides and arsenides are frequently altered to produce the secondary sulfates and arsenates. (Please see the following pages on sulfates and arsenates.)


4. Oxides and Hydroxides

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Simple and Multiple Oxides

      Members of the oxide class are minerals in which an oxygen anion is combined with one or more metals. The oxides can be divided into two categories consisting of simple oxides, which contain a single metallic element, and multiple oxides, which are compounds of oxygen and two or more metals.
      Simple oxides are compounds of a single metallic element and oxygen. Examples of such species include zincite (ZnO), tenorite (CuO), cuprite (Cu2O), rutile (TiO2), uraninite (UO2), corundum (Al2O3), hematite (Fe2O3), and magnetite (Fe3O4). Ice (H2O) provides an unusual example of a simple oxide in which hydrogen replaces a metal in order to act as the cation.
      Only one type of metal is present in combination with oxygen in each simple oxide species. Multiple oxides, in contrast, are compounds of oxygen with two or more metallic elements. Examples are ilmenite (FeTiO3), spinel (MgAl2O4), chromite (FeCr2O4), and chrysoberyl (BeAl2O4). Many of these species are colorful, relatively hard, and may be used as gemstones.
      The oxide class contains several metal ores of great economic importance. Among these are the iron ores, hematite and magnetite; chromite, an ore of chromium; manganite (MnO(OH)), which provides manganese; zincite, which contains zinc; and gibbsite (Al(OH)3), which offers a source of aluminum.
      The oxides can be usefully divided into the two categories of simple oxides, which contain only a single cation, and multiple oxides, which contain two or more metals. However, the oxides may alternatively be subdivided so that the chemical formula of each mineral species is categorized according to the ratio of metal to oxygen which it contains. If the capital letter A represents a metal atom, then the formulae AO, A2O, AO2, A2O3, and A3O4 each represent the chemical formulae of a group of mineral species. Examples of the AO type are provided by zincite (ZnO) and tenorite (CuO), while cuprite (Cu2O) offers an example of the A2O type. Species of the AO2 type are rutile (TiO2) and uraninite (UO2). The A2O3 type is represented by corundum (Al2O3), hematite (Fe2O3), and ilmenite (FeTiO3), while the A3O4 type is represented by spinel (MgAl2O4), magnetite (Fe3O4), chromite (FeCr2O4), and chrysoberyl (BeAl2O4).
AO
zincite (ZnO)
tenorite (CuO)   
A2O
cuprite (Cu2O)
ice (H2O)
AO2
rutile (TiO2)
uraninite (UO2)
A2O3
corundum (Al2O3)
hematite (Fe2O3)
ilmenite (FeTiO3)
A3O4
spinel (MgAl2O4)
magnetite (Fe3O4)
chromite (FeCr2O4)
chrysoberyl (BeAl2O4)

Hydroxides

      Hydroxides are compounds of metallic elements with water or the hydroxyl anion (OH)-. Examples of hydroxides are manganite (MnO(OH)), goethite (FeO(OH)), and gibbsite (Al(OH)3; one of the main components of bauxite). Minerals of the hydroxide class are typically softer than oxides and are of low to medium density.

      In terms of chemical composition, quartz (SiO2) is a member of the oxide class. However in terms of molecular structure, which in the Berzelian classification system is considered to be more fundamental than chemical composition, quartz is classified as a silicate. (Please refer to the following discussion of silicates.)


5. Halides

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Photos

      The halide class is composed of minerals in which an element of the halogen group such as fluorine (F), chlorine (Cl), bromine (Br), or iodine (I) provides the anion. These elements form relatively large, lightly charged, easily polarized ions. The halogen ions then bond to large, weakly polarized cations of sodium (Na), potassium (K), magnesium (Mg) or calcium (Ca), and sometimes aluminum (Al), copper (Cu), or silver (Ag). Halides are therefore constructed entirely of ionic bonds. Both cations and anions behave as almost spherical bodies which pack in a highly symmetric manner; most common halites possess isometric hexoctahedral crystalline structure.
      The halides tend to be soft, brittle, easily soluble in water, and possess medium to high melting points. They are poor conductors of heat and electricity when in solid state but good conductors when molten. (When molten, ions are liberated and may then transport charges. Molten halides thus conduct by electrolysis, or the transportation of charge by free-moving ions.)
      Example members of the halide class are the mineral species halite (NaCl; also known as rock salt), sylvite (KCl), fluorite (CaF2), and chlorargyrite (AgCl, also known as horn silver).


6. Carbonates

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      Mineral species which are members of the carbonate class are compounds of a metal or semimetal with the carbonate anion (CO3)2-. In these substances plane triangular (CO3)2- anion groups are linked together by various cations. Each oxygen atom is bonded more strongly to its associated carbon than to any other atom of the structure, and oxygen atoms are not shared between the carbonate anions. The plane triangular carbonate anions thus form the basic unit from which carbonate minerals are constructed.
      The bond between the carbon and the two oxygen atoms of the (CO3)2- anion is strong. However, when brought in contact with the hydrogen ion (H+) the carbonate radical decomposes, producing carbon dioxide and water. Minerals of the carbonate class thus react easily with acids such as hydrochloric acid (HCl). For example, calcite (calcium carbonate, CaCO3) effervesces when placed in an aqueous solution of HCl, producing carbon dioxide and calcium chloride:

CaCO3 (s) + 2HCl (aq) -------> CaCl2 (s) + CO2 (g) + H2O (l)

This reaction provides a means for the identification of carbonate species which is easily applicable in the field.

Anhydrous Carbonates

      The anhydrous carbonates lack both the hydroxyl anion (OH-) and water (H2O) in their chemical formulae. These carbonates are divided into three isostructural groups, called the calcite group, the dolomite group, and the aragonite group.
      The calcite group contains calcite (CaCO3), magnesite (MgCO3), rhodocrosite (MnCO3), and siderite (FeCO3). Each of these minerals is composed of the carbonate anion and cations of a single metal. Although the bonds within the carbonate anion are covalent, the bonds between the carbonate anion and the metal cation are ionic. The structure of the minerals of the calcite group is composed of alternate layers of metal cations and carbonate anions. All members of the group are isostructural, possessing unit cells which are completely analogous in structure and differ only in the identity of the constituent atoms. Because the calcite minerals are isostructural, they form solid solutions of varying proportions in which magnesium (radius 0.66 angstrom), manganese (0.80 angstrom) and iron (0.74 angstrom) substitute for one another. Calcium, however, does not substitute so readily for these elements because of its larger radius (0.99 angstrom). Members of the calcite group are also called the rhombohedral carbonates, because the unit cell and the cleavage both form rhombohedra.
      The dolomite group contains the minerals dolomite (CaMg(CO3)2) and ankerite (CaFe(CO3)2). Each of these species contains the semimetal calcium and a metal cation together with the carbonate anion. The structure of the dolomite minerals is composed of layers in which carbonate anions alternate with metal cations. In this way members of the dolomite group are similar in structure to the calcite group. However, the dolomites are more highly ordered than the calcites because their cation layers consist of layers of calcium alternating with layers of magnesium or iron. Due to its large atomic radius (0.99 angstrom) in comparison to the radii of iron (0.74 angstrom) or magnesium (0.66 angstrom) calcium is unable to form a solid solution with these metals, and the calcium separates out from the iron and magnesium to produce its own layer. Like the members of the calcite group, those of the dolomite group adhere to the rhombohedral crystal system.
      The aragonite group is composed of the orthorhombic carbonates, of which aragonite (CaCO3), strontianite (SrCO3) and cerussite (PbCO3) provide examples. The members of this group are isostructural and may form a solid solution in which strontium, lead, and calcium may substitute for one another.
      Like the members of the calcite group, the aragonite minerals each contain the carbonate anion and ions of a single metal. However, the members of the aragonite group adhere to the orthorhombic crystal system rather than the rhombohedral crystal system preferred by the calcite and dolomite groups. Structurally calcite and aragonite are thus quite different. (Please refer to the discussion of crystal system in Section 3 for details on the orthorhombic and rhombohedral (hexagonal) systems.)

Polymorphism and Pseudomorphism in Calcite and Aragonite

      The two different mineral species calcite and aragonite both possess the chemical formula CaCO3. Although chemically identical, these two species are structurally very different; calcite crystalizes according to the rhombohedral crystal system while aragonite crystallizes according to the orthorhombic crystal system. Aragonite is harder and has a higher specific gravity than calcite. Two mineral species such as calcite and aragonite which are chemically identical and yet differ structurally and possess different physical properties are said to be polymorphs.
      The structure of a carbonate is determined by the radius of its constituent cation. Those carbonates which contain smaller cations form rhombohedral structures while those possessing larger cations occupy orthorhombic structures. Magnesium, manganese, and iron are small cations, and the carbonates such as magnesite, rhodocrosite, and siderite which contain them adhere to the rhombohedral crystal system. Lead and strontium, on the other hand, are large cations, and species such as strontianite and cerussite which contain them belong to the orthorhombic crystal system. The radius of calcium occupies a unique position near the critical radius which distinguishes between the two possible crystal systems. Carbonate species which contain the calcium anion may thus crystallize according to either structural system. For this reason the compound calcium carbonate (CaCO3) crystallizes with rhombohedral crystal structure as calcite and with orthorhombic structure as aragonite. The fact that the calcium cation is of approximately the critical radius leads to the ability of calcium carbonate to crystallize in either system. This trait explains the polymorphism of calcite and aragonite, both of which possess the chemical formula CaCO3 but which crystallize according to the differing rhombohedral and orthorhombic crystal systems.
      Aragonite is less stable than calcite and therefore less common. Aragonite begins to invert to calcite at temperatures as low as 400° Celcius; in contact with water of aqueous solutions containing calcium carbonate it may even invert at room temperatures. For this reason pseudomorphs of calcite after aragonite are often observed. In such cases the unit cells of the original aragonite have been gradually replaced by those of calcite while the original crystal shape remains intact.

Hydrous Carbonates

      The hydrous carbonates consist of the copper carbonates, which contain both copper and the hydroxyl radical (OH-). Also contained within this subgroup are the hydrated carbonates such as thermonatrite (Na2CO3•H2O) and Trona (Na3H(CO3)2•2H2O, which contain water (H2O) within their lattice structures.
      The copper carbonates azurite (Cu3(CO3)2(OH)2), malachite (Cu2CO3(OH)2), and aurichalcite (Cu5(CO3)2(OH)6) contain both copper and the hydroxyl anion (OH-). The element copper is a strong pigmenting agent and renders azurite a bright, distinctive azure blue, malachite a vivid green, and aurichalcite green to blue. Both azurite and malachite are of the monoclinic crystal system while aurichalcite is of the orthorhombic crystal system. Malachite frequently forms a pseudomorph of azurite; azurite is less stable than malachite, and gradually inverts to it over long periods of time.


7. Nitrates

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      The nitrates are chemically closely akin to the carbonates. Nitrogen bonds to three oxygen atoms in a structure very similar to the carbonate anion group (CO3)2-. This nitrate radical, (NO3)-, forms the basic building block of the nitrates.
      Because nitrogen is more electronegative than carbon, the nitrate anion group (NO3)- is less stable than (CO3)2-. Mineral species of the nitrate class thus tend to occur less frequently in geologic formations than do the carbonates. (NO3)- is less charged than (CO3)2-; the nitrates tend therefore to be softer than the carbonates and also to possess lower melting points. The nitrates yield oxygen as the nitrate anion decomposes upon heating.
      Several mineral species of the nitrate class form structures analogous to certain of the carbonates. For example, nitratine (or soda niter; NaNO3) and the carbonate calcite (CaCO3) are isostructural and possess the same rhombohedral cleavage. (Nitratine is softer than calcite and melts ar a lower temperature, both of which are expected of a nitrate as compared to a carbonate). Niter (KNO3, known colloquially as saltpeter) is in turn isostructural with the orthorhombic carbonate aragonite (CaCO3). To complete the analogy with calcite and aragonite, nitratine (NaNO3) possesses a polymorph which is also of chemical formula NaNO3. The polymorph of rhombohedral nitratine is isostructural with orthorhombic niter (KNO3). Nitratine's polymorphism is thus analogous to that of the rhombohedral carbonate calcite and the orthorhombic carbonate aragonite, both of chemical formula CaCO3.


8. Borates

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      The boron atom joins to three oxygen atoms to form the borate radical, (BO3)3-. After this anion group has formed, each oxygen atom still has one electron available for bonding. Thus the oxygen atoms of the borate radical may, unlike those of the carbonate or nitrate radicals, be shared between anion groups. Borate radicals may therefore be linked into polymerized chains, sheets, or multiple groups. The most common borate minerals are composed of sheets of radicals which are linked together through the sharing of all three oxygen atoms. The sheets are then separated by layers of water molecules (H2O) and linked together by sodium (Na+) or calcium (Ca2+) ions.
      Two example members of the borate class are borax, which possesses the chemical formula Na2B4O7•10H2O, and colemanite, Ca2B5O11•5H2O.


9. Sulfates

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      The sulfur anion (S2-) may bond to a positive metallic or semimetallic ion, receiving two electrons in order to fill its valence shell. Mineral species of the sulfide class are compounds of this nature. Alternately, the six electrons of sulfur's unfilled valence shell may be ejected, resulting in the small, highly charged (S6+) cation. The sulfur cation may then form very strong bonds with four oxygen atoms, producing the anion group (SO4)2-. This sulfate radical forms the basic structural unit of the minerals of the sulfate class.
      All members of the sulfate class contain the tetrahedral sulfate radical (SO4)2-. These tightly bound anion groups do not share oxygen atoms and cannot polymerize.

Anhydrous Sulfates

      Anhydrous sulfates contain neither water molecules (H2O) nor the hydroxyl anion (OH-) within their lattice structures. The anhydrous sulfates consist of the barite group and the mineral species anhydrite (CaSO4). The members of the barite group include barite (BaSO4), celestite (SrSO4), and anglesite (PbSO4). These species are isostructural and may form a solid solution by exchanging atoms of strontium and lead for those of barium. The minerals of the barite group all possess orthorhombic symmetry, as does anhydrite. Anhydrite is not, however, isostructural with the barite minerals because the calcium ion possesses a radius smaller than that of barium, strontium, or lead.
      Barite (BaSO4) sometimes produces groups of divergent tabluar crystals known as "desert roses." The barite mineral anglesite (PbSO4) is formed through the oxidation of lead-bearing veins and in particular the alteration of the lead sulfide galena (PbS). Specimens of concretionary habit have been found consisting of concentric layers often containing a core of unaltered galena.

Hydrous Sulfates

Photos (Gypsum)

      Gypsum (CaSO4•2H2O) is the most prevalent and important of the hydrous sulfates. It possesses a sheetlike structure which consists of layers of calcium ions and sulfate anions separated by water molecules (H2O). Gypsum demonstrates perfect cleavage along planes parallel to its sheetlike layers. If the water is driven out of the gypsum structure, the mineral will collapse into the configuration of anhydrite (CaSO4) with a marked decrease in volume and loss of the perfect cleavage. Gypsum is frequently formed during the alteration of anhydrite.
      Chalcanthite (CuSO4•5H2O, also known as blue vitriol), melanterite (FeSO4•7H2O, known colloquially as copperas), epsomite (MgSO4•5H2O), antlerite (Cu3(OH)4SO4), linarite (PbCu(SO4)(OH)2), cyanotrichite (Cu4Al2SO4(OH)12•2H2O), and alunite (KAl3(OH)6(SO4)2; known as potassium alum) also belong to the hydrous sulfates.
      Chalcanthite, antlerite, linarite, and cyanotrichite are copper ores which are found in the oxidized and hydrothermally altered regions of copper veins. Due to the pigmenting effect of copper, chalcanthite, linarite and cyanotrichite are a deep azure blue in color. Antlerite may be emerald to very dark green and has a pale green streak. All four minerals may form as the product of secondary alteration of copper sulfides such as chalcocite (Cu2S). Melanterite is an iron ore which is formed by the oxidation and hydrothermal alteration of pyrite (FeS2) or other iron sulfide minerals. This species is yellow green to deep green due to the presence of iron.


10. Chromates

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      Minerals of the chromate class are compounds of metallic cations with the chromate anion group (CrO4)2-. The species of this class of minerals are very rare.
      The lead chromate crocoite (PbCrO4) is isostructural with the sulfates of the barite group (BaSO4). This rare mineral is found in the oxidized zones of lead-bearing veins, particularly in places where lead veins have encountered rocks containing chromite. Due to the strong pigmenting capabilities of chromium, crocoite is a bright orange-red in color while its streak is orange.


11. Molybdates and Tungstates

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      Just as sulfur and chromium form the anion groups (SO4)2- and (CrO4)2-, the ions of molybdenum (Mo) and tungsten (W) bond with oxygen atoms to create the anion groups (MoO4)2- and (WO4)2-. These anion groups then bond with metal cations to form the minerals of the molybdate and tungstate classes. However, the atomic radii of molybdenum and tungsten are larger than that of sulfur. For this reason the ionic groups containing molybdenum and tungsten do not form regular tetrahedra. Instead, the tetrahedra occupied by the metal cation and the oxygen atoms are flattened and deformed. Molybdenum and tungsten possess equal atomic radii and may therefore freely substitute for one another within the ionic groups (MoO4)2- and (WO4)2-, allowing the formation of series of solid solution. Molybdenum and tungsten may not, however, substitute for sulfur within the sulfate radical (SO4)2- or form solid solution with minerals of the sulfate class.
      Minerals containing the molybdate or tungstate anion groups are categorized as members of the molybdate or tungstate classes. These two classes are then further subdivided into the wolframite and scheelite groups. Minerals of either class may be placed into each group according to the relative size of the metallic cation which they contain. Species of the molybdate and tungstate classes are typically heavy, soft, and brittle. They tend to be dark or vividly colored.
      Mineral species belonging to the wolframite group contain small cations such as magnesium, manganese, iron, cobalt, and nickel. The tungstates huebnerite (MnWO4) and ferberite (FeWO4) are considered to be end members of a series of solid solution in which manganese may substitute for various quantities of iron and vice versa. Both huebnerite and ferberite belong to the wolframite group.
      Mineral species belonging to the scheelite group form compounds containing larger ions such as calcium. The tungstates scheelite (CaWO4) and stolzite (PbWO4) and the molybdates powellite (CaMoO4) and wulfenite (PbMoO4) are all members of the scheelite group. The lead molybdate wulfenite is a bright red color and is found in the oxidized portion of lead veins.
      Because molybdenum and tungsten possess equal atomic radii, they may freely substitute for one another within the ionic groups (MoO4)2- and (WO4)2-. One series of solid solution exists between powellite (CaMoO4) and scheelite (CaWO4), while another occurs between wulfenite (PbMoO4) and stolzite (PbWO4). However, despite the similarity in atomic radii the difference in atomic weights between the two elements is significant. Molybdenum possesses an atomic mass of 96 amu, while tungsten's mass is 184 amu. For this reason the two elements are often separated out by the force of gravity during the formation of the primary minerals which bear them. In primary mineral species, therefore, molybdate and tungstate species usually occur separately rather than in mixture. The two elements are more often found in solid solution in secondary minerals which result from the alteration of primary minerals.


12. Phosphates, Arsenates, and Vanadates

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      Like sulfur, the elements phosphorous (P), arsenic (As), and vanadium (V) form tetrahedral anion groups in combination with oxygen. The resulting phosphate radical, (PO4)3-, provides the basic structural unit of the minerals of the phosphate class; the arsenate and vanadate radicals (AsO4)3- and (VaO4)3- form the basic structural units of the arsenate and vanadate classes. The mineral species of these three classes are thus composed of the respective phosphate, arsenate, and vanadate radicals linked by various metal and semimetal cations. Phosphate, arsenic and vanadium ions may substitute for one another within the three anion groups, forming series of solid solution.
      The apatite group contains mineral species of each of the phosphate, arsenate, and vanadate classes. Among the minerals of the apatite group number the three lead ores pyromorphite (Pb5Cl(PO4)3), mimetite (Pb5Cl(AsO4)3), and vanadinite (Pb5Cl(VO4)3). Each of these species contains both lead and chlorine. The three minerals are isostructural, differing only according to whether they contain phosphor, arsenic, or vanadium within the radical. Pyromorphite is thus a phosphate, mimetite is an arsenate, and vanadinite is a vanadate. These three species form a complete series of solid solution in which arsenic and vanadium may replace phosphor. The mineral vanadinite is bright orange-red due to the presence of the chromophore vanadium; all three species are found in the oxidized areas of lead veins.
      The apatite group also contains the three species fluorapatite (Ca5F(PO4)3), chlorapatite (Ca5Cl(PO4)3), and hydroxylapatite (Ca5(OH)(PO4)3). These species are all calcium phosphates, because they contain the calcium cation (Ca2+) and the phosphate radical (PO43-). However, each species contains a different anion. Fluorapatite contains fluorine (F-), chlorapatite contains chlorine (Cl-), and hydroxylapatite contains the hydroxyl anion (OH-). The three minerals are isostructural, differing only according to which anion fills the requisite space in the unit cell. They form a complete series of solid solution in which chlorine or the hydroxyl anion may replace fluorine.
      Example phosphates which do not belong to the apatite group class are amblygonite (LiAlF(PO4)), the hydrous phosphate wavellite (Al3(PO4)2(OH)3), and the hydrous iron phosphate vivianite (Fe3(PO4)2•8H2O). Vivianite is colorless before exposure to light; after exposure to light it assumes a blue or green color due to the presence of iron. Its streak is blue. Tirquoise (CuAl6(PO4)4(OH)8•2H2O) is a hydrous copper and aluminum phosphate which may be sky blue, bright blue or tirquoise blue in color and is pigmented by copper. Lazulite ((Fe,Mg)Al2(OH)2(PO4)2) is a ferric and magnesian phosphate which is a characteristic azure blue in color.
      The mineral species erythrite (Co3(AsO4)2•8H2O is a hydrous cobalt arsenate. Erythrite is a vivid crimson or pink in color due to presence of the pigmenting agent cobalt; its streak is pale red or pink. The mineral is often found in the form of characteristic pink crusts called "cobalt bloom". Erythrite occurs as a secondary product of cobalt arsenides such as skutterudite (CoAs3). Nickel may substitute for the cobalt in erythrite to form the mineral annabergite (Ni3(AsO4)2•8H2O), sometimes called "nickel bloom". Annabergite is isostructural with erythrite; both minerals are hydrous arsenates and differ only according to whether they contain cobalt or nickel. Annabergite is a secondary mineral derived from nickel arsenides such as niccolite (NiAs). Nickel, like cobalt, is a chromophore: due to its presence, annabergite is light green in color.
      Conichalcite (CaCu(AsO4)(OH)) is a copper arsenate which is grass green in color and possesses a green streak. It is pigmented by copper. Olivenite (Cu2AsO4(OH)) is another copper arsenate which is usually olive green.
      The descloizite series includes the vanadates descloizite (PbZnVO4(OH)) and mottramite (PbCu(VO4(OH))). Both minerals are lead vanadates, containing the vanadium radical (VaO4)3- and a lead cation; they differ in that descloizite contains a zinc cation whereas mottramite contains a copper cation. Descloizite and mottramite are isostructural, forming a series of solid solution in which mottramite's copper substitutes for descloizite's zinc. Mottramite (PbCu(VO4)(OH)) is grass green or olive green in color due to the pigmenting action of copper; it possesses a green streak. Descloizite and mottramite are found in the oxidized portions of lead, copper, and vanadium veins.


13. Silicates

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      The fundamental constituent of the minerals of the silicate class is the silicate radical (SiO4)4-. This anion group is composed of one silicon atom bonded to four oxygen atoms, which occupy the apices of a regular tetrahedron. The bond which holds the silicon and oxygen atoms together within the silicate radical is partially ionic and partially covalent. This bond originates in the attraction of oppositely charged ions for one another, but it also involves the sharing of electrons and significant overlap of electron clouds.
      The various species of the silicate class are grouped according to their structural type. The silicate anions may exist as isolated tetrahedra, paired tetrahera, single or double chains, rings, sheets, or three-dimensional boxlike structures. Groups of minerals containing such structures are given the respective terms nesosilicates, sorosilicates, inosilicates, cyclosilicates, phyllosilicates and tectosilicates.
      Minerals of the silicate class are responsible for providing the bulk material out of which the earth's crust and mantle are formed. (Silicates form 95% of the crust and 97% of the mantle). Silicate minerals are usually of relatively great hardness, and single crystals are often translucent.

Polymerization

      The bond between silicon and oxygen utilizes only one of oxygen's available bonding electrons. It is therefore possible that each oxygen atom within a silicate radical may bond with another silicon ion, becoming part of a second silicate radical and cementing the two disseparate ion groups together. One, two, three, or four of the oxygen atoms in each silicate radical may bind to different external silicon anion groups in this way. (Two or more oxygen atoms are never shared between the same adjacent silicate tetrahedra.) This bonding system opens up many structural possibilities; silicate radicals may remain structurally isolate, join together in pairs, or link into rings, chains, sheets, or frameworks. The process of linking series of anion groups togehter into chains, sheets, or ring structures through the sharing of oxygens is called polymerization.
      In general, the higher the temperature at which a mineral specimen formed the lower the amount of polymerization which will be present. The silicate minerals in igneous rock undergo a definite sequence of crystallization beginning with those rocks possessing the least amount of polymerization and ending with those which possess most.
      The element aluminum plays a prominent role in the construction of silicate minerals. Aluminum may, like silicon, join to four oxygen atoms; however, it may also bond with six oxygens. An aluminum atom joined to four oxygen ions (AlO4) occupies the same volume of space which is needed by a silicate (SiO4)4- radical; the aluminum anion group may therefore replace the silicate anion within chains or polymers. When joined to six oxygen atoms, however, aluminum can instead cement separate silicate tetrahedra together by forming ionic bonds. The ionic bonds thus formed are much weaker than the ionic-covalent bonds within the silicate and aluminate radicals. Aluminum is not the only element which may bond with oxygen and help to join silicate radicals together. The elements magnesium (Mg), iron (Fe), manganese (Mn), aluminum (Al), and titanium (Ti) may all occur joined to six oxygen atoms within the structure of the silicate minerals.

Nesosilicates

      The nesosilicates or island silicates contain isolated (SiO4)4- tetrahedra. In the minerals of this group there is no direct linkage between separate silicate anion groups; the silicate tetrahedra are held together only by ionic bonds with various metallic cations.
      Examples of the nesosilicates are provided by members of the olivine group, which contains forsterite (Mg2(SiO4)) and fayalite (Fe2(SiO4)); as well as members of the garnet group; zircon (Zr(SiO4)); and topaz (Al2(SiO4)(F,OH)2).
      The garnets provide an interesting example of an isostructural group. The minerals of this group possess the general chemical formula A3B2(SiO4)3. The A lattice site is occupied by large ions such as calcium (Ca), magnesium (Mg), iron (Fe), and manganese (Mn) which possess two bonding electrons. The B lattice site is dedicated to smaller ions possessing three bonding electrons such as aluminum (Al), iron (Fe), titanium (Ti), and chromium (Cr). The magnesium, iron, and manganese inhabitants of the A lattice site are completely interchangeable because they possess almost equal atomic radii. Calcium, however, possesses a differing atomic radius and does not substitute for these three elements. Substitution between aluminum, iron and chromium occurs within the B lattice site.
      The pyralspite garnets all have aluminum in the B lattice position; magnesium, iron and manganese may substitute for one another in the A lattice site to produce pyrope (Mg3Al2(SiO4)3), almandite (Fe3Al2(SiO4)3), and spessartite (Mn3Al2(SiO4)3). The ugrandite garnets all contain calcium in the A lattice position; aluminum, iron, and chromium may substitute for one another in the B lattice position. The ugrandite subgroup thus contains grossularite (Ca3Al2(SiO4)3), andradite (Ca3Fe2(SiO4)3), and uvarovite (Ca3Cr2(SiO4)3). The six garnet species are isostructural, possessing lattice configurations which are completely analogous and differ only in the identity of their constituent atoms.

Sorosilicates

      In the sorosilicates or couplet silicates two silicate tetrahedra are linked into a pair by sharing a single oxygen ion. The most prevalent sorosilicates are members of the epidote group and are characterized by the mineral epidote, Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH).

Cyclosilicates

      The cyclosilicates or ring silicates are composed of (SiO4)4- tetrahedra linked into closed cyclic rings by the sharing of oxygen atoms. These rings possess a silicon to oxygen ratio of 1:3. Three possible ring configurations exist; these are the Si3O9 ring, observed only in the rare titanosilicate benitoite (BaTiSi3O9); the Si4O12 ring; and the Si6O18 ring, which is found in beryl and tourmaline. The Si6O18 rings are hexagonal and arranged in planar sheets. In beryl these parallel sheets are so strongly bonded together by beryllium and aluminum cations that they demonstrate only poor cleavage.
      Two example members of the cyclosilicate group are beryl (Be3Al2(Si6O18)), of which varieties are known as aquamarine, morganite, and emerald, and tourmaline (Na(Mg,Fe)3Al6(BO3)3(Si6O18)(OH,F)4).

Inosilicates

      The silicate anion groups of the inosilicates or chain silicates are linked together into single or double chains. In order to form single chains each silicate radical must share two out of its four oxygen atoms with neighboring radicals. In order to form double chains, half of the silicate radicals share three oxygens rather than two,while the other half continue to share two; the extra bonds affix two chains together into a double chain. Sheaves of parallel single or double chains of silicate tetrahedra are then bound together by metallic cations such as calcium (Ca), magnesium (Mg), or iron (Fe).
      Those inosilicate species which are composed of single chains are classified as the pyroxenes, while those which are constructed of double chains are termed the amphiboles. Both groups may contain the same set of cations, which means that many pyroxenes are analogous to a species of the amphibole group and vice versa. For example, the pyroxene hypersthene ((Mg,Fe)2(Si2O6)), which contains magnesium and iron in solid solution, is analogous to the amphibole anthophyllite ((Mg,Fe)7(Si8O22)(OH)2), which contains the same cations. The difference between the two species lies in the ratio of silicon to oxygen, which is a legacy of the respective single or double chain structures, and in the presence of the hydroxyl radical (OH-) characteristic of amphiboles.
      Example pyroxene species include the minerals enstatite (Mg2(Si2O6)), diopside (CaMg(Si2O6)), spodumene (LiAl(Si2O6)), and jadeite (NaAl(Si2O6)). The amphiboles include tremolite (Ca2Mg5(Si8O22)(OH)2), and glaucophane (Na2Mg3Al2(Si8O22)(OH)2). Wollastonite (Ca(SiO3)) and rhodonite (Mn(SiO3)) are also inosilicates. However, these two minerals share the structure of neither pyroxenes nor amphiboles, but are instead classified as members of the pyrorenoid group. Rhodonite is rose-red in color due to the presence of manganese.

Phyllosilicates

      The phyllosilicates or layer silicates contain sheetlike planes of (SiO4)4- tetrahedra in which three of the four possible oxygen atoms are shared between different silicate tetrahedra. The term 'phyllosilicate' is derived from the Greek word phyllon, or 'leaf'; because of their sheeted structure the phyllosilicates tend to cleave into flakes or leaves. All members of the phyllosilicate group therefore have platy or flaky habit and display a single, prominent cleavage. Such minerals are soft and possess a low specific gravity.
      Example members of the phyllosilicate class are kaolinite (Al4(Si4O10)(OH)4), which is a major component of clay, serpentine (Mg6(Si4O10)(OH)8), talc (Mg3(Si4O10)(OH)2), and muscovite or white mica (KAl2(AlSi3O10)(OH)2).

Tectosilicates

      The tectosilicates or framework silicates are constructed in the form of a three-dimensional framework of (SiO4)4- tetrahedra. In this structure all four of the oxygen ions belonging to each silicate tetrahedra are shared with nearby tetrahedra, forging a stable and strongly bonded network. The ratio of silicon to oxygen in this type of structure is 1:2.
      The silica minerals are a subgroup of the tectosilicates. These include the nine different polymorphs of silicon dioxide, SiO2: stishovite, coesite, quartz, keatite, cristobalite, tridymite, lechatelierite, and opal. (Please refer below for a discussion of polymorphism in the silica minerals.)
      The feldspar group forms another subgroup of the tectosilicates. The feldspars are aluminum silicates which contain potassium (K), sodium (Na), or calcium (Ca) cations. Orthoclase (K(AlSi3O8)) and albite (Na(AlSi3O8)) offer two examples of feldspar minerals. Aluminum cations couple to four oxygen atoms to form a tetrahedral anion group, (AlO4)5-, which is structurally equivalent to that of the silicate radical (SiO4)4-. This aluminum anion group may then substitute for silicate radicals within the three-dimensional tectosilicate framework. In order to maintain electrical neutrality during the substitution of (AlO4)5- for (SiO4)4-, metal cations such as potassium, sodium, or calcium must be included.
      A third subgroup of the tectosilicates is composed of the zeolites, which are hydrous aluminum silicates. Natrolite (Na2(Al2Si3O10)•2H2O) and stilbite (Ca(Al2Si7O18)•7H2O) are two examples of this group.

Polymorphism in the Silicas

      There are nine different ways in which the silicate tetrahedra may be arranged in a continuous and electrically neutral structure in order to share all four oxygen ions. Therefore there exist nine different polymorphs of silicon dioxide, SiO2. Each of these polymorphs possesses exactly the same chemical formula (SiO2) as all the others, yet is built according to a different structural plan. The polymorphs adhere to different crystal systems and possess differing specific gravities. Which polymorph is stable in a particular environment is determined by the ambient temperature of the environment and the lattice energy of the polymorph. Those forms which are stable at higher temperatures possess more open, expanded lattices with greater lattice energies. The higher-temperature polymorphs of silicon dioxide therefore tend to display lower density and specific gravity.
      The three major polymorphs of silicon dioxide are quartz, which possesses the most dense lattice and the lowest symmetry; tridymite, which has a less dense lattice and higher symmetry; and cristobalite, which has the most open lattice and the highest symmetry. A quantity of crystalline silicon dioxide may be transformed from one of these structural types to another only when the silicon-oxygen bonds are broken. The transition between cristobalite, tridymite and quartz thus takes place only over long periods of time or at extremes of temperature change. However, quartz, tridymite, and cristobalite also possess high- and low- temperature structural variations which are differentiated by shorter or longer bond lengths between the silicon and oxygen ions. Transitions between these high and low-temperature variations are fast and completely reversible.


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