1. Introduction to Berzelian Classification
Top
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.
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.
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.
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.
The native nonmetals include carbon (C), in the form of diamond and graphite, and sulphur (S).
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
Top
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.
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).
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.
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 |
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.)
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 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.)
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).
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:
This reaction provides a means for the identification of carbonate species which is easily applicable in the field.
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.)
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.
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.
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.
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.
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 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.
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.
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
Top
Photos
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
Top
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.
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.
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.
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.
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).
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).
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.
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).
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.
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.