The Classification of Minerals

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.

Native elements

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.

Metals

Gold group: These include the elements gold (Au), silver (Ag), copper (Cu), and lead (Pb).

Iron group: Contains iron (Fe) and nickel (Ni). 

Platinum group: Platinum (Pt), palladium (Pd), iridium (Ir), and osmium (Os)

2-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). 

3-Nonmetals 

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

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.

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Oxide

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.

Halide

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.

Carbonate

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,(CO₃)) effervesces when placed in an aqueous solution of HCl, producing carbon dioxide and calcium chloride:

CaCO_{3 (s)} + 2HCl _(aq) -------> CaCl_{2 (s)} + CO_{2 (g)} + H₂O _(l)

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

Nitrates

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.

Chromate

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.

Phosphate

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.

Silicate

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.