sulfate mineral, any naturally occurring salt of sulfuric acid. About 200 distinct kinds of sulfates are recorded in mineralogical literature, but most of them are of rare and local occurrence. Abundant deposits of sulfate minerals, such as barite and celestite, are exploited for the preparation of metal salts. Many beds of sulfate minerals are mined for fertilizer and salt preparations, and beds of pure gypsum are mined for the preparation of plaster of paris.
| name | colour | lustre | Mohs hardness | specific gravity |
|---|---|---|---|---|
| alum | colourless; white | vitreous | 2–2½ | 1.8 |
| alunite | white; grayish, yellowish, reddish, reddish brown | vitreous | 3½–4 | 2.6–2.9 |
| alunogen | white; yellowish or reddish | vitreous to silky | 1½–2 | 1.8 |
| anglesite | colourless to white; often tinted gray, yellow, green, or blue | adamantine to resinous or vitreous | 2½–3 | 6.4 |
| anhydrite | colourless to bluish or violet | vitreous to pearly | 3½ | 3.0 |
| antlerite | emerald to blackish green; light green | vitreous | 3½ | 3.9 |
| barite | colourless to white; also variable | vitreous to resinous | 3–3½ | 4.5 |
| botryogen | light to dark orange red | vitreous | 2–2½ | 2.1 |
| brochantite | emerald to blackish green; light green | vitreous | 3½–4 | 4.0 |
| caledonite | deep verdigris green or bluish green | resinous | 2½–3 | 5.8 |
| celestite | pale blue; white, reddish, greenish, brownish | vitreous | 3–3½ | 4.0 |
| chalcanthite | various shades of blue | vitreous | 2½ | 2.3 |
| coquimbite | pale violet to deep purple | vitreous | 2½ | 2.1 |
| epsomite | colourless; aggregates are white | vitreous; silky to earthy (fibrous) | 2–2½ | 1.7 |
| glauberite | gray; yellowish | vitreous to slightly waxy | 2½–3 | 2.75–2.85 |
| gypsum | colourless; white, gray, brownish, yellowish (massive) | subvitreous | 2 (a hardness standard) | 2.3 |
| halotrichite | colourless to white | vitreous | 1.5 | 1.7 (pick) to 1.9 (halo) |
| jarosite | ochre yellow to dark brown | subadamantine to vitreous; resinous on fracture | 2½–3½ | 2.9–3.3 |
| kainite | colourless; gray, blue, violet, yellowish, reddish | vitreous | 2½–3 | 2.2 |
| kieserite | colourless; grayish white, yellowish | vitreous | 3.5 | 2.6 |
| linarite | deep azure blue | vitreous to subadamantine | 2.5 | 5.3 |
| mirabilite | colourless to white | vitreous | 1½–2 | 1.5 |
| plumbojarosite | golden brown to dark brown | dull to glistening or silky | soft | 3.7 |
| polyhalite | colourless; white or gray; often salmon pink from included iron oxide | vitreous to resinous | 3.5 | 2.8 |
| thenardite | colourless; reddish, grayish, yellowish, or yellow brown | vitreous to resinous | 2½–3 | 2.7 |
| name | habit | fracture or cleavage | refractive indices | crystal system |
| alum | columnar or granular massive | conchoidal fracture | n = 1.453–1.466 | isometric |
| alunite | granular to dense massive | conchoidal fracture |
omega = 1.572 epsilon = 1.592 |
hexagonal |
| alunogen | fibrous masses and crusts | one perfect cleavage |
alpha = 1.459–1.475 beta = 1.461–1.478 gamma = 1.884–1.931 |
triclinic |
| anglesite | granular to compact massive; tabular or prismatic crystals | one good, one distinct cleavage |
alpha = 1.868–1.913 beta = 1.873–1.918 gamma = 1.884–1.931 |
orthorhombic |
| anhydrite | granular or fibrous massive; concretionary (tripestone) | two perfect, one good cleavage |
alpha = 1.567–1.580 beta = 1.572–1.586 gamma = 1.610–1.625 |
orthorhombic |
| antlerite | thick tabular crystals | one perfect cleavage |
alpha = 1.726 beta = 1.738 gamma = 1.789 |
orthorhombic |
| barite | usually in tabular crystals; rosettes (desert roses); massive | one perfect, one good cleavage |
alpha = 1.633–1.648 beta = 1.634–1.649 gamma = 1.645–1.661 |
orthorhombic |
| botryogen | reniform, botryoidal, or globular aggregates | one perfect, one good cleavage |
alpha = 1.523 beta = 1.530 gamma = 1.582 |
monoclinic |
| brochantite | prismatic to hairlike crystal and crystal aggregates; granular massive; crusts | one perfect cleavage |
alpha = 1.728 beta = 1.771 gamma = 1.800 |
monoclinic |
| caledonite | coating of small elongated crystals | one perfect cleavage |
alpha = 1.815–1.821 beta = 1.863–1.869 gamma = 1.906–1.912 |
orthorhombic |
| celestite | tabular crystals; fibrous massive | one perfect, one good cleavage |
alpha = 1.618–1.632 beta = 1.620–1.634 gamma = 1.627–1.642 |
orthorhombic |
| chalcanthite | short prismatic crystals; granular masses; stalactites and reniform masses | conchoidal fracture |
alpha = 1.514 beta = 1.537 gamma = 1.543 |
triclinic |
| coquimbite | prismatic and pyramidal crystals; granular massive |
omega = 1.536 epsilon = 1.572 |
hexagonal | |
| epsomite | fibrous or hairlike crusts; woolly efflorescences | one perfect cleavage |
alpha = 1.430–1.440 beta = 1.452–1.462 gamma = 1.457–1.469 |
orthorhombic |
| glauberite | tabular, dipyramidal, or prismatic crystals | one perfect cleavage |
alpha = 1.515 beta = 1.535 gamma = 1.536 |
monoclinic |
| gypsum | elongated tabular crystals (some 5 ft long; others twisted or bent); granular or fibrous masses; rosettes | one perfect cleavage |
alpha = 1.515–1.523 beta = 1.516–1.526 gamma = 1.524–1.532 |
monoclinic |
| halotrichite | aggregates of hairlike crystals | conchoidal fracture |
alpha = 1.475–1.480 beta = 1.480–1.486 gamma = 1.483–1.490 |
monoclinic |
| jarosite | minute crystals; crusts; granular or fibrous massive | one distinct cleavage |
omega = 1.82 epsilon = 1.715 |
hexagonal |
| kainite | granular massive; crystalline coatings | one perfect cleavage |
alpha = 1.494 beta = 1.505 gamma = 1.516 |
monoclinic |
| kieserite | granular massive, intergrown with other salts | two perfect cleavages |
alpha = 1.520 beta = 1.533 gamma = 1.584 |
monoclinic |
| linarite | elongated tabular crystals, either singly or in groups | one perfect cleavage; conchoidal fracture |
alpha = 1.809 beta = 1.839 gamma = 1.859 |
monoclinic |
| mirabilite | short prisms; lathlike or tabular crystals; crusts or fibrous masses; granular massive | one perfect cleavage |
alpha = 1.391–1.397 beta = 1.393–1.410 gamma = 1.395–1.411 |
monoclinic |
| plumbojarosite | crusts, lumps, compact masses of microscopic hexagonal plates | one fair cleavage |
omega = 1.875 epsilon = 1.786 |
hexagonal |
| polyhalite | fibrous to foliated massive | one perfect cleavage |
alpha = 1.547 beta = 1.560 gamma = 1.567 |
triclinic |
| thenardite | rather large crystals; crusts, efflorescences | one perfect, one fair cleavage |
alpha = 1.464–1.471 beta = 1.473–1.477 gamma = 1.481–1.485 |
orthorhombic |
All sulfates possess an atomic structure based on discrete insular sulfate (SO42-) tetrahedra, i.e., ions in which four oxygen atoms are symmetrically distributed at the corners of a tetrahedron with the sulfur atom in the centre. These tetrahedral groups do not polymerize, and the sulfate group behaves as a single negatively charged molecule, or complex. Thus, sulfates are distinct from the silicates and borates, which link together into chains, rings, sheets, or frameworks.
Sulfate minerals can be found in at least four kinds: as late oxidation products of preexisting sulfide ores, as evaporite deposits, in circulatory solutions, and in deposits formed by hot water or volcanic gases. Many sulfate minerals occur as basic hydrates of iron, cobalt, nickel, zinc, and copper at or near the source of preexisting primary sulfides. The sulfide minerals, through exposure to weathering and circulating water, have undergone oxidation in which the sulfide ion is converted to sulfate and the metal ion also is changed to some higher valence state. Noteworthy beds of such oxidation products occur in desert regions, such as Chuquicamata, Chile, where brightly coloured basic copper and ferric iron sulfates have accumulated. The sulfate anions generated by oxidation processes may also react with calcium carbonate rocks to form gypsum, CaSO4·2H2O. Sulfates formed by the oxidation of primary sulfides include antlerite [Cu3(SO4)(OH)4], brochantite [Cu4(SO4)(OH)6], chalcanthite [Cu2+(SO4)·5Η2Ο], anglesite (PbSO4), and plumbojarosite [PbFe3+6(SO4)4(OH)12].
Soluble alkali and alkaline-earth sulfates crystallize upon evaporation of sulfate-rich brines and trapped oceanic salt solutions. Such brines can form economically important deposits of sulfate, halide, and borate minerals in thick parallel beds, as the potash deposits at Stassfurt, Ger., and the southwestern United States. Many of the sulfate minerals are salts of more than one metal, such as polyhalite, which is a combination of potassium, calcium, and magnesium sulfates.
Sulfate minerals common in evaporite deposits include anhydrite, gypsum, thenardite (Na2SO4), epsomite (MgSO4·7H2O), glauberite [Na2Ca(SO4)2], kainite (MgSO4·KCl·3H2O), kieserite (MgSO4·H2O), mirabilite (Na2SO4·10H2O), and polyhalite [K2Ca2Mg(SO4)4·2H2O].
Groundwater carrying sulfate anions reacts with calcium ions in muds, clays, and limestones to form beds of gypsum. The massive material is called alabaster or plaster of paris (originally found in the clays and muds of the Paris basin). If such beds become deeply buried or metamorphosed (altered by heat and pressure), anhydrite may form by dehydration of the gypsum.
Numerous sulfates, usually simple, are formed directly from hot aqueous solutions associated with fumarolic (volcanic gas) vents and late-stage fissure systems in ore deposits. Noteworthy examples include anhydrite, barite, and celestine.