sulphate minerals

sulphate minerals Sulphate minerals are formed in a variety of environments, mainly at low temperatures. Of the more than one hundred species, many are rare and only one, gypsum, is of worldwide abundant occurrence; anhydrite is abundant at depth and barite (baryte) is fairly ubiquitous but generally occurs only in small amounts.

The basic structural unit is the sulphate group (SO₄)²⁻ linked by metal ions or by water molecules. In one anhydrous orthorhombic group, barite (BaSO₄), celestite (SrSO₄), and anglesite (PbSO₄) have the same structure, but anhydrite (CaSO₄), because of the smaller calcium ion, has a different structure. Some sulphate minerals, for example jarosite, contain the hydroxyl (OH) ion; others, such as the rare epsomite and gypsum (CaSO₄·2H₂O), contain the water molecule.

Gypsum and anhydrite are closely related; both are calcium minerals and both are formed early (after carbonate) in the sequence of minerals when sea water is evaporated or in sabkha environments, as in the present-day Persian Gulf. Anhydrite is not precipitated below 42 °C unless the salinity of the water is high. Gypsum is generally formed at the surface and is stable down to about a kilometre or so, where it inverts to anhydrite. On uplift, or with the ingress of water, anhydrite turns into gypsum. In this process there is a 61 per cent increase in volume, which often results in intense deformation of the evaporite bed. Under certain conditions the growth of gypsum, resulting from the oxidation of pyrite, the formation of sulphuric acid, and its reaction with the carbonate of shelly material in clays and shales, can cause the heaving of floors in buildings.

Gypsum of the variety known as selenite is monoclinic; its hardness is 2 on Mohs scale and it has one perfect cleavage yielding flexible but not elastic laminae; other, rarer cleavages may result in rhombohedral masses. Twin crystals are common; they show swallow-tail or arrow-head forms and more rarely cruciform growths, and may grow to 3–4 m long. Gypsum grows in four different habits: selenite as just described; alabaster, variously coloured in nodular granular masses; satin spar, which is usually white and finely fibrous; and desert rose, like selenite in form but enclosing many quartz grains. Gypsum has many commercial uses: as a retarder in Portland cement, as a filler in paper, textiles, and paints, in the manufacture of sulphuric acid and fertilizers, and in the production of plaster of Paris.

Barite is primarily a gangue mineral in hydrothermal veins but is also widespread in nodules, veins, and lenses in limestones, sometimes growing with a cockscomb habit. Most commonly white or colourless, its density makes it an important constituent of drilling muds; it is also used as a filler in paints, in the sugar industry, and as the primary source of barium.

Celestite (celestine) has similar properties to barite, but usually has a characteristic pale blue colour. It occurs as an evaporite mineral associated with gypsum and halite, in veins of hydrothermal origin, or in veins and cavities in limestones; the strontium derives from the alteration of aragonite to calcite. It is used for its red colour in pyrotechnics, in paints, and in the refining of sugar beet.

Anglesite is fairly rare; it commonly forms white granular skins around silvery-grey galena as a result of the alteration of galena in the zone of oxidation.

R. Bradshaw