572 MINERALS/Sulphates Sulphates G Cressey, The Natural History Museum, London, UK Copyright 2005, Natural History Museum All Rights Reserved Introduction Gypsum (CaSO4 Á 2H2O), the most common of the sulphate minerals, is also known as alabaster (a finegrained massive form), satin spar (a fibrous variety of gypsum), or selenite (colourless transparent gypsum crystals) Gypsum is often found in considerable thicknesses within evaporite sequences and in association with limestones and shales Large deposits of alabaster gypsum are commonly observed at the sole of many large-scale tectonic dislocations and thrust faults Gypsum is of great importance as a raw material in the production of cement and plaster for the building industry Crystal Structure of Gypsum In the crystal structure of gypsum, calcium ions are coordinated by six oxygens from sulphate (SO4) tetrahedral groups, and by two oxygens from water molecules (H2O) Two layers of SO4 tetrahedra are Figure The structure of gypsum (view looking almost down the z axis): SO4 tetrahedral units (green) and calcium ions (red) form double layer sheets perpendicular to the y axis At each side of these sheets, water molecules (with oxygen in blue and hydrogen shown as small black dots) form weak hydrogen bonds to the next layer in the structure bound together by calcium ions forming a doublelayer sheet Water molecules are located at each side of these double-layer sheets The water molecules are oriented such that one of the two hydrogens of each water molecule points almost perpendicular to the sheets; this facilitates weak hydrogen bonding to the next layer in the structure These hydrogen bonds point approximately along the y-axis of the crystal, forming a layer of weak hydrogen bonding perpendicular to the y-axis, and explain the easy (010) cleavage of gypsum The proton (Hỵ) positions in gypsum have been determined in deuterated gypsum by neutron diffraction, as weakly scattering proton positions cannot be determined accurately by X-ray diffraction methods (Figure 1) Physical Properties of Gypsum The weak hydrogen bonding in the structure of gypsum, occurring in layers perpendicular to the y-axis, accounts for the perfect (010) cleavage and is also responsible for the anisotropic thermal expansion behaviour of gypsum The b dimension of the unit cell increases the most with temperature, and this is principally because the hydrogen bonds oriented across the sheets easily lengthen with increasing temperature The static compressibility of gypsum is also highly anisotropic and, as is the case with the thermal expansion, is related to the hydrogen-bonding orientation across the sheet structure; these hydrogen bonds more easily shorten with pressure and therefore the b lattice parameter has the greatest compressibility Upon heating to about 65 C, gypsum dehydrates to form the hemihydrate phase, bassanite (CaSO4 Á 0.5H2O), with its z-axis parallel to that of the gypsum; this is known as plaster of Paris Further heating dehydrates the sulphate completely, forming g-CaSO4 (a polymorph of anhydrite, CaSO4) at about 95 C g-CaSO4 is also known as ‘soluble anhydrite’, as unlike anhydrite it can be re-hydrated Anhydrite has a different structure and cannot be easily hydrated Commercial plaster of Paris consists mainly of calcium sulphate hemihydrate (bassanite) and is made by heating gypsum to about 200 C Heating gypsum at higher temperatures produces anhydrite and this change is irreversible At the stage where bassanite and/or g-CaSO4 are produced, re-hydration to gypsum (CaSO4 Á 2H2O) easily occurs; this is the process of the setting of plaster of Paris to form an interlocking mass of gypsum crystals