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Gypsum [CaSO4.2H2O]
Structure Monoclinic
Space Group : I12/C1
(No. 15)
a=5.6740 Å, b=15.1049 Å, c=6.4909 Å
a=90.0, b=118.513, g=90.0
Z=4
Atomic Positional Parameters
H 8f 0.2537 0.1666 0.5077
Ca 4e 0.5000 0.0800 0.2500
S 4e 0.0000 0.0767 0.7500
O 8f 0.9641 0.1328 0.5503
O 8f 0.7576 0.0219 0.6643
O 8f 0.3834 0.1824 0.4618
H 8f 0.4055 0.2448 0.4932
Reference
PF Schofield, KS Knight and IC Stretton (1996) Thermal expansion of gypsum investigated by neutron powder diffraction. American Mineralogist, 81, 847-851
Mineral Chemistry
Gypsum, the most common of the sulphate minerals, is also known as satin spar, alabaster, or selenite, which is a colourless and transparent variety of gypsum. Upon heating, gypsum dehydrates up to about 65oC to form a CaSO4.0.5 H2O phase. Further heating dehydrates the sulphate completely, giving a polymorph of anhydrite (CaSO4), g-CaSO4, at about 95oC. Gypsum, along with anhydrite, is a constituent of evaporites, sediments formed by precipitation of minerals through evaporation of either marine or terrestrial waters. Gypsum may also be formed by contact of sulphuric acid with calcium bearing rocks. Sulphuric acid may be derived from weathered or oxidised sulphide deposits, or from sulphurous emissions of volcanic origin.
In the gypsum crystal structure, calciums are coordinated by six oxygens from sulphate (SO4), and by two oxygens from water (H2O). Two sheets of sulphates are bound together by calciums forming double sheet layers. At each side of these layers are water molecules, which form weak hydrogen bonds to the next layer in the structure.
To identify proton (H+) positions, gypsum was deuterated before examination at a temperature of 4.2 K (-269oC) by neutron diffraction at the 'POLARIS' experimental station, Rutherford Appleton Laboratory, UK (Schofield et al. 1996).
The CHIME figure shows calcium ions as large grey spheres, and sulphurs as large yellow spheres. Oxygens are shown as small red spheres, and hydrogens as white sticks. Note the positions of the water (H2O) molecules in the crystal structure. The unit cell is outlined. Use the CHIME menu 'Display' to select other viewing options. Clicking on 'Reload' in the web browser menu restores the original configuration.