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European Journal of Mineralogy; July, August 2006; v. 18; no. 4; p. 449-462; DOI: 10.1127/0935-1221/2006/0018-0449
© 2006 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Articles

The thermoelastic properties of MgSO4·7D2O (epsomite) from powder neutron diffraction and ab initio calculation

Andrew D. FORTES1,*, Ian G. WOOD1, Maria ALFREDSSON1, Lidunka VOCADLO1 and Kevin S. KNIGHT2,3

1 Research School of Geological and Geophysical Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom
2 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 OQX, United Kingdom
3 Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom

* E-mail: andrew.fortes{at}ucl.ac.uk

time-of-flight powder neutron diffraction has been used to measure the molar volume of MgSO4.7D2O (i) from 1.8 — 300 K at ambient pressure, (ii) from 50 — 290 K at 1.4, 3.0, and 4.5 kbar, (iii) from 0 — 5.5 kbar at 290 K, and (iv) from 0 — 4.5 kbar at 50 K. The data have allowed us to determine the temperature dependence of the incompressibility, ({partial}K/{partial}T)p, (thermodynamically equivalent to the pressure dependence of the thermal expansion, ({partial}{alpha}/{partial}P)T) of epsomite throughout its stability field. We observed that the a-axis exhibits negative thermal expansion, {alpha}a, from 30 — 250 K at room pressure, turning positive above 250 K and being zero below 30 K. However, each of the crystallographic axes exhibits a sharp change in ({partial}{alpha}/{partial}T) at ~125 K, and this appears to correspond to significant changes in the axial incompressibilities with the a- and c-axes softening, and the b-axis stiffening considerably below ~125 K.

Our thermoelastic results are in agreement with ab initio calculations at zero Kelvin; however the calculations offer no obvious insight into the mechanism responsible for the change in behaviour at low temperature.

Key-words: epsomite, neutron diffraction, high-pressure, incompressibility, thermal expansion.






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