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Mineralogical Magazine; February 2004; v. 68; no. 1; p. 135-146; DOI: 10.1180/0026461046810176
© 2004 Mineralogical Society of Great Britain and Ireland
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The structure and thermal expansion behaviour of ikaite, CaCO3.6H2O, from T = 114 to T = 293 K

A. R. Lennie*, C. C. Tang and S. P. Thompson

Daresbury Laboratory, Warrington, Cheshire WA4 4AD, UK

* E-mail: a.lennie{at}dl.ac.uk

The hydrous calcium carbonate mineral ikaite (CaCO3.6H2O) forms in nature at low temperature in carbonate- and Ca-rich waters. Ikaite crystallizes in the spacegroup C2/c, and consists of CaCO3.6H2O units with Ca ions coordinated by eight oxygens, six from H2O and two from the carbonate group. Hydrogen bonding links CaCO3.6H2O moieties to form the crystal structure.

We have used synchrotron X-ray powder diffraction at T = 243 K to refine the monoclinic structure of ikaite, and have measured unit-cell parameters of ikaite between T = 114 K and T = 293 K. Anisotropic thermal expansion in ikaite is evident, with the smallest relative increase occurring along the b direction parallel to 2-fold axes. The contribution of hydrogen bonding to thermal expansion is assessed by comparison of our data with previously published data for deuterated ikaite, ice and gypsum. Ikaite exhibits a coefficient of volume expansion intermediate between that of ice (Ih) and of deuterated gypsum (CaSO4.2D2O) between T = 114 K and T = 293 K.

KEYWORDS: ikaite, carbonate, crystal structure, thermal expansion, hydrogen bonding




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