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Re-investigation of the crystal structure of whewellite [Ca(C2O4)·H2O] and the dehydration mechanism of caoxite [Ca(C2O4)·3H2O]

Published online by Cambridge University Press:  05 July 2018

T. Echigo ,
M. Kimata ,
A. Kyono ,
M. Shimizu  and
T. Hatta
T. Echigo*
Affiliation:
Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-8572, Japan
M. Kimata
Affiliation:
Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-8572, Japan
A. Kyono
Affiliation:
Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-8572, Japan
M. Shimizu
Affiliation:
Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-8572, Japan
T. Hatta
Affiliation:
Japan International Research Center for Agricultural Sciences, Independent Administrative Institute, Ohwashi 1-1, Tsukuba 305-8686, Japan
*
*E-mail: echigo@arsia.geo.tsukuba.ac.jp
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Abstract

The crystal structure of whewellite [Ca(C2O4)·H2O] and the dehydration mechanism of caoxite [Ca(C2O4)·3H2O] have been studied by means of differential thermal analysis, X-ray diffraction (powder and single-crystal) analysis and infrared analysis. The first and second analyses confirmed the direct transformation of caoxite into whewellite without an intermediate weddellite [Ca(C2O4)·2H2O] stage. Infrared spectra obtained from caoxite, weddellite and whewellite emphasize the similarity of the O–H-stretching band and O–C–O-stretching band in whewellite and caoxite and the unique bands of weddellite. The structure refinement at low temperature (123 K) reveals that all the hydrogen atoms of whewellite form hydrogen bonds and the two water molecules prop up the crystal structure by the hydrogen bonds that cause a strong anisotropy of the displacement parameter.

Comparing the structural features of whewellite with those of weddellite and caoxite suggests that caoxite and whewellite have a sheet structure consisting of Ca2+ ions and oxalate ions although weddellite does not. It is additionally confirmed that the sheets of caoxite are corrugated by hydrogen bonds but whewellite has flat sheets. The corrugated sheets of caoxite would be flattened by dehydration so the direct transformation of caoxite into whewellite would not occur via weddellite. Essential for this transformation is the dehydration of interlayered water molecules in caoxite leading to the building of the crystal structure of whewellite on its intralayered water molecules. The difference in conformation of water molecules between those two crystal structures may explain the more common occurrence of whewellite than of caoxite in nature.

Keywords

whewellitecaoxite, weddellitedehydration mechanismcrystal structureX-ray diffractioninfrared analysisdifferential thermal analysis
Type
Research Article
Information
Mineralogical Magazine , Volume 69 , Issue 1 , February 2005 , pp. 77 - 88
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2005

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