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The crystal structure of calcite III

1997; American Geophysical Union; Volume: 24; Issue: 13 Linguagem: Inglês

10.1029/97gl01603

1944-8007

Joseph R. Smyth, Thomas J. Ahrens,

Crystallography and molecular interactions

The crystal structure of calcite III has been deduced from existing high pressure powder X‐ray diffraction patterns, based on the assumption that it is a displacive modification of the calcite I structure. The structure is monoclinic with space group C2 and a Z of 6. There are two Ca and two C positions, and five O positions, and atom coordinates have been refined by distance‐least‐squares methods to give reasonable octahedral coordination for Ca and parallel, planar CO 3 groups. Unit cell parameters refined from a published powder diffraction pattern at 4.1 GPa are: a = 8.746(8)Å; b = 4.685(5)Å; c = 8.275(8)Å; and β= 94.4°. The structure has a calculated density of 2.949 Mg/m³ at 4.1 GPa which is less than that of aragonite at this pressure and consistent with early piston cylinder studies. This implies that calcite III is indeed a metastable intermediary between calcite I and aragonite.