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A high-pressure polytypic transformation in type-I chlorite

¹ Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K.
² European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP220, F-38043 Grenoble, France

Correspondence: ^* E-mail: mdw{at}nhm.ac.uk

The compressional behavior of a natural chromian-clinochlore-Ia polytype has been studied by synchrotron powder X-ray diffraction to 8 GPa at 298 K under hydrostatic conditions. A reversible polytypic transformation occurs at 6 GPa from the Ia-4 to a Ib polytype in which all a-type interlayers are converted to b-type by sheet translation across the interlayer, thus removing the high degree of cation superposition between the brucite-like sheet and the adjacent 2:1 layer that is associated with a-type interlayers. The isothermal bulk modulus for the Ia polytype to 5.9 GPa was determined by fitting pressure-volume data to a second-order Birch-Murnaghan equation of state: K₀ = 78.7 ± 1.4 GPa. This value is close to that of clinochlore-IIb (K₀ = 81 GPa). Elastic moduli for a, b, and d₀₀₁ obtained from second-order fits to a Birch-Murnaghan equation of state are: K_a0 = 113(5) GPa, K_b0 = 91(2) GPa, and K_001,0 = 54(1) GPa. A possible relationship is discussed between the 6 GPa polytypic transformation observed in chromian-clinochlore and the non-polytypic transformation at 9 GPa in clinochlore-IIb reported recently. Both transformations are dominated by contraction of the interlayer, with negligible in-plane compression of the polyhedral sheets. Both may also involve an increase in oxygen close-packing across the interlayer. However, in chromian-clinochlore, the destabilization of a-type interlayers is a further significant structural factor that may cause the transformation to occur at a lower pressure. Both transformations also involve an increase in the compressibility of chlorite. Although the transformation in chromian-clinochlore is influenced by the vertical superpositions of cations across the a-type interlayer, the hydrogen bonding is robust and does not reorganize until a considerable pressure (6 GPa) is reached.

This article has been cited by other articles:

				D. P. Dobson, A. A. De Ronde, M. D. Welch, and P. G. Meredith The acoustic emissions signature of a pressure-induced polytypic transformation in chlorite American Mineralogist, February 1, 2007; 92(2-3): 437 - 440. [Abstract] [Full Text] [PDF]