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Vlasovite: a second occurrence and a triclinic to monoclinic inversion

Published online by Cambridge University Press:  14 March 2018

S. G. Fleet  and
J. R. Cann
S. G. Fleet
Affiliation:
Department of Mineralogy and Petrology, Downing Place, Cambridge
J. R. Cann
Affiliation:
Department of Mineralogy and Petrology, Downing Place, Cambridge
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Summary

Vlasovite, Na2ZrSi4O11, previously reported as monoclinic by Pyatenko and Voronkov from an occurrence in the Kola Peninsula, Russia, has been found in rocks from Ascension Island in a triclinic twinned modification. The obliquity is very slight (α and γ, differ from 90° by less than 1°), and X-ray photographs show arrays of paired spots corresponding to diffraction by different parts of the crystal related by the twinning operation, reflexion across (010).

On heating, this vlasovite is found to transform to the monoclinic variety at 29° C. The phase change has been followed on a single-crystal X-ray heating camera, and the obliquity is found to have successively smaller values as the crystal is heated from temperatures below room temperature to 29° C. The obliquity is characteristic of the particular temperature over the transformation range and is not affected by thermal history.

The transformation appears to be a displacive one, similar in some respects to α → β tridymite and α → β cristobalite, and some information has been obtained by X-ray structural investigations about the nature of the atomic movements involved.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 36 , Issue 278 , June 1967 , pp. 233 - 241
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1967

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