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Aegirine of possible authigenic origin in Middle Devonian sediments in Caithness, Scotland

Published online by Cambridge University Press:  05 July 2018

N. J. Fortey  and
U. McL. Michie
N. J. Fortey
Affiliation:
Institute of Geological Sciences, 64/78 Gray's Inn Road, London WCIX 8NG
U. McL. Michie
Affiliation:
Institute of Geological Sciences, I54 Clerkenwell Road, London ECIR 5DU
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Summary

An occurrence of aegirine in Middle Devonian lacustrine beds in Caithness is described. The mineral occurs as euhedral to subhedral prismatic crystals within a finely laminated siltstone. Microprobe analyses reveal a remarkably ‘pure’ composition consisting almost entirely of Na, Fe3+, Si, and O. X-ray diffractometry yielded the unit cell data: a 9·657 Å, b 8·800 Å, c 5·296 Å, and β 107·37°. Comparison with other natural aegirine compositions shows marked differences from those of igneous or metamorphic origin but a close similarity to a unique occurrence of authigenic aegirine in the lacustrine Green River Formation of Wyoming, U.S.A. Consideration of the nature of the Caithness occurrence, and comparison of the Devonian Orcadian basin with the Eocene Green River basin indicate similar geological environments. It is suggested that the mineral formed in hypersaline conditions in which sodium may have been derived from contemporaneous alkaline volcanism within the Orcadian basin.

Type
Research Article
Information
Mineralogical Magazine , Volume 42 , Issue 324 , December 1978 , pp. 439 - 442
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1978

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