Aegirine of Possible Authigenic Origin in Middle Devonian Sediments in Caithness, Scotland

N. J. Fortey and U. McL. Michie
Institute of Geological Sciences, 64/78 Gray's Inn Road, London WC1X 8NG
Institute of Geological Sciences, 154 Clerkenwell Road, London EC1R 5DU

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.

Mineralogical Magazine; December 1978 v. 42; no. 324; p. 439-442; DOI: 10.1180/minmag.1978.042.324.05
© 1978, The Mineralogical Society
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