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An occurrence of stevensite and kerolite in the Devonian Crousa gabbro at Dean Quarry, The Lizard, Cornwall, England

Published online by Cambridge University Press:  09 July 2018

N. J. Elton ,
J. J. Hooper  and
V. A. D. Holyer
N. J. Elton
Affiliation:
Tregears, Little Treviscoe, St A ustell, Cornwall PL26 7QL
J. J. Hooper
Affiliation:
48 St Julitta, Luxulyan, Bodmin, Cornwall PL30 5ED
V. A. D. Holyer
Affiliation:
The Cornerstone, 7 Tregonning Park, St Keverne, Helston, Cornwall TR12 6QT, UK
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Abstract

Stevensite has been found at Dean Quarry, The Lizard, Cornwall, associated intimately with the disordered, hydrated variety of talc known as kerolite. Both minerals occur as pseudomorphs after pectolite and as microcrystalline anhedral masses filling cavities and veins in gabbro. Associated minerals include calcite and prehnite. The XRD data show that stevensite occurs as a physical mixture with kerolite, rather than as mixed-layer stevensite-kerolite. Kerolite is unaffected by glycerol treatment whilst stevensite readily expands: this test provides the only reliable distinction between the two minerals. Differences in IR and chemical data for stevensite and kerolite are subtle; data are broadly similar to those in the literature. Both minerals are readily decomposed to amorphous silica by hot concentrated HCl. Stevensite dehydroxylated at 400°C for 1.5 h is virtually indistinguishable from kerolite. The Dean Quarry occurrence is similar to many others worldwide. Stevensite and kerolite formed after prehnite, calcite, analcime, pectolite and natrolite during a phase of decomposition and alteration of these earlier minerals. ‘Stevensite’ from the first British occurrence in the Whin Sill, North Tyne, has been re-examined and appears to be kerolite or ‘hydrated-talc’.

Type
Research Article
Information
Clay Minerals , Volume 32 , Issue 2 , June 1997 , pp. 241 - 252
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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