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Low-potash gismondine from Ireland and Iceland

Published online by Cambridge University Press:  14 March 2018

G. P. L. Walker*
Affiliation:
Imperial College, London S.W. 7

Summary

Gismondine is recorded from the Tertiary basalts of the North Atlantic region, from a total of some 40 localities in Antrim and Iceland. It occupies a well-marked position in the sequence of zeolite zones in the lavas, and the characteristic associates are chabazite, thomsonite, and phillipsite. Five new chemical analyses of gismondine from these localities show a negligible content of potash, and indicate plagioclase-type substitution, giving a series ranging from near Ca8Al16Si16O64.34H2O to near Na2·5Ca5·5Al13·5Si18·5O64.34H2O. Substitution of NaSi for CaAl leads to a decrease in refringence, birefringence, and density.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1962

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