Low-Potash Gismondine from Ireland and Iceland

G. P. L. Walker and I. S. E. Carmichael
Imperial College, London S.W. 7.
With chemical analyses by I. S. E. Carmichael, M. A. 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.

Mineralogical Magazine; September 1962 v. 33; no. 258; p. 187-201; DOI: 10.1180/minmag.1962.033.258.02
© 1962, The Mineralogical Society
Mineralogical Society (www.minersoc.org)