gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1965.035.269.08 Alteration Products of Olivine and Pyroxene in Basalt Lavas from the Isle of Mull1 Fawcett J. J. B.Sc., Ph.D. Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C.2

The work described in this paper formed part of a Ph.D. thesis submitted to the University of Manchester in 1961.

Present address: Dept. of Geological Sciences, University of Toronto, Toronto 5, Canada.

 3 1965 35 269 55 68 18 2 1964 Copyright © 1965, The Mineralogical Society 1965 Summary

Flood basalts of the Isle of Mull were sampled in areas outside the metamorphic aureole of the central intrusions. Alteration products of olivine and pyroxene in these rocks are talc, chlorite, serpentine, iron oxides, brucite, calcite, and ‘iddingsite’. Plagioclase feldspar in the same rocks is unaltered. ‘Iddingsite’ is less common than mixtures of chlorite and serpentine.

In two flows the only alteration product near the bottom is ‘iddingsite’ but at higher levels in the same flow green ehloritic material replaces olivine. This variation is accompanied by an increase in the FeO/Fe2O3 ratio in the rock towards the flow top. Green alteration products may be converted to material optically identicaI with ‘iddingsite’ by heating in air at 600° C. Zoned patches of chloritic material have formed by crystallization of the late stage magmatic fluid, modified in composition by reactions with olivine and pyroxene.