The Plagioclase, Forsterite, Diopside, Liquid Equilibrium in the System CaO-Na2O-MgO-Al2O3-SiO2

G. M. Biggar and D. J. Humphries
Department of Geology, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW

Abstract: Liquids in the system CaO-Na2O-MgO-Al2O3-SiO2 which crystallize plagioclase and forsterite, often with diopside and sometimes with spinel, were studied. The movement of the plagioclase-forsterite saturation surface to higher plagioclase contents as albite replaces anorthite, and the concurrent expansion of the forsterite and diopside primary fields, are documented. The maximum temperature of the liquid in equilibrium with plagioclase, diopside, and forsterite decreases as the anorthite to albite ratio decreases and as enstatite is added, and was experimentally determined over part of its range from 1278 to 113°C The decrease was not linear, being generally small even when 67 mole% albite was present, and thereafter, it must decrease rapidly to reach the values reported for the system albite-diopside-forsterite.

Mineralogical Magazine; September 1981 v. 44; no. 335; p. 309-314; DOI: 10.1180/minmag.1981.044.335.11
© 1981, The Mineralogical Society
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