Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XX, Nos.2-3


Phase Relations in the Chromium-Poor Part of the System Diopside-Anorthite-Forsterite-MgCr2O4 in Air at 1 atm

Onuma, Kosuke

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The effect of chromium on the silicate system was studied at 1 atm by adding MgCr2O4 to the join diopside-anorthite-forsterite which is a thermal divide in the system CaO-MgO-Al2O3-SiO2. When a small amount of MgCr2O4 (0.2 - 0.5 wt%) is added to the join, the liquidus surface for spinel expands at the expense of anorthite. Thus, the assemblage diopside + forsterite + anorthite + liquid at a ternary invariant point becomes unstable and the assemblages of diopside + anorthite + spinel + liquid and diopside + forsterite + spinel + liquids appear on the liquidus surface. The points at which these assemblages are present are not "invariant" point but "piercing" point, indicating that the join diopside-anorthite-forsterite with a small amount of chromium is not thermal divide, and that the residual liquid in the crystallization can move through the join diopside-anorthite-forsterite.