Publication Date:
2011-04-01
Description:
Qandilite (Mg2TiO4) and magnesioferrite occur in forsterite - spinel - calcite skarn ejecta from Mt. Vesuvius, Italy, with an exceptionally large compositional range that outlines the miscibility gap in the system spinel - qandilite - magnesioferrite (with a small amount of Fe2+), at present solely determined on the spinel-qandilite binary at T 〉 1000{degrees}C. The analyzed spinel- qandilite and spinel-magnesioferrite pairs are consistent with the solvus and tie-lines (except for a temperature offset) derived from the thermochemical model of spinel solid-solutions. Temperatures of formation in the range 650-700{degrees}C are inferred from the petrological study of the skarn-forming processes involved, which typically include two types of metasomatic reactions, i.e., formation of spinel - forsterite - calcite endoskarns by desilication of aluminosilicate bodies at the contact of dolostone wallrocks, and reaction of such pre-existing endoskarns with new influxes of magma {+/-} fluid. Calculated phase-relations among qandilite, perovskite and geikielite show that qandilite with moderate magnesioferrite contents is the high-temperature Ti mineral stable in magnesian and extremely silica-deficient surroundings under oxidized conditions.
Print ISSN:
0008-4476
Topics:
Geosciences
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