Abstract
Synthetic boron-free kornerupine obtained at 7 kbar water pressure and 830°C has the formula (Mg 3.72 Al 5.21 ) [6] (Al 1.28 Si 3.72 ) [4] O 19.76 (OH) 2.24 . It thus lies exactly on the join enstatite-corundum, midway between the M:A:S ratios 4:3:4 and 1:1:1. By extending the work of Seifert (1975) towards higher water pressures, the upper pressure stability limit of this phase was determined to lie between about 11.5 and 13 kbar for T = 800-900°C. The high-pressure breakdown assemblage is enstatite + corundum, and the reaction curve has a flat positive slope. Thus dehydration of B-free kornerupine at this pressure proceeds toward lower temperatures. The lower temperature stability limit in the range 7-11 kbar is defined by breakdown to the more hydrated assemblage chlorite + talc + corundum between 735°C and 750°C, the reaction curve possessing a steep, probably positive slope
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