Boron-free kornerupine; its upper pressure stability limit in the system MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O (MASH)

Synthetic B-free kornerupine obtained at 830 degrees C and 7 kbar P (sub H2O) has the formula (Mg (sub 3.72) Al (sub 5.21) ) (super [6]) (Al (sub 1.28) Si (sub 3.72) ) (super [4]) O (sub 19.76) (OH) (sub 2.24) . It thus lies on the join enstatite-corundum, midway between the M:A:S ratios 4:3:4 and 1:1:1. The upper P limit of this phase lies between approx 11.5 and 13 kbar for T 800-900 degrees C. The high-P breakdown assemblage is enstatite + corundum, and the reaction curve has a flat positive slope. Thus dehydration of B- free kornerupine at this P proceeds towards lower T. The lower T stability limit in the range 7-11 kbar is defined by breakdown to the more hydrated assemblage chlorite + talc + corundum between 735 and 750 degrees C, the reaction curve having a steep, probably positive slope.