Dehydration of diasporite to corundite in nature and experiment

Abstract

The diasporite-corundite rock transformation, which releases 6–8 wt% H 2 O in an average metabauxite, was studied experimentally. The results are compared with petrological observations on the island of Naxos (Greece), where the transformation occurred in metakarst bauxites during prograde regional metamorphism. Dehydration experiments were started with fine-grained natural diasporite embedded in marble. The samples were first annealed in the diaspore stability field, then slowly brought to the final pressure-temperature ( P - T ) conditions in the corundum field and kept there five to seven days. Overstepping the diaspore-corundum equilibrium by ∼30 °C at 8 kbar resulted in partial dehydration of diaspore. As with the corundum-in isograd on Naxos, the corundum grew preferentially along the bauxite-marble contact. Experiments at 17–40 kbar with T oversteps of 40–150 °C resulted in complete diaspore breakdown. A high-porosity zone containing corundum and silicates developed along the bauxite-marble boundary, resulting from the solid volume decreases associated with the diaspore-corundum and decarbonation reactions. In nature, the marble similarly behaved as a barrier for liberated fluid, as indicated by coarse corundum- chloritoid segregations along metabauxite rims. In the 30–40 kbar experiments, the porous contact zone acted as fluid pathway, allowing partial dissolution of metabauxite. This demonstrates pronounced Al, Fe, and Ti mobility at high P and T of 600–800 °C.