HIGH-PRESSURE DEFORMATION OF CALCITE MARBLE AND ITS TRANSFORMATION TOARAGONITE UNDER NONHYDROSTATIC CONDITIONS

We conducted deformation experiments on Carrara marble in the aragonit e and calcite stability fields to observe the synkinematic transformat ion of calcite to aragonite, and to identify any relationships between transformation and deformation or sample strength. Deformation-induce d microstructures in calcite crystals varied most significantly with t emperature, ranging from limited slip and twinning at 400-degrees-C, l imited recrystallization at 500-degrees-C, widespread recrystallizatio n at 600 and 700-degrees-C, to grain growth at 800-900-degrees-C. Vari ations in confining pressure from 0.3 to 2.0 GPa have no apparent effe ct on calcite deformation microstructures. Aragonite grew in 10(-6)-10 (-7) s-1 strain rate tests conducted for 18-524 h at confining pressur es of 1.7-2.0 GPa and temperatures of 500-600-degrees-C. As in our pre viously reported hydrostatic experiments on this same transformation, the aragonite nucleated on calcite grain boundaries. The extent of tra nsformation varied from a few percent conversion near pistons at 400-d egrees-C, 2.0 GPa and 10(-4) s-1 strain rate in a 0.8 h long experimen t, to 98% transformation in a 21-day test at a strain rate of 10(-7) s -1, a temperature of 600-degrees-C and a pressure of 2.0 GPa. At 500-d egrees-C, porphyroblastic 100-200 mum aragonite crystals grew at a rat e faster than 8 x 10(-10)m s-1. At 600-degrees-C, the growth of aragon ite neoblasts was slower, approximately 6 x 10(-11)m s-1, and formed ' glove-and-finger' cellular-precipitation-like textures identical to th ose observed in hydrostatic experiments. The transformation to aragoni te is not accompanied by a shear instability or anisotropic aragonite growth, consistent with its relatively small volume change and latent heat in comparison with compounds that do display those features.