Length scale dependence of high-pressure amorphization: the static amorphization of anorthite SIMON A. T. REDFERN Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK Abstract High-pressure amorphization of anorthite has been observed by energy-dispersive X-ray diffraction of powdered samples held under static pressure in a diamond anvil cell. The onset of amorphization is accompanied by a significant reduction in the intensity of Bragg reflections at pressures between 10 and 14 GPa, and anorthite becomes completely X-ray amorphous between 14 and 20 GPa. These pressures are significantly lower than those suggested by earlier birefringence studies. The discrepancy can be reconciled in terms of a model of high-pressure amorphization in which partially amorphized anorthite can be regarded as a spatially heterogeneous anti-glass, with long-range order maintained but translational disorder dominating at shorter correlation lengths. KEYWORDS:high-pressure amorphization, anorthite, anti-glass, metamictisation. Introduction coordinated arrangement upon increasing pressure. They inferred amorphization from the changes in DIAPLECTIC feldspar glasses have been recovered at birefringence of the recovered samples, which many meteorite impact sites, and shock-compression became isotropic after compression above ~25 has long been recognized as a route to the production GPa. The latest results on the amorphization of of highly disordered non-crystalline material leading, anorthite come from in situ Raman spectroscopic for example, to the amorphization of plagioclase (e.g. experiments (Daniel et al., 1995a,b), which suggest Arndt et al., 1982; Staffler and Hornemann, 1972). that vitrification proceeds above II GPa and is More recently, Williams and Jeanloz (1989) demon- complete by 17 GPa. Daniel et al. (1995a, b) found strated that room-temperature static compression of that samples pressurized above 20 GPa transformed anorthite in the diamond anvil cell could also induce to an amorphous glass state irreversibly, while the breakdown of crystallinity and formation of a samples that were pressurized to lower peak dense amorphous phase. Their experiments consisted pressures reverted to a disordered state with some of measurements of changes in infrared absorption of lattice symmetry on decompression. single crystals of anorthite conducted in situ at high- Such amorphization at modest pressures and pressure. Alongside this, they made observations of ambient (or low) temperature has now been observed changes in birefringence of the same samples, in a plethora of materials. Early X-ray and Raman conducted at ambient pressure after the samples spectroscopic work showed that a-quartz transforms had been recovered from the diamond cell, in which to a dense amorphous phase on increasing pressure, they had been subjected to known peak pressures. and recent molecular dynamics simulations suggest Anorthites were studied at pressures between 1 bar that the high-pressure amorphization of Si02 phases and 33 GPa. The authors noted that the infrared involves a change in coordination of Si from four to absorption associated with T-O-Tbends and stretches six (Zhang and Ong, 1993; Binggeli et al., 1994a,b) changed reversibly over this pressure range, and and a reduction in interstitial void space. These interpreted the changes as an increase in T-site studies suggii:Stthat the transition from crystalline coordination from four-fold to a more highly Si02 to a high-pressure glass is associated with an Mineralogical Magazine, June ]996, Vol. 60, pp. 493-498 ~ Copyright the Mineralogical Society