EXPERIMENTAL DYNAMIC METAMORPHISM OF MINERAL SINGLE-CRYSTALS

This paper is a rcview of some of the rich and varied interactions bet ween non-hydrostatic stress and phase transformations or mineral react ions, drawn mainly from results of experiments done on mineral single crystals in our laboratory or our co-authors'. The state of stress and inelastic deformation can enter ''plicitly into the equilibrium phase relations and kinetics of mineral reactions. Altematively, phase tran sformations can have prominent effects on rheology and on the nature o f inelastic deformation. Our examples represent five types of structur al phase changes, each of which is distinguished by particular mechani cal effects. In increasing structural complexity, these include: (1) d isplacivephase transformations involving no bond-breaking, which may p roduce anomalous brittle behavior. A primary example is the alpha-beta quartz transition which shows anomalously low fracture strength and t ertiary creep behavior near the transition temperature; (2) martensiti c-like transformations involving transformation strains dominated by s hear deformation. Examples include the orthoenstatite --> clinoenstati te and wurtzite --> sphalerite transformations; (3) coherent exsolutio n or precipitation of a mineral solute from a supersaturated solid-sol ution, with anisotropy of precipitation and creep rates produced under non-hydrostatic stress. Examples include exsolution of corundum from MgO . nAl2O3 spinels and Ca-clinopyroxene from orthopyroxene; (4) orde r-disorder transformations that are believed to cause anomalous plasti c yield strengthening, such as MgO . nAl2O3 spinels; and (5) near-surf ace devolatilization of hydrous silicate single-crystals that produces a fundamental brittleness thought to be connected with dehydration at microcracks at temperatures well below nominal macroscopic dehydratio n temperatures. As none of these interactions between single-crystal p hase transformations and non-hydrostatic stress is understood in detai l, this paper serves as a challenge to field structural geologists to test whether interactions of these types occur in nature, and to thcor eticians to reach a deeper understanding of the complex relations betw een phase transformations, the local state of stress and associated de formation and deformation rates.