HIGH-TEMPERATURE DEFORMATION OF DIOPSIDE CRYSTAL .3. INFLUENCES OF O-P(2) AND SIO2 PRECIPITATION

Single crystals of gem quality diopside (with Fe/(Ca+Mg+Fe) approximat ely 0.02) were deformed in a dead load apparatus under controlled oxyg en partial pressure (PO2), in the range 8 x 10(-14) - 2 x 10(-9) MPa, at two temperatures T1 = 1100-degrees-C and T2 = 1200-degrees-C. The a im of these experiments was to investigate the sensitivity of diopside creep rate to pO2 at these two temperatures. T1 and T2 are on both si des of a critical temperature T(c) congruent-to 1130-degrees-1140-degr ees-C at which the activation energy E of the creep rate decreases (f rom 442 to 48 kJ/mol) with rising temperature (Raterron and Jaoul, 199 1) when siliceous microdroplets (approximately 0.1 mum in size) form ( Ingrin et al., 1991). Specimens were deformed with axial compressive s tress sigma (110-143 MPa) along [010]; with this setting, the {110}1/2 [a +/- b] slip systems are symetrically activated, and strain rates ep silon are in the range 2 x 10(-8) - 2 X 10(-7) S-1. At T1 and under lo w pO2, we find epsilon is-proportional-to pO2(-0.200+/-0.033) for samp les that lack SiO2-rich precipitates in the host. At T2 and at the hig hest pO2 explored, epsilon becomes insensitive to pO2 for samples that contain SiO2-rich precipitates in the matrix. Electrical conductivity sigma(e) shows similar sensitivities to pO2 (Huebner and Voigt, 1988) . We propose a point defect model based on the chemistry of nonstoichi ometric compounds with cationic vacancies and ferric iron Fe3+ as majo rity point defects. The model predicts the critical values of T(c) and pO2, beyond which the increasing abundance of the majority point defe cts promotes SiO2 precipitation. T(c) and pO2, values are interdepende nt; they are also functions of Fe content iii diopside and of its init ial nonstoichiometry. This model offers an explanation of the pO2 depe ndencies of point defects concentrations as well. A comparison with ex perimental epsilon and simga(e) sensitivities to pO2 suggests that int erstitial divalent cations, which are minority defects, control electr ical transport and diffusion-assisted dislocation glide. The model als o shows that the occurrence Of SiO2 precipitation does not necessarily imply a supersilicic starting material.