High‐temperature deformation of diopside crystal: 3. Influences of pO2 and SiO2 precipitation

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Abstract
Single crystals of gem quality diopside (with Fe/(Ca+Mg+Fe) ≃ 0.02) were deformed in a dead load apparatus under controlled oxygen partial pressure (pO2), in the range 8×10−14–2×10−9 MPa, at two temperatures T1 = 1100°C and T2 = 1200°C. The aim of these experiments was to investigate the sensitivity of diopside creep rate to pO2 at these two temperatures. T1 and T2 are on both sides of a critical temperature Tc ≃ 1130°–1140°C at which the activation energy E* of the creep rate decreases (from 442 to 48 kJ/mol) with rising temperature (Raterron and Jaoul, 1991) when siliceous microdroplets (≃0.1 μm in size) form (Ingrin et al., 1991). Specimens were deformed with axial compressive stress σ (110–143 MPa) along [010]; with this setting, the {110}1/2〈a±b〉 slip systems are symetrically activated, and strain rates are in the range 2×10−8–2×10−7 s−1. At T1 and under low pO2, we find for samples that lack SiO2‐rich precipitates in the host. At T2 and at the highest pO2 explored, becomes insensitive to pO2 for samples that contain SiO2‐rich precipitates in the matrix. Electrical conductivity σe shows similar sensitivities to pO2 (Huebner and Voigt, 1988). We propose a point defect model based on the chemistry of nonstoichiometric compounds with cationic vacancies and ferric iron Fe3+ as majority point defects. The model predicts the critical values of Tc and pO2c beyond which the increasing abundance of the majority point defects promotes SiO2 precipitation. Tc and pO2c values are interdependent; they are also functions of Fe content in diopside and of its initial nonstoichiometry. This model offers an explanation of the pO2 dependencies of point defects concentrations as well. A comparison with experimental and σe sensitivities to pO2 suggests that interstitial divalent cations, which are minority defects, control electrical transport and diffusion‐assisted dislocation glide. The model also shows that the occurrence of SiO2 precipitation does not necessarily imply a supersilicic starting material.

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