A reinvestigation by transmission electron microscopy (TEM) of magnetite and amphibole precipitates occurring in black star diopside from Nammakal (India) underlines the intimate relationships between both types of precipitated phases. A detailed examination shows that the small platelet-shaped crystals of magnetite occur only inside or along the amphibole lamellae. The phases are inferred to have coprecipitated, governed by the diffusion of H into the sample (diopside1 + H2 → diopside2 + amphibole + magnetite). Such a reaction implies that the amounts of the precipitated phases are completely determined by the initial A1 content of the host pyroxene; measurements of these amounts are in quantitative agreement with these predictions.

Furthermore, two families of platelet-shaped precipitates of magnetite are observed; both have epitaxial relations with the diopside matrix such that their larger dimension is approximately parallel to either c or a of diopside. The angle between the two directions of the magnetite platelets evolves with the size of the precipitates from 103.8° to 105.5°. The phase-boundary thermometer of Fleet et al. (1980) leads to a reaction temperature increasing from 500 to 700 °C. We suggest that the magnetite and amphibole precipitates formed during reheating to this temperature range.

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