Abstract

The temperature of the displacive triclinic/monoclinic transformation, Tdispl, of synthetic plagioclases and ternary feldspars was determined from high-temperature X-ray powder photographs. Tdispl increases with increasing Ab + An content in ternary feldspars and with increasing An content in the plagioclase series such that plagioclase with more than ≈ 15 mol% An cannot become monoclinic before melting. The temperatures of the displacive and diffusive transformation of plagioclases coincide. K-exchange experiments are used to discuss the Al,Si topochemistry of the samples.

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