High-Temperature Albite and Contiguous Feldspars

Synthetic albite is found to have X-ray and optical properties different than those of the very pure natural albites from pegmatites, but these natural albites can, by heating, be converted into a form identical with the synthetic product. High- and low-temperature modifications of albite are thus indicated. The inversion temperature is believed to be in the neighborhood of 700° C. Optical and X-ray studies show that the inversion extends into the plagiocla.se series approximately to An₃₅. These high-temperature plagioclases are common as phenocrysts in extrusive rocks but have not been found in plutonic rocks.