Scientia Sinica

wep!

182 SCIENTIA: SINICA Vol. V

states that xanthophyllite (or clintonite), the Mg-Ca brittle mica, does occur in contactmetamorphic deposits in schists on the Western Urals, while chloritoid is found, although only in small amounts, in contact-metamorphic deposits in the marble. Gustavson!” also concludes, that chloritoid, the Fe brittle mica, could occur as a distinctly hydrothermal mineral. It is to be noted that in the cases described by Betechtin brittle micas formed most likely in the process of thermal metamorphism rather than regional metamorphism. This of course, is in accord with the findings of our experiments.

In the Western Hills of Peking, China, chloritoid is a widespread rockforming mineral, occurring in beds ranging in age from the Sinian up to the Jurassic. It is interesting to note that while much of the chloritoid probably formed as a result of low-grade regional metamorphism, part of it occurs distinctly in veins.”

However, the occurrence of brittle micas as a hydrothermal alteration product or in hydrothermal veins is much rarer than their occurrence as a product of regional metamorphism. In addition, brittle micas are considerably less abundant than ordinary micas in nature. All this may be partly due to the comparably narrow range of stability of brittle micas with respect to temperature, concentration, and the presence of radicals, such as CO3. Our experimental results indicate that phlogopite can be hydrothermally synthesized at temperatures as low as 250°C”, while the lowest temperature limit at which margarite could be made lies much higher (350-370°C). The change of acidity might more seriouly effect the stability range of brittle micas than that of ordinary K-micas. In the case of margarite high basicity probably renders it unstable and tends to form feldspathoids (cancrinite and others). This tendency is likely to be strengthened if CO; and other radicals are present in even small amounts. This does not seem to have occurred with the synthesis of the ordinary K-micas. All this may partly explain the comparative rarety of margarite as compared with K-micas. In addition, the factor of unfavourable ionic size of Ca in the margarite structure might render t less stable than a K-mica. Similar reason was pointed out by Gruner"! to explain partly the rare occurrence of paragonite in nature.

Our experimental findings show that while margarite was readily synthesized, we had failed to prepare other members of the Fe-free brittle micas (Na-margarite, clintonite). The explaination may lie in the following facts. No “pure” Na-margarite has ever been recorded in geological literature. The most sodic margarite ever reported appears to be the mineral ephesite from the Postmasburg district, $. Africa, containing as much as 8.657% Na,O and 14% CaO™!, The other sodic margarites contain considerably less Na. In

1) Oral communication by K. Yang and T. L, Ho.

2) Results of synthesis of phlogopite will be published in a separate paper.