Abstract

Kyanite, andalusite, and sillimanite are found together in the cordierite-bearing mica schist of the Prichard formation of the Precambrian Belt series exposed in the Boehls Butte quadrangle in the southern part of the Idaho panhandle. Microscopic studies of this schist suggest that the following inversions took place: (1) sillimanite—»kyanite, (2) sillimanite—»andalusite, (3) kyanite—»andalusite, (4) kyanite—»sillimanite, (5) andalusite—»sillimanite. These inversions can be in part related to the fluctuation of temperature and stresses during the complex regional and thermal metamorphism to which the schist was subjected. In some thin sections all three modifications occur side by side, suggesting that they were crystallized close to the physical-chemical conditions in which all three may exist together. The association of epidote and plagioclase (An36) in the calcium-rich beds of the same area suggests that the temperature during the crystallization was close to 400° C.

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