Abstract
Several grossulars from Adirondack calc-silicates have been found to contain up to 0.76 weight percent fluorine substituted in the structure, equivalent to a 1.5 mole percent substitution of fluorine for oxygen. These garnets occur with secondary prehnite and form through retrogradation of wollastonite after the peak of Grenville regional metamorphism. Normalization of a microprobe analysis for one garnet yields an inferred formula of:
The unit cell parameter, a = 11.858(5)Å, and the refractive index, n = 1.726(3), are compatible with the presence of both OH and F in solid solution. The analytical data may be explained by several coupled substitutions:
The new hypothetical garnet endmembers: may be derived from these substitutions.
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