Three substances occur in some of the Geco gneisses: a colorless to pale brown translucent material occurring in biotite, dark brown to black clots associated with sillimanite in quartz-dominated matrices, and massive felted brown material forming a constituent of fibrolite patches. Electron microprobe analysis reveals a range of hydrous alumino silicates containing variable small amounts of impurities. The compounds range from kaolinite or halloysite through sillimanite. In some instances the precursor developed from earlier partial kaolinization of biotite, thus accounting for the common intergrowth of sillimanite and biotite in metamorphic rocks.--Modified journal abstract.

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