Allophane with an exceptionally high content of alumina occurs in granodiorite in the northwest part of the Bass Lake quadrangle, Mariposa County, California. Chemical analysis of the mineral gives SiO2 15.1, Al2O3 47.7, CaO 0.29, Na2O 0.24, K2O 0.14, H2O+ 20.8%, H2O- 15.7%, (alumina determined by difference), corresponding to the formula SiO2 · 2Al2O3 · 8H2O. The allophane is optically isotropic and is amorphous to infrared and X radiation. Occurrence as cores to plagioclase crystals suggests a secondary origin, and it is probable that development of allophane represents the first stage of weathering of the rock.

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