Twenty samples of hornblendes from rocks of 14 plutonic units in the central Sierra Nevada and Inyo Mountains, California, have been studied in detail. Optical, density, single-crystal and powder X-ray diffraction, and major and minor element chemical data are reported.The compositions of the hornblendes show only limited correlation with the chemistry of the rocks in which they occurred. Hornblendes from granitic rocks of the eastern Sierra Nevada and Inyo Mountains have a wide range of tetrahedral aluminum content which is often as low as three-quarters of an atom per formula unit, whereas hornblendes from younger granitic rocks elsewhere in the Sierra Nevada batholith contain more than one atom of tetrahedral aluminum per formula unit. Because an increase of aluminum in tetrahedral co-ordination is considered indicative of higher temperatures of crystallization, the observed differences in the hornblendes suggest that older plutonic rocks of the batholith may have been metamorphosed regionally or may have been affected by widespread hydrothermal action prior to consolidation of later granitic rocks.
