A group of eight hornblendes from the Piedmont region of Pennsylvania and Delaware were studied with special reference to their genetic relationships. Chemical analyses and optical data are presented. The relationship of optical properties and composition are discussed generally. Substitution of heavy metals for magnesium has the greatest influence on optical properties. Both refractive index and optic angle (+2V) increase with increasing heavy metal substitution. Substitution of aluminum for silicon has a similar effect on refractive index, but decreases the optic angle.

Potash and soda content of the hornblendes is correlated with degree of granitization of the host rock. Both total potash and the K/Na ratio in the vacant positions of the lattice increase with greater alteration.

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