Hornblendes in amphibolite interlayers in the paragneiss of the northwest Adirondack Mountains undergo systematic changes in color, composition, and density during progressive metamorphism from almandine-amphibolite to hornblende-granulite facies. In contrast, indices of refraction of the hornblendes remain about constant.
In the almandine-amphibolite facies the amphibolite layers have the bulk composition of a saturated basalt and consist of bluish-green hornblende, andesine, and quartz. As these layers are traced into the hornblende-granulite facies, their composition undergoes a progressive change to that of an olivine basalt with brownish-green hornblende, clinopyroxene and orthopyroxene, and calcic andesine as major constituents.
Compositional changes in the hornblendes with increasing grade of metamorphism include increases in Ti, Na, K, Cr, V, and Sc. Decreases occur in the amounts of Mn, Zn, OH + F + Cl, and in the ratios Fe2O3/FeO and Fe/Mg.
Density of the hornblendes increases from 3.260 to 3.278 with the increasing grade of metamorphism.
These changes in the hornblendes with increasing T and P, although well denned, are less pronounced than those measured in biotites and garnets of the enclosing paragneiss. Large variations in the physical and chemical properties of hornblendes in metamafic rocks reconstituted above the epidote-amphibolite facies appear to be induced principally by critical changes in the bulk composition of the total rock, and not by the regional gradients in T, P, or by changes in kind, or composition, of the coexisting minerals.