Abstract

The paragenetic sequence of the minerals of the Barton garnet mine shows that the crystallization of the mine rock started with the formation of a hornblende-diorite matrix and ended essentially with the crystallization of the large garnet grains and/or crystals within the hornblende rims. These rims or envelopes, which were formerly considered “reaction rims,” are paragenetically earlier than the enclosed minerals. The hornblende envelopes and the minerals within the envelopes developed from retained isolated pegmatitic or deuteric solutions having more-or-less spherical outlines en.losed within the hornblende diorite. The crystallization of these isolated solutions produced first the hornblende envelopes followed by terminated crystals of hornblende and hypersthene, large grains of plagioclase, and intergrowths of hornblende and feldspar growing from the inside of the envelopes toward the center. The last mineral to form in any quantity was garnet, which covered the minerals attached to the interior of the envelopes and which also enclosed crystals of biotite, apatite, hypersthene, and rounded grains of pyrite which developed contemporaneously with the garnet.

There is no evidence of late introduction of material or of metasomatic action between any of the minerals present.

The term dsuterocryst is proposed to apply to those crystals or large grains as hornblende, hypersthene, biotite, and garnet which developed from late pegmatitic or deuteric solutions retained within the parent rock. Such crystals are late in development in contrast to phenocrysts which appear early and are retained as conspicuous crystals or grains.

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