Shallow-seated, high-temperature sanidine pegmatites of Tertiary age occur in the Black Range, Grant County, New Mexico. The pegmatites are small bodies within a rhyolite porphyry plug that has been injected into rhyolite tuffs of Tertiary age. The pegmatites consist dominantly of quartz and sanidine with accessory quantities of cleavelandite, biotite, sphene, magnetite, and ilmenite. The sanidine, which occurs in crystals up to one or two feet on a side, is the moonstone variety and displays limpid blue and white play of colors. Microcline is absent, and on the basis of the sanidine the pegmatites are believed to have crystallized at temperatures higher than those of the normal plutonic pegmatites. Textural and structural evidence indicates that the pegmatites crystallized prior to the enclosing rhyolite porphyry. The origin and classification of pegmatites are reviewed.

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