The Brown Derby No. 1 pegmatite, of Gunnison County, Colorado, contains a highly diversified suite of micas: biotite, muscovite, zinnwaldite, rose muscovite, sericite, and five major varieties of lepidolite. Each of the micas was formed in a unit of characteristic mineralogy and texture. Chemical analyses demonstrate that the Li micas show nonsystematic variations in both major and minor elements with respect to their positions in the pegmatite and probably with respect to relative time of formation. These data coupled with nonsystematic variations in texture and mineral composition, and cross-cutting structural relationships indicate that the albite-Li mica units are not primary zones but are replacement layers formed by reactions between zonal pegmatite rock and deuteric, residual pegmatitic fluids in a closed-system environment.

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