BERYL-TANTALITE PEGMATITES OF NORTHEASTERN BRAZIL
Microcline-muscovite-quartz pegmatites in Northeastern Brazil have yielded approximately 600 metric tons of tantalite and 8000 metric tons of beryl, between 1937 and 1944. Other economic minerals are cassiterite, muscovite, amblygonite, and spodumene. The regional rocks are pre-Cambrian schists and quartzites intruded by granites. The pegmatites range from homogeneous textures without notable internal differentiation to heterogeneous textures with pronounced internal zoning. In the latter type, the essential minerals—microcline, muscovite, and quartz—form gigantic crystals, and the accessory minerals—tantalite, beryl, spodumene, and amblygonite—crystallize on a comparable scale. Aside from late albitization of the microcline, there appears to be little evidence that the differentiation in the heterogeneous pegmatites resulted from a series of complex replacements. The field evidence appears, rather, to indicate deposition in a single cycle with a progressive increase in crystal size from walls to center. Forty-two characteristic heterogeneous pegmatites are described, and plane-table sketch maps for some of them are reproduced.