Abstract
SummaryThe location and some characteristics of nine veins containing nigerite are reported. These veins are part of the province of tin-tantalum veins of central Amapá. They are intrusive in andalusite schist and commonly consist of pegmatite in the centre and of quartz-mica rock at the margins. The nigerite can occur in the pegmatite as well as in the quartz-mica rock; its presence in a sample of quartz-sillimanite rock is also mentioned. In both pegmatite and quartz-mica rock, the common associated translucent heavy minerals are cassiterite, tourmaline, chrysoberyl, and occasionally andalusite, sillimanite, and garnet. Associated minerals in the quartz-sillimanite rock are cassiterite and chrysoberyl. Nigerite and cassiterite commonly occur together but are not found in the presence of gahnite. A possible explanation for this is given.The nigerite of Amapá occurs as thin hexagonal plates, transparent and golden brown, amber-yellow or colourless, with diameters from microscopic up to a few millimetres. The sizes and the optical sign of this nigerite are the same as those of Nigeria and of Siberia, whereas Portuguese nigerite occurs in very fine grains and is mostly optically negative. The existence of alluvial nigerite is reported.
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