Breithauptite

This specimen, a sawn and polished slab from a dendritic mass of nickeline and associated arsenide minerals from the Trotter mine, is the one in which Franklin breithauptite in microscopic grains was first discovered. Nickeline (NiAs) is the coppery red mineral along the lower edge of the slab. As shown in the photograph, the nickeline dendrites toward their bulbous outer margins are partly replaced by pararammelsbergite (NiAs₂, the pale gray mineral). The replacement texture is particularly evident at lower left, where only “ghost” remnants of nickeline are seen, but their original dendritic texture is still clearly visible. Succeeding the pararammelsbergite in outward growth of the dendrites are rammelsbergite (also NiAs2, darker gray) and small amounts of gersdorffite (NiAsS), skutterudite (CoAs₃), and safflorite ((Co,Ni,Fe)As₂). The black mineral in thin rims around the margins of the dendrites is stilpnomelane. Associated minerals, in addition to those mentioned, include fluorite (white to violet, upper right and lower left), magnetite (large black grains at left), and white calcite (upper left). This specimen for years was in the collection of Lee Areson (LA 275) and then passed to the collection of Earl R. Verbeek (ERV-1895) upon the sale in 1988-1989 of the Areson collection by Richard Hauck. Small fragments were removed from the irregular rear face of this slab, mounted in plastic, and polished for reflected-light microscopy. The photomicrographs shown below were obtained from these polished sections.

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