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March-April 2011

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Thirty-seventh Rochester Mineralogical Symposium: Contributed Papers in Specimen Mineralogy—Part 1

THE TWENTY-SEVENTH TECHNICAL SESSION of the Rochester Mineralogical Symposium was held on 16 April 2010. Eight abstracts were accepted and presented as oral papers. The review panel consisted of Dr. Steven Chamberlain, New York State Museum; Dr. Carl Francis, Harvard University; Dr. Sarah Hanson, Adrian College; Dr. Marian Lupulescu, New York State Museum; and Dr. George Robinson, Michigan Technological University.

NEW DATA ON MINERALS FROM THE KEWEENAW COPPER DISTRICT, MICHICAN. T. W. Buchholz1, A. U. Falster2, W. B. Simmons2. 11140 12th St. N, Wisconsin Rapids, WI 54494; 2Dept. of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148.

Recent work on material from several former mines on the Keweenaw Peninsula, Michigan, has revealed the presence of several minerals not previously reported from the district.

Noting the suggestion of the possible occurrence of anilite (Cu7S4) near Painesdale, Michigan (Heinrich and Robinson 2004), carbonate matrix was partially removed from samples of chalcocite-bearing vein material from the Baltic mine exposing the sulfides. Several samples carried small grains of a soft, dark blue sulfide showing excellent cleavage and evidence of multiple twinning. X-ray diffraction (XRD) analysis of a small crystal fragment yielded a pattern showing good agreement with anilite. The anilite seems restricted to quartz-rich portions of thin carbonate-quartz-sulfide veinlets, is intimately associated with chalcocite, and is usually spatially associated with late postmineralization cross-cutting fractures. Additional minerals present in the veinlets are native copper, metallic grains of Cu-As alloys showing highly variable Cu-As ratios, and spongy-appearing masses of Cu-arsenide phases that seem to be composed of fine-grained domains with widely variable compositions. Work continues to attempt to characterize these phases.

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