Boron-Bearing Kornerupine from Fiskenæsset, West Greenland: A Re-Examination of Specimens from the Type Locality

Edward S. Grew, Richard K. Herd* and Nicholas Marquez
Department of Geological Sciences, 110 Boardman Hall, University of Maine, Orono, Maine 04469, U.S.A.
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
Materials Analysis Department, Aerospace Corporation, Los Angeles, California 90009, U.S.A.
*Geological Survey of Canada Contribution No. 44786.

Abstract: In 1884, Lorenzen proposed the formula MgAl2SiO6 for his new mineral kornerupine from Fiskenæsset and did not suspect it to contain boron. Lacroix and de Gramont (1919) reported boron in Fiskenæsset kornerupine, while Herd (1973) found none. New analyses (ion microprobe mass analyser and spectrophotometric) of kornerupine in three specimens from the type locality, including the specimens analysed by Lorenzen and Herd, indicate the presence of boron in all three, in amounts ranging from 0.50 to 1.44 wt.% B2O3, e.g. (Li0.04 Na0.01 Ca0.01) (Mg3.49 Mn0.01 Fe0.17 Ti0.01 Al5.64)Σ9.30 (Si3.67 Al1.02 B0.31)Σ5 O21 (OH0.99 F0.01) for Lorenzen's specimen. Textures and chemical compositions suggest that kornerupine crystallized in equilibrium in the following assemblages, all with anorthite (An 92–95) and phlogopite (XFe = atomic Fe/(Fe + Mg) = 0.028–0.035): (1) kornerupine (0.045)-gedrite (0.067); (2) kornerupine (0.038–0.050)-sapphirine (0.032–0.035); and (3) kornerupine (0.050)-hornblende. Fluorine contents of kornerupine range from 0.01 to 0.06%, of phlogopite, from 0.09 to 0.10%. In the first assemblage, sapphirine (0.040) and corundum are enclosed in radiating bundles of kornerupine; additionally sapphirine, corundum, and/or gedrite occur with chlorite and pinite (cordierite?) as breakdown products of kornerupine. Kornerupine may have formed by reactions such as: gedrite + sapphirine + corundum + B2O3 (in solution) + H2O = kornerupine + anorthite + Na-phlogopite under conditions of the granulite facies. Boron for kornerupine formation was most likely remobilized by hydrous fluids from metasedimentary rocks occurring along the upper contact of the Fiske⇒set gabbro-anorthosite complex with amphibolite.

Keywords: boron • kornerupine • Fiskenæsset • Greenland • ion microprobe • spectrophotometry

Mineralogical Magazine; December 1987 v. 51; no. 363; p. 695-708; DOI: 10.1180/minmag.1987.051.363.10
© 1987, The Mineralogical Society
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