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Wagnerite with isokite from the Benson Mines, west-central Adirondack Highlands, New York

Published online by Cambridge University Press:  05 July 2018

Howard W. Jaffe ,
Leo M. Hall  and
Howard T. Evans Jr.
Howard W. Jaffe
Affiliation:
Department of Geology and Geography, University of Massachusetts, Amherst, Massachusetts 01003
Leo M. Hall
Affiliation:
Department of Geology and Geography, University of Massachusetts, Amherst, Massachusetts 01003
Howard T. Evans Jr.
Affiliation:
U.S. Geological Survey, National Center 959 Reston, Virginia 22092, U.S.A.
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Abstract

The rare fluophosphate minerals wagnerite, ideally Mg2(PO4)F, and isokite, ideally CaMg(PO4)F, are intimately associated with magnetite-hematite deposits in sillimanite-, garnet-, and pyroxene-rich paragneisses and migmatites at the Benson Mines, near Star Lake in the west-central Adirondack Highlands of New York State. Coarsely crystalline wagnerite occurs in lenticular masses, typically 4 × 8 cm, delineated by sharply cross-cutting, sinuous, 2 cm-wide veins of fine-grained, fibrous to platy isokite and granular fluorapatite. These also penetrate transverse fractures across wagnerite lenses. Isokite formed from the introduction of Ca- and O-rich hydrothermal solutions into wagnerite. Both minerals are monoclinic: wagnerite crystallises in space group P21/a with a = 11.945, b = 12.717, c = 9.70 Å, β = 108.18°, V = 1400.2 Å3, D(calc) = 3.291 g/cm3 for Z = 16; isokite crystallises in space group A2/a with a = 6.909, b = 8.746, c = 6.518 Å, β = 112.20°, V = 364.7 Å3, D(calc) = 3.248 for Z = 4. Optical properties for wagnerite are: α = 1.5845, β = 1.5875, γ = 1.6010, 2V = 51°(calc.) disp = r < v weak, absorption α < β > γ with α = col., β = pale yel., γ = v. pale yel. For isokite only a mean index of refraction, n = 1.598, could be measured. Wet chemical analysis of wagnerite containing a calculated 11.4% of isokite as fine lamellae, gave the formula:

Keywords

wagneriteisokiteBenson minesAdirondack HighlandsGrenville ProvinceNew York

Information

Type
Mineralogy
Information
Mineralogical Magazine , Volume 56 , Issue 383 , June 1992 , pp. 227 - 233
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1992

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Footnotes

*

Present address: R.R.1 Box 540 Range Road, Underhill, Vermont 05489-9402, U.S.A.

†

Deceased

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