A unique manganese-rich silicic edenite in the Grenville Marble, Fowler, New York

A manganiferous pod in the Grenville marble of the Arnold open pit of the Gouverneur Talc Company at Fowler, New York, consists of alternating layers composed mostly of polycrystalline braunite and a chemically unique Mn-rich silicic edenite which has the chemical formula [(Na_0.47K_0.01Ca_0.03)(Ca_1.03Mn_0.97)(Mg_4.55Mn_0.31Al_0.10Fe_0.04)(Si_7.49Al_0.51)O_22.13-(OH)_1.87], based on electron microprobe analyses. This amphibole belongs to space group C2/m and has unit-cell edges a = 9.802(2) Å, b = 18.025(4) Å, c = 5.281(1) Å; β = 104.39(1)°. The measured density is 3.28 g/cm³; the calculated density is 3.1l4 g/cm³. The difference is attributed to about 10% by volume ofbraunite that occurs as minute inclusions in the edenite. The optical properties of this edenite are γ = 1.642, β = 1.632, Å = 1.620, all ± 0.002; 2V_calc = 84°; weakly pleochroic with Z = pale pinkish brown, Y = pale orange brown, X = colorless. Electron microprobe analyses of the garnet (82 mol% spessartine) and the braunite associated with the manganoan silicic edenite are presented.