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 [(Na0.47K0.01Ca0.03)(Ca1.03Mn0.97)(Mg4.55Mn0.31Al0.10Fe0.04)(Si7.49Al0.51)O22.13-(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; 2Vcalc = 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.