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 ų, 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 ų, 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: [Mg1.530 Fe0.199 2+ Ca0.111 Mn0.091 Fe0.034 3+ Al0.031 Na0.004 Ti0.001 ]Σ2.001 [ P0.944 Al0.047 Si0.003 C0.006 ]Σ1.000 [O3.965 (OH) 0.035] Σ4.000 [ F0.948 (OH) 0.052 ] Σ1.000.