Laphamite, ideally As₂(Se,S)₃, is monoclinic, P2₁/n, with a = 11.86(1), b = 9.756(9), c = 4.265(9) Å, β = 90.17°, V = 493.5 ų, Z = 4, D (calc.) = 4.60, D (meas.) = 4.5(1) g/cm³. Microprobe analysis yielded: As 47.0, Se 43.7, S 8.7, sum = 99.4 wt. %. In plane polarized light in polished section at about 3200 K laphamite is moderately bireflectant from white to grey. Reflection pleochroism is absent but fiery red internal reflections are characteristic as are golden yellow reflections along scratches. Anisotropy is moderate with grey rotation tints, masked in some sections by internal reflections. Reflectance spectra in air and oil between 400 and 700 nm are given. Colour values for the CIE illiminant C in air are, for R₁ followed by R₂: Y% 29.0, 33.5, λ_d 480, 478; P_e% 9.1, 8.7. Laphamite forms dark red crystals, up to 5.0 mm, associated with orpiment, arsenolite, and an unnamed phase, NH₄AlF₄, in a burning coal waste-dump at Burnside, Northumberland County, Pennsylvania.