Fulbrightite, the Arsenate Analog of Sincosite

Fulbrightite (IMA2019–032), Ca(VO)2(AsO4)2·4H2O, is a new mineral from the Packrat mine, near Gateway, Mesa County, Colorado, USA, and from the Rovnost mine, Jáchymov, Czech Republic. It is a low-temperature secondary phase. The mineral most typically occurs in shades of light green and forms rosettes of roughly square (pseudotetragonal) plates. The streak is colorless to pale green and the luster is vitreous to pearly. The Mohs hardness is about 2½. Crystals are brittle, but slightly flexible in thin plates. Cleavages are (001) perfect, (100) and (010) excellent, (110) and fair. Fracture is stepped, irregular, and curved. The measured density is 3.12(2) g/cm3. The mineral is optically biaxial (–), α = 1.675(3), β = 1.718(3), and γ = 1.718(3) (white light); 2V ≈ 5°; orientation: X ≈ c; pleochroism: X colorless, Y and Z pale green (X < Y = Z). Electron-microprobe analyses gave the empirical formulae Ca0.99(V4+1.00O)2[(As5+0.98V5+0.02)O4]2·4(H2.005O) (Packrat mine) and (Ca1.02Fe0.01Ba0.01)Σ1.04(V4+O)1.96[(As5+0.99P0.01)O4]2·4.04H2O (Rovnost mine). X-ray powder diffraction (coupled with the chemical analyses) showed fulbrightite to be the arsenate analog of sincosite. The mineral is triclinic, space group P1, with cell parameters a = 6.434(8), b = 6.480(8), c = 6.718(8) Å, α = 107.90(6), β = 94.06(4), γ = 90.06(3)°, V = 265.8(6) Å3, and Z = 1. The Raman and infrared spectra of fulbrightite and sincosite are consistent with them being arsenate and phosphate analogs, respectively.