Abstract: Wycheproofite is a new hydrated sodium aluminium zirconium phosphate from a pegmatite vein in granite at Wycheproof, in northwestern Victoria, Australia. The mineral occurs as compact, finely fibrous masses in small cavities in the quartz/feldspar/muscovite/schorl-bearing pegmatite. The fibrous crystals are between 5 and 10 µm wide and up to several mm long. Accompanying minerals include two other zirconium phosphates — kosnarite and a new species, selwynite, the K-analogue of gainesite — as well as wardite, eosphorite, cyrilovite, leucophosphitc, rockbridgeite, a kidwellite-like mineral and saleeite. The wycheproofite aggregates are pale pinkish to brownish orange, with a vitreous to pearly lustre. The streak is colourless, fracture rough, cleavage not observed and the Mohs hardness is between 4 and 5. Optical data are incomplete due to the fibrous nature of the mineral; the indices of refraction are in the range 1.62–1.64. The measured density is 2.83 g cm⁻³. Chemical analysis gave (wt.%) Na₂O 6.36, K₂O 0.44, CaO 0.66, FeO 0.36, MnO 0.21, Al₂O₃ 12.03, Cs₂O 0.03, ZrO₂ 32.43, HfO₂ 1.24, P₂O₅ 35.85, SiO₂ 0.23, F 0.34, H₂O 9.0, less 0=F 0.14, Total 99.04. The simplified formula is NaAlZr(PO₄)₂(OH)₂·H₂O. Wycheproofite is triclinic with unit cell parameters a=10.926(5) Å, b = 10.986(5) Å, c = 12.479(9) Å, α= 71.37(4)°, β = 77.39(4)°, γ= 87.54(3)° V = 1375.9 ų. For Z= 6, the calculated density is 2.81 g cm^-3. The strongest lines in the X-ray powder diffraction pattern are [d_obs (Å), I_obs,hkl] 2.603 (100) 040; 4.128 (80) 121; 3.711 (65) 023; 3.465 (60) 030; 8.865 (40) 101; 3.243 (35) 132. The crystal structure has not been solved due to the finely fibrous nature of the material available. The name is for the locality, which in the local Australian Aboriginal language means ‘witchie bushes growing on a hilltop’.

Data on the third occurrence of kosnarite, KZr₂(PO₄)₃, at Wycheproof are also given.