Abstract
Phaunouxite is triclinic P[-1] with a = 12.563(7), b = 12.181(6), c = 6.205(4) Å, α = 88.94(3), ß = 91.67(3), γ = 113.44(4)°, Z = 2. The strongest lines in the X-ray powder pattern are : 11.5(10) (100), 6.23(9) ([-2]10), 5.42(8) (011, 101, 0[-1]1), 3.276(9) (2[-3]1, 310), 2.963(6) (221), 2.810(6) (112, [-2]12), 2.443(7) (1[-3]2, [-5]30, [-3]02), 2.026(6) (0[-1]3, 103). The [001] needle crystals are colourless, vitreous, and with {100} dominant ; they occur in fan-shaped clusters at Gabe-Gottes vein in Sainte-Marie-aux-Mines (Vosges) with picropharmacolite, ferrarisite and aragonite. Cleavage {100} perfect ; Dmeas = 2.28(2) g.cm⁻³, Dcalc = 2.275 g. cm⁻³. Refractive indices are : nγ = 1.556(2), nß = 1.542(2), nα = 1.532(2). 2V ≃ 80°. Only Ca, As, O and H are among the chemical constituents ; wet chemical analysis gave CaO 28.2, As₂O₅ 39,0. H₂O 31,5 %. The ideal chemical formula, Ca₃(AsO₄)₂ • 11 H₂O, was also established through crystal structure determination and the identification of the final dehydration product Ca₃(AsO₄)₂, which is formed at 570 °C and is preceded by two unidentified phases at 60 and 110°C. Phaunouxite is always intergrown with rauenthalite, Ca₃(AsO₄)₂ • 10 H₂O, from which it can be distinguished only through the strongest lines of their powder patterns. The name is for Phaunoux, the french name of the Rauenthal valley where the mine is, and has been approved by the IMA Commission.
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