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Calciopharmacoalumite, ideally Ca 0.5 Al 4 (AsO 4 ) 3 (ОН) 4 ∙5H 2 O, the first Ca-dominant pharmacosiderite-supergroup mineral from the Obdilya deposit, Kyrgyzstan

Year: 2025

Type: article

Abstract: Calciopharmacoalumite, ideally Ca 0.5 Al 4 (AsO 4 ) 3 (ОН) 4 •5H 2 O, was found at the Obdilya gold ore deposit located within the Chauvai ore field (Kyrgyzstan).Calciopharmacoalumite occurs as clusters of light-brown "iron-bearing" cubic crystals and white iron-free polycrystalline masses overgrown with prismatic vladimirite crystals.Calciopharmacoalumite is associated also with mansfieldite, talmessite, tapiaite and various amorphous K-Ca-Fe-Al arsenates.Calciopharmacoalumite is cubic P-43m, a = 7.758(5) Å, V = 801.9(7)Å 3 , R 1 = 0.043.The eight strongest lines of the X-ray powder diffraction pattern are [I-d(Å)-hkl]: 100-7.76-100;60-4.47-111;33-3.880-200;30-3.166-211;51-2.746-220;16-2.457-310;19-2.339-311.The chemical composition determined by electron-microprobe analysis is (wt.%): As 2 O 5 48.05,Al 2 O 3 20.89;Fe 2 O 3 7.42; CaO 5.09; MgO 2.33; SrO 0.33; K 2 O 1.24; H 2 O 17.97, total 103.32.The empirical formula calculated on the basis of As = 3 atoms per formula unit is (Ca 0.65 K 0.19 Sr 0.02 )1.53+ Σ0.86 (Al 2.94 Fe 3+ 0.66 Mg 0.40 ) 11.60+ Σ4.00 (AsO 4 ) 3 9-((OH) 3.87 O 0.13 ) 4.13- Σ4.00 •5.19(H 2 O).Calciopharmacoalumite is the first Ca-dominant member of the pharmacosiderite supergroup.The origin of the new mineral is assumed to be lowtemperature hydrothermal or hypergenic.

Source: Mineralogical Magazine

DOI: 10.1180/mgm.2025.10181