Galliskiite, Ca₄Al₂(PO₄)₂F₈·5H₂O, a new mineral from the Gigante granitic pegmatite, Córdoba province, Argentina

Abstract

Galliskiite, ideally Ca₄Al₂(PO₄)₂F₈·5H₂O, is a new mineral found at the Gigante granitic pegmatite, Punilla department, Córdoba Province, Argentina. It is named for Argentine mineralogist and pegmatite specialist Miguel Ángel Galliski. Galliskiite is triclinic, P1̄, a = 6.1933(7), b = 9.871(1), c = 13.580(2) Å, α = 89.716(3), β = 75.303(4), γ = 88.683(4)°, Z = 2. The strongest lines in the X-ray powder diffraction pattern are [d in Å, (I)]: 7.904 (70), 5.994 (100), 3.280 (58), 3.113 (30), 2.945 (85), 2.887 (44), 2.483 (20), 2.262 (27), 2.150 (23), 1.821 (27), and 1.798 (25). It occurs as crude platy crystals elongated along [001] and flattened on {010}, with frosty surfaces. Simple contact and polysynthetic twinning on {100} by rotation about [010] is ubiquitous. It is colorless and transparent, has white streak and vitreous luster, and is nonfluorescent under ultraviolet radiation. It has a Mohs hardness of 2½, conchoidal to irregular fracture and two fair cleavages at approximately 90°. The measured density is 2.67(3) g/cm³, and the calculated density is 2.670 g/cm³. Galliskiite dissolves slowly in concentrated HCl. The mineral is biaxial (+), α = 1.493(1), β = 1.495(1), γ = 1.520(1), 2V_meas = 33(5)°, 2V_calc = 32°; dispersion, r < v; orientation Z ≈ b, X and Z at 40-50° from a and c. No pleochroism is observed. Analysis by electron microprobe (average of 12 analyses given in wt%) provided CaO 34.71, MgO 0.01, FeO 0.10, MnO 0.17, Al₂O₃ 15.92, SiO₂ 0.06, TiO₂ 0.01, P₂O₅ 21.94, F 21.35, H₂O (calculated by stoichiometry) 15.08, less F≡O 8.99, total 100.39 wt%. The empirical formula, based on 21 (F+O), is (Ca_3.98Mn_0.02Fe_0.01)_Σ4.0Al_2.01(P_1.99Si_0.01O₈)F_7.23(OH)_0.77·5H₂O. The crystal structure, solved and refined using single-crystal data to R₁ = 0.033, consists of double chains of alternating corner-sharing AlF₃O₃ octahedra and PO₄ tetrahedra along the a axis. The chains are joined into a framework via bonds to four distinct Ca atoms. Calcium atoms are also linked by sharing isolated F atoms and H₂O molecules. The double-chain motif in the structure of galliskiite is distinct from that in any other known phosphate.