Arrojadite-(BaNa), BaNa3(Na,Ca)Fe2+13Al(PO4)11(PO3OH)(OH)2, A New Phosphate Mineral From the Luna Albite Pegmatite, Dorio Commune, Lecco Province, Italy

Abstract

Abstract Arrojadite-(BaNa), BaNa 3 (Na,Ca)Fe 2+ 13 Al(PO 4 ) 11 (PO 3 OH)(OH) 2 , is the Na-, Ba-rich member of the arrojadite group. This primary phosphate was found at the Luna albite pegmatite, Dorio, Lecco province, Italy where it occurs associated with fluorapatite in the blocky-albite unit of the pegmatite. This new phosphate forms greenish or yellowish translucent masses, or rough crystals up to 4–5 cm in diameter hosted by albite. Arrojadite-(BaNa) is anisotropic with α 1.656(2), β 1.660(2), and γ 1.664(2), and is non-fluorescent under both short-wave (254 nm) and long-wave (366 nm) ultraviolet light. The mineral is brittle with an irregular fracture and a good cleavage parallel to {110}; its Mohs hardness is 4–5. Its measured specific gravity is 3.54(2). The empirical formula, calculated on the basis of 12 (P + Si + As) atoms per formula unit is (Ba 0.62 K 0.27 Pb 0.13 Sr 0.07 ) Σ1.09 Na 3 (Na 1.19 Ca 0.85 ) Σ2.04 (Fe 2+ 9.82 Mg 1.92 Mn 2+ 1.64 ) Σ13.38 Al 1.01 (PO 4 ) 11 (HPO 4 )(OH 1.75 F 0.25 ) Σ2 , with the water content calculated considering 2 (OH – + F) per formula unit. Arrojadite-(BaNa) is monoclinic with space group C 2/c; its unit-cell parameters are a 16.4984(6) A, b 10.0228(3) A, c 24.648(1) A, β 105.850(4)°, and V 3920.8(2) A 3 , for Z = 4. The eight strongest lines in the X-ray powder diffraction pattern are [ d in A ( I / I 0 ) hkl ]: 3.137 (100) 5 1 0, 2.818 (61) 3 1 6, 3.303 (46) 3 2, 2.667 (35) 2 0 8, 2.878 (32) 3 3 1, 3.488 (28) 1 1 6, 4.621 (22) 1 3, and 2.936 (22) 3 3 0. The mineral, which has been approved by the CNMNC under number IMA 2014-071, is named arrojadite-(BaNa) since it corresponds to the Ba-Na-rich member of the arrojadite mineral group, in which Ba is the principal element at the X5 (= A1) site and M Fe * ≤ 0.5 apfu . Sodium is the unique element at the cation X1 (= Ca) site.