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

Arrojadite-(BaNa), BaNa3(Na,Ca)Fe132+Al(PO4)(11)(PO3OH)(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 alpha 1.656(2), beta 1.660(2), and gamma 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 (Ba0.62K0.27Pb0.13Sr0.07)(Sigma 1.09)Na-3(Na1.19Ca0.85)(Sigma 2.04)(Fe9.822+Mg1.92Mn1.642+)(Sigma 13.38)Al-1.01(PO4)(11)(HPO4)(OH1.75F0.25)(Sigma 2), with the water content calculated considering 2 (OH- + F) per formula unit. Arrojadite-(BaNa) is monoclinic with space group C2/c; its unit-cell parameters are a 16.4984(6) angstrom, b 10.0228(3) angstrom, c 24.648(1) angstrom, beta 105.850(4)degrees, and V 3920.8(2) angstrom(3), for Z = 4. The eight strongest lines in the X-ray powder diffraction pattern are [d in angstrom (I/I-0) hkl]: 3.137 (100) 5 1 0, 2.818 (61) 3 1 6, 3.303 (46) (1) over bar 3 2, 2.667 (35) 2 0 8, 2.878 (32) 3 3 1, 3.488 (28) 1 1 6, 4.621 (22) (3) over bar 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 Fe-M* <= 0.5 apfu. Sodium is the unique element at the cation X1 (= Ca) site.