Aluminotaipingite-(CeCa), (Ce₆Ca₃)Al(SiO₄)₃[SiO₃(OH)]₄F_3, a new member of the cerite-supergroup minerals

Abstract Aluminotaipingite-(CeCa), (Ce 6 Ca 3 )Al(SiO 4 ) 3 [SiO 3 (OH)] 4 F 3 , is a new member of the cerite-supergroup minerals, whose general chemical formula is A 9 XM [ T O 3 Ø] 7 Z 3 , ( A = REE, Ca, Sr, Na and □; X = □, Ca, Na and Fe 2+ ; M = Mg, Fe 2+ , Fe 3+ , Al and Mn; T = Si and P; Ø = O and OH; Z = □, OH and F). It was found in cavities of a leucogranitic orthogneiss at the Casette quarry, Montoso, Bagnolo Piemonte, Cuneo Province, Piedmont, Italy. Crystals of aluminotaipingite-(CeCa) are light pink to pink, transparent or semi-transparent, with a vitreous lustre. It forms pyramidal crystals up to 0.07 mm in size and observed forms are {0 0 1}, {1 0 $\bar{2}$ }. The tenacity is brittle, no distinct cleavage is observed and the fracture is uneven. The mineral does not fluoresce in long- or short-wave ultraviolet light. The streak is white. Hardness (Mohs) = 5. The calculated density is 4.476 g cm –3 . The mineral is trigonal, space group R 3 c , with a = 10.658(3), c = 37.865(9) Å, V = 3725(2) Å 3 and Z = 6. The eight strongest powder X-ray diffraction lines are [ d obs, Å ( I, % ) ( h k l )]: 8.38(29)(0 1 2), 4.499(28)(2 0 2), 3.282(41)(2 1 4), 2.936(100)(0 2 10), 2.816(51)(1 2 8), 2.669(37)(2 2 0), 2.207 (29)(3 0 12) and 1.935(35)(2 3 8). The structure was refined to R =0.0306 for 2297 reflections with I >2σ( I ). The crystal structure of aluminotaipingite-(CeCa) contains two nine-fold coordinated sites ( A 1 and A 2), which are occupied mainly by lanthanides, and a third nine-fold coordinated A 3 site containing almost equal amounts of lanthanides and Ca. The X site is vacant and at the octahedral M site aluminium prevails over Fe 3+ . Among the three independent T sites, T 2 belongs to a (SiO 4 ) 4– anion, whereas T 1 and T 3 belong to (SiO 3 OH) 3– anions. Fluorine is involved in coordination with the A 1 and A 3 sites.