Aluminocerite-ce : a new species from Baveno, Italy : description and crystal-structure determination

Aluminocerite-(Ce), ideally (Ce,Ca)9Al(SiO 4)3[SiO3(OH)]4(OH)3, is isostructural with cerite-(Ce) and cerite-(La). The holotype was found at the Ratti quarry, near Baveno, Italy, in millimeter-sized secondary cavities hosted in aplite-pegmatite veins and pods within pink granite. Aluminocerite-(Ce) forms aggregates of pseudo-octahedral to rhombohedral crystals flattened on the c axis. The cotype of aluminocerite-(Ce) was discovered at the Locatelly quarry, also near Baveno, where it occurs in centimeter-sized miarolitic cavities in pink granite. The mineral is pale pink to pink-reddish, with a white streak, and is translucent with a vitreous luster. Aluminocerite-(Ce) is non-fluorescent. The hardness based on the Mohs scale is 5, and the tenacity is brittle. Neither cleavage, fracture, or twinning were observed. Calculated density is 4.675 g/cm3. It is uniaxial, optically positive, with no = 1.810-1.816 and ne = 1.812-1.822 (λ= 589 nm) and non-pleochroic. The average of 15 electron microprobe analyses for the holotype gave (wt%): Ce2O3 23.37; Nd2O3 15.59; La2O3 7.43; Sm2O3 4.38; Pr2O3 3.54; Gd2O3 3.12; Y 2O3 1.68; Dy2O3 0.46; Yb 2O3 0.07; CaO 8.31; Fe2O3 0.47; Al2O3 2.47; SiO2 24.01; and H2O 3.63 (calculated from crystal-chemical constraints), total 98.53 wt%, corresponding to the empirical formula (Ca2.60Ce249Nd 1.62La0.80Sm0.44Pr0. 38Gd 0.30Y0.26Dy0.04Yb0.01) Σ8.94(Al3 +0.85Fe3+0.10)Σ0..95(SiO4)3 [SiO 3(OH)]4 (OH)3.06, calculated on the basis of Si = 7. Aluminocerite-(Ce) is trigonal, space group R3c, with a =10.645(1), c = 38.019(5) Å, V = 3731 Å3. The strongest eight lines in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 3.405(27)(122), 3.250(26)(124), 2.914(100)(02,10), 2.647(58)(220), 2.198(40)(03,12), 1.923(34) (238), 1.826(24)(051), and 1.732(46)(03,18). The crystal structure has been refined to R1 = 0.056 for 745 observed reflections. The name is an allusion to the fact that it is the Al-dominant analog of cerite-(Ce).