CERITE-(La), (La,Ce,Ca)9(Fe,Ca,Mg)(SiO4)3[SiO3(OH)]4(OH)3, A NEW MINERAL SPECIES FROM THE KHIBINA ALKALINE MASSIF: OCCURRENCE AND CRYSTAL STRUCTURE

Abstract

Cerite-(La), ideally (La,Ce,Ca)(9)(Fe,Ca,Mg)(SiO4)(3)[SiO3(OH)](4)(OH)(3), was found in an aegirine-natrolite-microcline vein in foyaite, Mt. Yuksporr, Khibina massif, Kola Peninsula, Russia. It forms porous pseudomorphs (up to 7 cm long) after an unidentified hexagonal prismatic mineral. Within the pseudomorphs, cerite-(La) occurs as a boxwork-like aggregates of equant to tabular crystals (flattened on c, up to 2 mm across), with dominant rhombohedral and pinacoidal faces. Associated minerals are aegirine, anatase, ancylite-(Ce), barylite, catapleiite, cerite-(Ce), chabazite-Ca, edingtonite, fluorapatite, galena, ilmenite, microcline, natrolite, sphalerite, strontianite and vanadinite. The mineral is brittle, with a conchoidal fracture. It is light yellow to pinkish brown with a white streak, and translucent with a vitreous luster. Cleavage and parting were not observed. The Mobs hardness is 5; the density is 4.7(1) g cm(-3) (meas.) and 4.74 g cm(-3) (calc.). It is uniaxial, optically positive, n(o) 1.810(5), n(e) 1.820(5) (for lambda=589 nm). An average result of seven electron-microprobe analyses for seven different crystals (each one reported as an average of 6-10 points from each crystal) gave La2O3 37.57, Ce2O3 23.67, Pr2O3 0.61, Nd2O3 1.48, Sm2O3 0.10, Gd2O3 0.24, SrO 1.97, CaO 5.09, Fe2O3 1.40, MgO 0.51, SiO2 22.38, P2O5 0.63, H2O 3.20 (determined by the Penfield method), total 98.85, which corresponds to the empirical formula (La4.23Ce2.65Ca1.37Sr0.35Nd0.16Pr0.07Gd0.02Sm0.01)(Sigma8.86) (Fe0.32Ca0.30Mg0.23)(Sigma0.85) [SiO4](3)[(Si0.84P0.16)(Sigma1.00)O-3(OH)](4)(OH)(2.78), calculated on the basis of (Si+P)=7. The mineral is trigonal, R3c, with a 10.7493 (6), c 38.318 (3) Angstrom, V 3834.36 Angstrom(3), Z=6. The, strongest eight lines in the X-ray powder-diffraction pattern [d in Angstrom(I)(hkl)] are: 3.53(26)(10.10,211), 3.47(40)(122), 3.31(38)(214), 3.10(25)(300), 2.958(100)(02.10), 2.833(37)(128), 2.689(34)(220), 1.949(34)(238,13.13). The crystal structure has been refined on the basis of F-2 for all unique reflections collected using a CCD area detector, to an R1 of 0.036, calculated for the 1544 unique observed reflections (F(o)greater than or equal to4sigmaF(o)). Cerite-(La) is structurally related to cerite-(Ce). The mineral is named as the La-dominant analogue of cerite-(Ce). The structural formula for these minerals can be written as REE9MCax[SiO4](3)[SiO3((OH)(1-x))](3)[SiO3(OH)](1-x)(OH)(3), where REE stands for La or Ce, and where M represents Fe, Mg, Ca.