DIVERSILITE-(Ce), Na_2(Ba,K)_6Ce_2Fe~(2+)Ti_3[Si_3O_9]_3[SiO_3OH]_3(OH,H_2O)_9, A NEW SILICATE WITH HETEROGENEOUS TETRAHEDRAL COMPLEXES FROM THE KHIBINA ALKALINE MASSIF, KOLA PENINSULA, RUSSIA

摘要

Diversilite-(Ce) was found in a waste dump from an underground outcrop on Mt Yukspor, Khibiny alkaline massif, Kola peninsula, Russia, in a strongly mineralized rischorrite-ijolite-urtite pegmatite. Associated minerals are: nepheline, sodalite, potassium feldspar, natrolite, pectolite, aegirine, scherbakovite, lamprophyllite, magnesium astrophyllite, delindeite, wadeite, umbite and kostylevite. It occurs as a fan-shaped intergrowth (2--3 mm) of subhedral platy [on (001)] crystals (1--2 mm). Color yellowish orange. Streak white. Lustre vitreous. Transparent or translucent. Brittle. Fracture conchoidal. H. 5. D (meas.) 3.86(2) g/cm~3, D (cale.) 3.72 g/cm~3. Optically uniaxial or slightly biaxial, positive: omega = 1.705(2), epsilon = 1.708(2). Strongly pleochroic: X pale yellow, Z yellowish orange, absorption Z > X. Trigonal, space group R 3-bar, a = 10.713(1), c =60.67(1) A, Z = 6. Chemical composition (microprobe, H_2O wet chemical) at Si = 12 corresponds to the empirical formula Na_(1.98)(Ba_(3.60)K_(2.74))_(SIGMA 6.34) (REE_(1.85)Mn_(0.12)Sr_(0.09)Ca_(0.04))_(SIGMA2.10)(Fe_(0.96)~(2+)Mn_(0.04))_(SIGMA 1.00)(Ti_(2.55)Nb_(0.27)Mn_(0.18))_(SIGMA 3.00)Si_(12)O_(36)[(OH)_(7.88)(H_2O)_(3.74)]_(SIGMA 11.62). Simplified formula Na_2(Ba,K)_6Ce_2Fe~(2+)Ti_3Si_(12)O_(36) (OH,H_2O)_(12). The results of crystal structure determination at Z = 6 correspond to the crystal chemical formula Na_2(Ba,K)_6Ce_2Fe~(2+)Ti_3 [Si_3O_9]_3[SiO_3OH]_3(OH,H_2O)_9. Diversilite-(Ce) is the first structurally studied silicate mineral with isolated triple [Si_3O_9] rings and discrete [SiO_3OH] tetrahedra, as reflected in its name (from Latin diversus -- heterogeneous -and silicate). On the basis of the X-ray powder diffraction pattern and other properties, it is similar to ilimaussite-(Ce) but shows a predominance of Ti over Nb and K over Na.