Ferriallanite-(Ce), CaCeFe3+AlFe2+(SiO4)(Si2O7)O(OH), a new member of the epidote group:: Description, X-ray and Mossbauer study

Ferriallanite-(Ce), ideally CaCeFe3+AlFe2+(SiO4)(Si2O7)O(OH), is the analogue of allanite-(Ce) with Fe3+ dominant in the octahedral site M(1). It is of metasomatic origin and occurs in an alkaline granitic pegmatite of Mount Ulyn Khuren in the Altai Range (about 48degrees32'N, 92degrees55'E), Mongolian People's Republic. Ferri allanite-(Ce) is black, opaque or translucent, with a brown streak; the luster is resinous. The Mobs hardness is 6; it is brittle, without observed cleavage or parting. The fracture is conchoidal to uneven. Its density is 4.22 g/cm(3). It is biaxial (-), alpha 1.825(2), beta 1.855(5), gamma 1.880(5), 2V(calc) 83degrees. The strongest five peaks in the X-ray powder-diffraction pattern [d(Angstrom)(I)(hkl)] are: 2.93(65)((1) over bar 13), 2.72(80)(120,013), 2.63(60)((3) over bar 11), 2.18(100)(122,(1) over bar 23,(4) over bar 01), 2.14(80)(221,(4) over bar 03, (2) over bar 23 ,014). The crystal structure has been refined to R = 0.026 using 2296 observed reflections collected with a four-circle X-ray diffractometer [a 8.962(2), b 5.836(2), c 10. 182(2) Angstrom, beta 115.02(1)degrees, space group P2(1)/m]. The position of the hydrogen atom has been found. On the basis of the chemical data, structure refinement, Mossbauer spectroscopy and charge-distribution calculations, the cations have been assigned to the independent crystallographic sites and the following crystal chemical formula has been obtained: (Ca0.97Ce0.03)(Ce0.89Ca0.11)(Fe0.803+Ti0.144+Al0.06)(Al0.56Fe0.443+) (Fe0.932+Mn0.07)(Si-0.94 Al0.06O4)(Si2O7)O(OH). In the same sample, a single-point electron-microprobe analysis reveals an ideal composition. CaCeFe23+ Fe2+(SiO4)(Si2O7)O(OH), but not enough material is available for a full characterization of this potentially new epidote-group mineral.