Gatelite-(Ce), a new REE-bearing mineral from Trimouns, French Pyrenees: Crystal structure and polysomatic relationships with epidote and törnebohmite-(Ce)

Gatelite-(Ce), ideally (Ca1REE3)e=4[Al2(Al,Mg)(Mg,Fe,Al)]e=4[Si2O7][SiO4]3(O,F) (OH,O)2, is a newly identified mineral from the Trimouns talc deposit, Luzenac, Ariege, French Pyrenees. The mineral occurs as striated colorless crystals finely intergrown with minute lamellae of tornebohmite-(Ce). Associated minerals include pyrite, aeschynite-(Y), dolomite, tornebohmite-(Ce), dissakisite-(Ce), talc, and quartz. Gatelite-(Ce) is insoluble in HCl, relatively hard (Mohs hardness 6–7), brittle with irregular fracture, and exhibits good {100} and imperfect {001} cleavages. Gatelite-(Ce) is monoclinic, space group P 21/ a , with the following unit-cell parameters: a = 17.770(4), b = 5.651(1), c =17.458(4) A, β =116.18(2) ° , V = 1573.3(6) A3, and Z = 4. The strongest five powder-diffraction lines [ d in A ( I/I o) ( hkl )] are 15.67 (87) (001); 3.49 (50) (4−12); 2.97 (100) (2−15); 2.83 (44) (020); and 2.61 (56) (6−12). Electron-microprobe analysis supported by single-crystal structure determination yielded the following empirical formula: (Ca1.09La0.54Ce1.36Pr0.14Nd0.75Sm0.11Dy0.01Y0.04)e4.04 (Al3.06Mg0.51 Fe0.322+Nb0.01)e3.90 Si5.06 O20.26 (OH)1.60 F0.14 The calculated density (from the empirical formula) is 4.51 g/cm3. The structure was solved by direct methods and refined to R obs = 4.65%. It consists of edge-sharing octahedral chains running along the b axis, cross-linked by SiO4 and Si2O7 groups. The remaining large cavities are occupied by Ca and REE. The structure of gatelite-(Ce) can be easily described as a regular alternance of slabs of epidote-type structure (E) and slabs of tornebohmite-type structure (T) parallel to the (001) plane. Gatelite-(Ce) can be regarded as a ET polisome within a polysomatic series having epidote and tornebohmite as end-members.