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Gatelite-(Ce), ideally (Ca 1 REE 3 ) Σ=4 [Al2(Al,Mg)(Mg,Fe,Al)] Σ=4 [Si 2 O 7 ][SiO 4 ] 3 (O,F) (OH,O) 2 , is a newly identified mineral from the Trimouns talc deposit, Luzenac, Ariège, French Pyrenees. The mineral occurs as striated colorless crystals finely intergrown with minute lamellae of törnebohmite- (Ce). Associated minerals include pyrite, aeschynite-(Y), dolomite, törnebohmite-(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 P2 1 /a, with the following unit-cell parameters: a = 17.770(4), b = 5.651(1), c =17.458(4) Å, b =116.18(2)°, V = 1573.3(6) Å 3 , and Z = 4. The strongest five powder-diffraction lines [d in Å (I/Io) (hkl)] are 15.67 (87) (001); 3.49 (50) (4̅12); 2.97 (100) (2̅15); 2.83 (44) (020); and 2.61 (56) (6̅12). Electron-microprobe analysis supported by single-crystal structure determination yielded the following empirical formula: (Ca 1.09 La 0.54 Ce 1.36 Pr 0.14 Nd 0.75 Sm 0.11 Dy 0.01 Y 0.04 ) Σ4.04  (Al 3.06 Mg 0.51  Fe 2+  0.32 Nb 0.01 ) Σ3.90  Si 5.06  O 20.26  (OH) 1.60  F 0.14 . The calculated density (from the empirical formula) is 4.51 g/ cm 3 . The structure was solved by direct methods and refined to R obs  = 4.65%. It consists of edgesharing octahedral chains running along the b axis, cross-linked by SiO 4  and Si 2 O 7  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 törnebohmitetype structure (T) parallel to the (001) plane. Gatelite-(Ce) can be regarded as a ET polisome within a polysomatic series having epidote and törnebohmite as end-members.

american mineralogist

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