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

¹ Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira 4, I-50121 Firenze, Italy
² Laboratoire de Minéralogie, Museum National d’Histoire Naturelle, 61 rue Buffon, F-75005 Paris, France

Correspondence: ^* E-mail: pbcry{at}steno.geo.unifi.it

Gatelite-(Ce), ideally (Ca₁REE₃)₌₄[Al₂(Al,Mg)(Mg,Fe,Al)]₌₄[Si₂O₇][SiO₄]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 P2₁/_a, with the following unit-cell parameters: a = 17.770(4), b = 5.651(1), c =17.458(4) Å, ß =116.18(2)°, V = 1573.3(6) ų, and Z = 4. The strongest five powder-diffraction lines [d in Å (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: (Ca_1.09La_0.54Ce_1.36Pr_0.14Nd_0.75Sm_0.11Dy_0.01Y_0.04)_4.04 (Al_3.06Mg_0.51 Fe_0.32²⁺Nb_0.01)_3.90 Si_5.06 O_20.26 (OH)_1.60 F_0.14 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 törnebohmite-type 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.

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