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European Journal of Mineralogy; May, June 2005; v. 17; no. 3; p. 475-481; DOI: 10.1127/0935-1221/2005/0017-0475
© 2005 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Articles

Crystal structure and chemistry of trilithionite-2M2 and polylithionite-2M2

Maria Franca BRIGATTI1,*, Enrico CAPRILLI1, Daniele MALFERRARI1, Luca MEDICI2 and Luciano POPPI1

1 Dipartimento di Scienze della Terra dell'Università di Modena e Reggio Emilia, Via S. Eufemia 19, I-41100 Modena, Italy
2 CNR – Istituto di Metodologie per l'Analisi Ambientale, C.da S. Loja, I-85050 - Tito Scalo (PZ), Italy

* E-mail: brigatti{at}unimore.it

The crystal chemistry of three Li-bearing mica-2M2 crystals from pegmatites has been studied by chemical analyses and single crystal X-ray diffraction; their belonging to the trilithionite-polylithionite join is highlighted by the following compositional ranges in atoms per formula unit [based on O12-(x+y) (OH)xFy]: 3.198 ≤ Si ≤ 3.538, 0.462 ≤ [IV]Al ≤ 0.811, 1.195 ≤ [VI]Al ≤ 1.390, 0.031 ≤ (Fe+Mn) ≤ 0.072, 1.522 ≤ Li ≤ 1.757, 0.872 ≤ K ≤ 0.906, 0.030 ≤ Na ≤ 0.073, 0.000 ≤ (Cs+Rb) ≤ 0.099, 1.541 ≤ F ≤ 1.722. The correlation between F and Li content is confirmed, as observed in Li-rich micas.

Crystal structure refinements were carried out in space group C2/c (R values vary between 0.030 and 0.031). The crystal chemistry is mostly influenced by tetrahedral chemical composition. Increasing [IV]Al content, {alpha} and {psi}M1 parameters increase; Si content involves a lowering of the interlayer separation and tetrahedral thickness. Li content affects octahedral thickness. The stability of 2M2 polytype seems to be induced by a relative increase of {Delta}z tetrahedral parameter, which reduces the repulsion between basal tetrahedral oxygen atoms. Unlike Li-bearing muscovite, trioctahedral Li-bearing mica crystals show an octahedral occupancy not related to octahedral charge.

Key-words: mica, trilithionite, polylithionite, lithium, 2M2 polytype.




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