Metamorphic cookeite in Alpine metapelites from Rif, northern Morocco, and the Betic Chain, southern Spain
Goffé, Bruno; Azañon, Jοsé Miguel; Bouybaouene, Mοhamed Larbi; Jullien, Michel
European Journal of Mineralogy Volume 8 Number 2 (1996), p. 335 - 348
published: Apr 26, 1996
manuscript accepted: Nov 22, 1995
manuscript received: Mar 28, 1995
ArtNo. ESP147050802006, Price: 29.00 €
Abstract Cookeite, LiAl4[AlSi3O10](OH)8 is reported for the first time from Permo-Triassic metapelites of the Sebtides and Alpujarrides nappes in the Rif and Betic mountains. Cookeite occurs in low-pressure, low-temperature as well as high-pressure, low-temperature metamorphic terranes. In the Rif chain, it occurs in the upper (Tizgarine) and lower (Beni Mzala) units of the Beni Mzala window. In the Betic chain its occurrences are widespread in the lower Alpujarrides units (Escalate and Salobrena units) between Granada and Almeria. Cookeite is associated with pyrophyllite in the lowest-grade units and is essentially a product of kyanite alteration in the highest-grade units. Electron and ion microprobe analyses show large compositional variations: Si ranges from 3 to 3.76 atoms per formula unit, Al from 4.51 to 5 and R2+ (= Fe + Mg + Mn) from 0.03 to 0.87 while the Li2O content varies from 1.95 to 2.70 wt%. These variations are interpreted as the combination of intergrowth of pyrophyllite with cookeite, of partial alteration to kaolinite and of the R2+SiAl-2 (Tschermak) and LiAlR2+-2 (cookeite-sudoite) substitutions. The substitution toward the sudoite end-member reaches 30 mole %. The structural and compositional variations revealed by TEM observations could be related to the metamorphic variations. Under low-grade, low-pressure metamorphic conditions, cookeite is expected to be highly substituted and structurally disordered while it is near end-member and highly ordered under high-pressure and low-temperature conditions.