Metamorphic cookeite in Alpine metapelites from Rif, northern Morocco, and the Betic Chain, southern Spain

Cookeite is reported from low-T, low-P and from low-T, high-P Permo-Triassic metapelites of the Sebtides and Alpujarrides nappes in the Rif and Betic mountains. It occurs associated with pyrophyllite in the lowest-grade units and is essentially a product of kyanite alteration in the highest-grade units. EPMA and ion microprobe analyses show large compositional variations: Si ranges from 3 to 3.76 atoms pfu, Al from 5.51 to 5, and R (super 2+) from 0.03 to 0.87, while Li (sub 2) O ranges 1.95-2.70 wt.%. These variations are interpreted as a combination of the intergrowth of pyrophyllite with cookeite, partial alteration to kaolinite and the R (super 2+) SiAl (sub -2) (Tschermak) and LiAlR (super 2+) (sub -2) (cookeite-sudoite) substitutions; the substitution towards the sudoite end-member reaches 30 mole %. The structural and compositional variations revealed by TEM could be related to metamorphic variations; under low-grade, low-P metamorphic conditions, cookeite is expected to be highly substituted and structurally disordered whereas it is near end-member composition and highly ordered under high-P, low-T conditions.