Polytypism of cookeite in low-grade metapelites of the Cameros Basin, Spain: Lack of correlation of well-ordered polytypes with pressure

Cookeite occurs in low-grade, chloritoid-bearing metapelites of the Cameros Basin, northern Iberian Range, Spain, as thin coatings on poikiloblastic pyrite crystals <25 cm on edge. Textural relations imply formation via hydrothermal fluids in a syn- to late-regional metamorphic environment. Selectedarea electron diffraction patterns of cookeite display a wide range of stacking order-disorder, from patterns with streaking parallel to c* in k ≠ 3n reflections to sharply defined reflections having one-, two-, and greater than two-layer periodicities. Well-ordered polytypes in crystals up to 0.1 µm thick are of type Ia, as determined by powder X-ray diffraction. Coexisting chamosite likewise displays a range of order in polytypism from complete disorder to the well-ordered IIb polytype which is typical of low-grade metamorphism.