Two orientations of white micas with subordinate chlorite have been observed in a fine-grained (50 Angstrom to 2 mu m) matrix of a Silurian lower anchizonal mudrock from central Wales: one parallel to bedding and one parallel to cleavage that is approximately 30 degrees-50 degrees to bedding. Bedding-parallel micas consist of small (50-200 Angstrom thick) deformed packets (1M(d) polytype) and larger(100 Angstrom-2 mu m) strain-free grains (2M(1), polytype). All strained micas and some strain-free grains have compositions varying from Mu(86)Pg(14) to Mu(58)Pg(42), intermediate to muscovite and paragonite, and falling within the Mu-Pg solvus. Individual packets of layers are chemically homogeneous and some of them give only one set of 001 reflections (d approximate to 19.6 Angstrom). Micas with such intermediate compositions are metastable. Some packets of coarse, strain-free micas have compositions of approximately Mu(93)Pg(7) or Mu(11)Pg(89). Split pairs of 001 reflections with d-values of 20 Angstrom and 19.6 Angstrom, and 20 Angstrom and 19.2 Angstrom, respectively, were observed in some SAED patterns, suggesting coexistence of muscovite and intermediate Na/K mica (similar to Mu(60)Pg(40)), and of discrete muscovite and paragonite, consistent with the splitting of the basal reflections of micas as observed in bulk-rock XRD patterns. Cleavage-parallel micas (2M(1) and 3T polytypes) occur as strain-free large grains (200 Angstrom to 2 mu m) of discrete muscovite (Mu(100)Pg(0)) and paragonite (Mu(6)Pg(94)), often with subhedral to euhedral cross-sections. The data suggest that bedding-parallel metastable micas with disordered interlayer K and Na were initially derived from alteration of smectite during burial diagenesis. They subsequently underwent dissolution, with crystallization of more evolved bedding-parallel micas during deep burial. Discrete grains of stable muscovite and paragonite then crystallized in the cleavage orientation through tectonic stress-induced dissolution of bedding-parallel matrix micas. Combined XRD and TEM/AEM data further show that the so-called 6:4 ordered mixed-layer paragonite/muscovite actually corresponds to cation-disordered, homogeneous mica of intermediate composition.
