The diagenetic to low-grade metamorphic evolution of matrix white micas in the system muscovite-paragonite in a mudrock from central Wales, United Kingdom

Two orientations of white micas with subordinate chlorite have been observed in a fine-grained (50 Å to 2 μ m) matrix of a Silurian lower anchizonal mudrock from central Wales: one parallel to bedding and one parallel to cleavage that is approximately 30°-50° to bedding. Bedding-parallel micas consist of small (50-200 Å thick) deformed packets (1M d polytype) and larger (100 Å-2 μ 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 00 l reflections (d ≈ 19.6 Å). 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 00 l reflections with d-values of 20 Å and 19.6 Å, and 20 Å and 19.2 Å, respectively, were observed in some SAED patterns, suggesting coexistence of muscovite and intermediate Na/K mica (∼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 Å to 2 μ 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.