The Diagenetic to Low-Grade Metamorphic Evolution of Matrix White Micas in the System Muscovite-Paragonite in a Mudrock from Central Wales, United Kingdom*

Gejing Li1, Donald R. Peacor1, R. J. Merriman2 and B. Roberts3
1 Department of Geological Sciences, University of Michigan Ann Arbor, Michigan 48109-1063
2 British Geological Survey, Keyworth, Nottingham NG12 5GG, United Kingdom
3 Geology Department, Birkbeck College, Malet Street, London WCIE 7HX, United Kingdom
*Contribution No. 497 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109-1063.

Abstract: Two orientations of white micas with subordinate chlorite have been observed in a fine-grained (50 Å to 2 µm) 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 (IMd polytype) and larger (100 Å-2 µm) strain-free grains (2M1 polytype). All strained micas and some strain-free grains have compositions varying from Mu86Pg14 to Mu58Pg42, 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 00l reflections (d ≈ 19.6 Å). Micas with such intermediate compositions are metastable. Some packets of coarse, strain-free micas have compositions of approximately Mu93Pg7 or Mu11Pg89. Split pairs of 00l 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 (∼ Mu60Pg40), 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 (2M1 and 3T polytypes) occur as strain-free large grains (200 Å to 2 µm) of discrete muscovite (Mu100Pg0) and paragonite (Mu6Pg94,), 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.

Key Words: Central Welsh basin • Diagenesis • Intermediate Na/K mica • Low-grade metamorphism • Modulated structure • Muscovite • Paragonite • Solvus

Clays and Clay Minerals; August 1994 v. 42; no. 4; p. 369-381; DOI: 10.1346/CCMN.1994.0420401
© 1994, The Clay Minerals Society
Clay Minerals Society (www.clays.org)