TEM and AEM constraints on he origin and significance of chlorite-mica stacks in slates: an example from central

Abstract--Chlorite-mica stacks (grains of intergrown chlorite and white mica) in a matrix of fine-grained white mica and chlorite have been studied using XRD, SEM, EMPA, TEM and AEM methods. The stacks occur in a weakly cleaved Llandoverian mudstone, central Wales, which has a white mica (illite) crystallinity index of 0.35 ° A20 corresponding to the lower anchizone. White mica occurs as packets (100/ ~ to 8 #m thick) interleaved with dominant chlorite packets in stacks, with both apparent coherent interfaces or cross-cutting small angle boundaries with chlorite layers. It is well-crystallized 2M ~ polytype with phengitic composition and low paragonite component [Na/(Na+K)-< 0-0.07]. Chlorite in stacks is Fe-rich and relatively homogeneous. TiO2 crystals urround stacks and occur within chlorite, and white mica is Ti-rich compared to matrix white mica. Fine-grained white mica and chlorite in the matrix have two different orientations: one parallel to bedding and one parallel to cleavage, which is approximately 30-50 °to bedding. Matrix white mica is predominantly a 2M1 polytype, but some cleavage-parallel white mica is 3 T and some bedding-parallel white mica is 1Md. It is Na-rich [Na/(Na+ K)- 0.14-0.42] and relatively heterogeneous; some discrete paragonite and phengitic muscovite are observed to coexist in the cleavage orientation. Matrix white mica and chlorite each contain less Fe than corresponding white mica and chlorite in stacks. Both matrix and stack chlorite are one-layer polytypes. The data imply that chlorite in the stacks is largely derived from the replacement of volcanogenic biotite an