XRD reflection-intensity ratios in slate slabs as detectors of incipient calcite fabric in slates

Lattice preferred orientation (LPO) of calcite is difficult to measure in rocks rich in mica, since the "basal" 00·1 and "prismatic" hk·0 X-ray reflections of calcite are weak or masked by mica reflections. The calcite reflections 01·8 at 47.5° and 20·2 at 43.2 °2Θ (CuKα radiation) correspond to a flat (e) and a steep (M) rhombohedron, their intensities are oppositely affected by LPO, and they do not coincide with strong reflections of other common phases. The departure of their intensity ratio, IR_{47.5°/43.2°}, in cleavage-parallel (C), bedding-parallel (B), and intersection-normal (⊥I) slabs from that in randomly oriented calcite can therefore be used as an approximate indicator of incipient calcite LPO in phyllosilicate-rich, fine-grained rocks, in which calcite fabric is hard to detect by other means. Sixteen samples of Tertiary flysch with incipient cleavage from the Helvetic zone of the Swiss Alps were investigated. All show low IR_47.5°/43.2° in the ⊥I slabs, indicating a calcite c-axis or e-pole LPO subnormal to I. High IR_{47.5°/43.2°} in the C slabs of most of the samples indicate a large angle of this fabric to cleavage. Relatively high IR_{47.5°/43.2°} in the B slabs in some of the samples reflect persistence of an earlier bedding-normal calcite c-axis/e-pole fabric, in many cases a bedding-parallel cleavage. In the case of small cleavage-bedding (C<B) angles, these cases cannot be unequivocally distinguished, due to the convergence of the IR_{47.5°/43.2°} in the C and B slabs. The method gives the best results at high C<B angles; additional information on the orientation of the c-axis/e-pole LPO fabric is obtained from plots of I_47.5°^*, against I_43.2° ^*, as normalized against _48.6° as a parameter of calcite content in the slabs. Plots of IR_{47.5°/43.2°C}/ IR_{47.5°/43.2°⊥I} vs. IR_{47.5°/43.2°C}/ IR_{47.5°/43.2°B} can be used to assess the intensity of tectonic calcite fabric development in samples with different C<B angles and different pre-deformational bedding-normal calcite c-axis LPO fabrics; the IR_{47.5°/43.2°} C/⊥I ratio is tentatively proposed as an indicator of this intensity. The calcite fabrics detected are much weaker than those of the phyllosilicates, suggesting that the phyllosilicates reorient largely by passive mechanical rotation through removal of matrix by pressure solution.