Mica (110) Pole Figures in Support of Marchian Behaviour of Phyllosilicates during the Development of Phyllosilicate Preferred Orientation

摘要

Measuring mica (001) preferred orientation is insufficient when a determination of the complete, three-dimensional orientation distribution of phyllosilicates in a phyllosilicate fabric is aimed at. The missing information concerns a possible crystallographically controlled orientation anisotropy within the basal planes of the phyllosilicates. Such an anisotropy may be indicative of contributions of crystalplastic deformation or differential solution-precipitation mechanisms in the overall development of a phyllosilicate preferred orientation. Measuring mica (110) preferred orientation might provide the missing information. The combined knowledge of the preferred orientation of both basal and prism planes of phyllosilicates permits to portray the three-dimensional orientation distribution of the phyllosilicates in a phyllosilicate fabric.rnIn a survey applying X-ray pole figure goniometry, basically two mica (110) pole figure patterns are obtained. Both are linked to a specific mica (001) pole figure pattern. The first type of mica (110) pole figure pattern exhibits perfect girdle symmetry. This girdle is situated at exactly 90° with regard to the corresponding axially symmetrical mica (001) pole figure maximum. In the second type of mica (110) pole figure pattern a pole figure maximum is present in a rather weakly developed girdle. The corresponding mica (001) pole figure pattern shows an orthorhombic symmetry. The short axis of the latter orientation distribution coincides with the mica (110) pole figure maximum.rnTogether with their corresponding mica (001) pole figure patterns, the two types of mica (110) pole figure patterns clearly demonstrate the absence of any crystallographically-controlled orientation anisotropy within the basal planes of the phyllosilicates. On the one hand, the axial symmetry of both mica (001) and corresponding mica (110) pole figure patterns implies a complete rotational freedom of the mica crystals around the pole of the basal plane. On the other hand, the mica (110) pole figure maximum in the second type has to be interpreted as a 'zone axis'. Such a zone axis originates from a superposition of girdle-type mica (110) orientation distributions, related to the different basal plane orientations in the orthorhombic mica (001) pole figure pattern. From this survey it can be concluded that the sole crystallographic control on the phyllosilicate preferred orientation is the determination of the shape anisotropy of the phyllosilicate crystals by the basal planes. In no other way a crystallographic control is apparent. Development of phyllosilicate preferred orientation is completely determined by the shape anisotropy. The obtained three-dimensional orientation distribution of phyllosilicates, as represented by both mica (001) and mica (110) pole figures, shows that the development of a phyllosilicate preferred orientation is still best explained by assuming a Marchian behaviour of the phyllosilicate crystals.