Kurt Helming

Complete texture analysis of a deformed amphibolite: comparison between neutron diffraction and U-stage data

Abstract This paper describes a new technique of analysing complex neutron diffraction patterns, allowing the quantitative texture analysis of each rock-forming mineral up to trielinic lattice symmetry in polymineralic rocks. The method is demonstrated using hornblende and plagioclase fabrics in Alzenau amphibolite from the Spessart Mountains (Mid-German Crystalline Rise). Hornblende and plagioclase exhibit a pronounced shape preferred orientation with respect to the macroscopic fabric elements. For control purposes, crystallographic preferred orientations were also determined independently by individual grain measurements using a U-stage. From the neutron texture analyses we obtain only superposed pole figures (hornblende and hornblende, hornblende and plagioclase, and plagioclase with plagioclase) of low-indexed lattice planes. Therefore, the critical crystallographic directions were recalculated by the orientation distribution function (ODF) which is given by texture components. The optical and neutron-derived pole figures are in a good agreement. All pole figures display strong preferred orientation, although the plagioclase texture is less pronounced. The hornblende texture consists of a [001] maximum parallel to the stretching lineation, with [010] showing a tendency to develop a girdle around the lineation. [100]-axes are distributed with a clearly split maximum representing the monoclinic angle between [100] and [001]. The plagioclase texture is a rare one with (001) subparallel within the foliation and [100] oriented subparallel to the lineation. [010] of plagioclase is concentrated normal to lineation within the foliation. The technique outlined here is potentially the most useful one for texture analysis of polyphase rocks, independent of crystal symmetry.

Texture Analysis with Area Detectors

Starting from simple geometric considerations concerning directions and orientations, intelligent strategies for pole figure measurements were developed for the area detector. The amount and quality of texture information contained in measured or available data sets can be directly controlled. The texture approximation is done by the component method. The method does not have any restrictions concerning the grids of sample directions in the pole figures. An almost constant information depth can be obtained at a low angle of incidence of the primary beam for the study of thin surface layers.