Individual orientation measurements in quartz polycrystals: advantages and limitations for texture and petrophysical property determinations

Abstract

Individual orientation determination of quartz grains by electron channelling in principle gives the complete orientation. However, in routine analysis the noise level in electron channelling patterns (ECPs) does not permit the determination of handedness of a quartz grain in a polycrystal. In practice, all quartz grains are arbitrarily indexed as right-handed. Hence, Dauphiné twins can be identified, but not Brazil twins. This practice also means that only the centrosymmetric petrophysical properties can be determined from texture measurements. These include most geologically relevant properties (e.g. thermal conductivity, thermal expansion and elasticity). However, other properties (e.g. piezoelectricity) which are not centrosymmetric cannot be calculated from such texture measurements. Some texture-forming processes (e.g. dislocation glide) can also be considered to be centrosymmetric in quartz, whereas others (e.g. grain boundary migration) may not be. The method of quantitative texture analysis from individual measurements is briefly recalled. As an example, 382 grains from Tongue quartzite are used to illustrate the advantages of texture analysis from ECPs. The orientation distribution function (ODF) is calculated from ECPs and X-ray pole figures of the same sample. The agreement is found to be good between the two methods, proving that ECPs can be used for quantitative analysis. The methods used in local texture analysis and the definitions of the various misorientation distribution functions (MODFs) are given. Data collected from a traverse of a quartzo-feldspathic shear zone in Lewisian gneiss (Torridon ‘quartzite’) are used to illustrate local texture analysis. Examples from a region of shear strain of about one are given of core and mantle subgrains and Dauphiné twins. Dispersion trails of the crystallographic axes within a single grain show an apparent rotation about the intermediate structural axis Y. Detailed analysis of the subgrain misorientation axes in specimen and crystallographic co-ordinates show an important scatter, implying that the subgrains resulted from local incompatibility strains rather than specimen-scale kinematics. The method of calculation of physical properties from individual orientation measurements is given for second-and fourth-order tensors. Using the texture data from Tongue quartzite we have calculated thermal conductivity, thermal expansion and seismic velocities. All these properties are extremely anisotropic in quartz. However, it is emphasized that the presence of a second phase on grain boundaries (e. g. water, graphite) may completely alter a physical property (e.g. electric) and render the values calculated from texture measurements inappropriate.