INDIVIDUAL ORIENTATION MEASUREMENTS IN QUARTZ POLYCRYSTALS - ADVANTAGES AND LIMITATIONS FOR TEXTURE AND PETROPHYSICAL PROPERTY DETERMINATIONS

Individual orientation determination of quartz grains by electron chan nelling in principle gives the complete orientation. However, in routi ne analysis the noise level in electron channelling pattems (ECPs) doe s not permit the determination of handedness of a quartz grain in a po lycrystal. In practice, all quartz grains are arbitTarily indexed as r ight-handed. Hence, Dauphine twins can be identified, but not Brazil t wins. This practice also means that only the centrosymmetric petrophys ical properties can be determined from texture measurements. These inc lude most geologically relevant properties (e.g. thermal conductivity, thermal expansion and elasticity). However, other properties (e.g. pi ezoelectricity) which are not centrosymmetric cannot be calculated fro m such texture measurements. Some texture-forming processes (e.g. disl ocation 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 brie fly recalled. As an example, 382 grains from Tongue quartzite are used to illustrate the advantages of texture analysis from ECPs. The orien tation distribution function (ODF) is calculated from ECPs and X-ray p ole figures of the same sample. The agreement is found to be good betw een the two methods, proving that ECPs can be used for quantitative an alysis. The methods used in local texture analysis and the definitions of the various misorientation distribution functions (MODFs) are give n. 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 Dauphine twins. Dispersion trails of the crystallographic axes within a single grain show an app arent rotation about the intermediate structural axis Y. Detailed anal ysis of the subgrain misorientation axes in specimen and crystallograp hic co-ordinates show an important scatter, implying that the subgrain s resulted from local incompatibility strains rather than specimen-sca le kinematics. The method of calculation of physical properties from i ndividual orientation measUTements is given for secondand fourth-order tensors. Using the texture data from Tongue quartzite we have calcula ted thermal conductivity, thermal expansion and seismic velocities. Al l 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. el ectric) and render the values calculated from texture measurements ina ppropriate.