An optical method for the determination of 〈a〉 axis orientations in deformed aggregates of quartz

Knowledge of 〈a〉 axis orientation is of paramount importance to the understanding of deformation mechanisms in quartz and by extension, quartz rich rocks. The positions of the 〈a〉-axis cannot be measured by standard optical (U-stage) techniques since quartz is optically uniaxial. Many quartz grains, however, contain abundant fluid inclusions and in many instances, these fluid inclusions occupy negative crystal-shaped cavities for which the 〈a〉-axis can be optically determined. This study has utilized these negative crystals to locate 〈a〉-axis positions using a U-stage. In order to test the validity of this approach a blind comparison was performed on a deformed quartz aggregate from the Simplon Fault Zone (Central Alps: Switzerland). Both 〈a〉 and 〈c〉 axis positions for this sample were measured using the proposed optical technique and via an X-ray texture goniometer. The locations for 〈a〉 axis maxima contoured on a Schmidt net using the two techniques agree closely, demonstrating that the optical technique was successful. Advantages of the optical technique are (1) that it is simple and requires inexpensive equipment and (2) that 〈a〉 axis positions can be determined on a grain-by-grain basis allowing for the detailed study of crystallographic orientation differences across grain boundaries in deformed aggregates of quartz.