The development of grain boundary orientation distribution in quartz.

Abstract

The fabric of high-angle boundaries in recrystallized quartz aggregates deformed under distinct strain and metamorphic conditions has been investigated in samples from iron formations recrystallized under lower greenschist to lower amphibolite facies conditions. Orientation of grain boundaries was measured using U-stage, while crystallographic fabric and misorientation between grains were measured using EBSD. In domains where grains were recrystallized under lower greenschist facies conditions, grain boundaries are sutured, composed of several small straight segments, whose traces can be parallel to traces of any crystallographic plane. In contrast, in domains of upper-greenschist–lower-amphibolite facies, grain boundaries are predominantly composed of long straight segments and belong to girdle of rhombohedral planes, while segments within girdle of basal or prism planes are rare. The longest segments (larger than 270 mm) are commonly within girdle of rhombohedral planes for the two grains that share a boundary surface, possibly representing Dauphine´ twin boundaries. Under higher temperature condition of the lower amphibolite facies (500–550 C), boundary segments are longer and tend to represent coincident crystallographic orientations for the two grains that share a boundary.