Nucleation and growth of new grains in recrystallized quartz vein : an example from banded iron formation in Iron Quadrangle, Brazil.

Abstract

Intracrystalline microcracks developed in quartz single crystals deformed in greenschist metamorphic conditions. A detailed study of samples collected in tabular to lens shape quartz vein was carried out to investigate how the microcracks initiated and how the microstructures evolved with the progressive deformation. A combination of light and EBSD (electron backscatter diffraction) techniques was used to analyze the microstructures and determine the crystallographic orientation of quartz grains. The crystallographic orientations of microcracks indicate that they might have initiated parallel to the direction of one of the rhombohedral planes of the host crystals. It is suggested that new grains nucleated by rotation of broken fragments from the host grains. c-axes the of host are distributed in a small-circle close to the foliation plane while the c-axes of the new grains in microcracks are more scattered when compared with the host orientations. New grains grew with their c-axes approximately perpendicular to the shortening direction.