Interphase misorientation as a tool to study metamorphic reactions and crystallization in geological materials

Abstract

Interphase boundaries are planar defects that separate two different minerals, which in general have different compositions and/or crystalline structures and may play an important role as a pathway for fluids in rocks and affect their physical properties. For the proper characterization of interphase boundaries, one needs to define the misorientation between adjacent grains and the orientation of the grain boundary plane, but the analysis performed here are only limited to the misorientation characterization and the trace of the interphase boundary. Although the determination of possible orientation relationships between the two adjacent phases is routinely performed by selected area electron diffraction in the transmission electron microscope, this method lacks statistical representativeness. With the advent of techniques like electron backscatter diffraction (EBSD), it is possible to calculate orientation relationships not only in single pairs of crystals, but in full thin sections, and not limited to single phases, but also between different minerals. The interphasee misorientation is calculated from two orientations of two adjacent crystals of different phases. A set of single misorientations is then used to calculate the misorientation distribution function (MDF), from where it is possible to identify a maximum, and their crystallographic interpretation. If we then know the misorientation and the unit cell parameters of the individual phases, the crystallographic relationships between them can be described with the pairs of parallel crystallographic planes and the pairs of crystallographic directions. We present examples of the use of interphase misorientation analysis on the transformation of calcite-aragonite, olivine-antigorite, magnetite-hematite, and on the study of orientation relationships between plagioclase-olivine-ilmenite in mid-ocean ridges gabbros (ODP Hole 735).