EBSD AND OPTICAL MICROSCOPE LINKED THROUGH OPTICAL MODELLING OF CRYSTALLOGRAPHICAL DATA FROM EBSD AND 3D VISUALIZATION OF OPTICAL PROPERTIES

The optical microscope has played a key role in mineral identification by optical properties and texture/microstructure description. Inference color is a key optical property, which varies with mineral orientation and sample thickness. The characteristic birefringence corresponds to the maximum value (n-gamma - n-alpha) and which varies with mineral orientation, and requires several measurements to estimate a characteristic value. This poses a challenge for most geology students as they are looking at a 3D property in a 2D section, with minerals cut in different directions. To improve the understanding of this a MatLab based code was developed to illustrate the 3D character of the interference color. We implemented in the using the toolbox MTEX [2]. This enables several simulations to be calculated based on EBSD data of crystal orientations, such as simulation of optical crossed polarised images, and interference color [1], combined with the MTEX functions [2]. The simulated crossed polar images can also be used to verify that reference frame has been set up correctly for the ESBD data collection because the inference color depends on both the mineral and the orientation and the simulated and observed optical images should match. Another application is to simulate the 3D optical properties on spherical projections of inference colors. This also allows demonstrating how the isogyres in the optical interference figure moves as a function of mineral rotation on the spherical projection. Examples calcite, quartz and olivine are presented, but the method can be extended to other mineral groups where the refractive indexes and the relation to crystal axes are known. Using 3D inversion procedure we might be able to determine optical data with constraints from EBSD data and quantify lattice preferred orientations from optical images.

1. Sørensen BE 2013, European Journal of Mineralogy. Volume 25, Number 1, 5-10.
2. Bachmann F, et al. 2010, Solid State Phenomena, 160, 63-68.