Electron Backscatter Diffraction (EBSD) Analysis of Bassanite Transformation Textures and Crystal Structure Produced from Experimentally Deformed and Dehydrated Gypsum

Dehydration reactions have important effects on fluid flow, pore pressure and brittle failure in the Earth. The rates and mechanisms of reaction are relevant to understanding those effects. The microstructures of a set of samples containing bassanite and gypsum from dehydration experiments on Volterra gypsum have been analysed by conventional transmitted light microscopy and electron backscatter diffraction (EBSD). These techniques have been used to characterize the varying crystal morphology and to measure the crystallographic preferred orientation (CPO) of the samples. In some samples bassanite grains show two distinct morphologies; large (0.5-2 mm), euhedral crystals and small (< 50 mu m), acicular crystals: these represent the two different bassanite crystal habits. Bassanite was indexed using EBSD for the first time, and problems related to significant mis-indexing resulting from the pseudotrigonal symmetry of bassanite are resolved. The original starting material has a strong, local and measurable CPO. A strong CPO in post-experiment bassanite is found to mimic the original gypsum c-axis orientation. Bassanite grain size is related to the degree of reaction overstep. Spatial heterogeneity in bassanite distribution may indicate the development of fluid pathways within the samples.