A STUDY OF DEFORMATION-ENHANCED DISSOLUTION IN THEORY, EXPERIMENT AND NATURE BASED ON MICROSTRUCTURAL EVIDENCE (CRYSTALLOGRAPHY)

Evidence of a heterogeneous diffusive mass transfer mechanism, often called "pressure solution", has been observed in rocks deformed under conditions ranging from diagenetic to medium grade metamorphic. Solution cleavage seams in the Scaglia Rossa limestone from the Umbria-Marche Appenines, Italy have been attributed to "pressure solution". This transmission electron microscope (TEM) study investigates primarily the limestone microstructures associated with these seams, but also describes some phyllosilicate features. Although no evidence of permanent deformation was observed around the seams with an optical microscope, the TEM resolved dislocations concentrated within limited areas of single grains. Microstructures from the natural samples were compared with experimentally deformed calcite microstructures and with computer simulations of TEM dislocation contrast. Observations suggested that selective dissolution of localized dislocation tangles was responsible for solution cleavage formation. Similar microstructural evidence associated with the stress corrosion cracking, most commonly documented in alloys, supported this interpretation. Further evidence indicated that a solution cleavage surface propagates laterally from its tip, like a fracture.; A comparative experimental study investigated a single crystal's response to a stress gradient and its relationship to crystal orientation. The occurrence and location of heterogeneous permanent deformation depended on crystal orientation, and could be predicted by a method that could be applied to any crystal. In addition, dense concentrations of r-slip dislocations preceded evidence of macroscopic strain in one set of cylinders. Notably, the calculated resolved shear stress for r-slip remained far below the corresponding experimentally determined critical resolved shear stress (CRSS) after causes of apparent low stress measurements were considered. The significant difference underscored the need to distinguish dislocation activation stress from experimental determinations of CRSS.; This study attempted to reconcile the grain scale TEM evidence with the aggregate scale optical evidence to explain seam morphology and orientation dependence on regional stress. It was suggested that solution cleavage resembled dissolution related forms of slow stable crack propagation and may be unique to "transition" zones between fracture and flow styles of deformation. There, grains have the greatest potential for heterogeneous permanent deformation, which provides the relative metastability for heterogeneous dissolution.