ALBITIZATION OF PLAGIOCLASE CRYSTALS IN THE STEVENS SANDSTONE (MIOCENE), SAN-JOAQUIN BASIN, CALIFORNIA, AND THE FRIO FORMATION (OLIGOCENE), GULF-COAST, TEXAS - A TEM AEM STUDY

Conventional Transmission Electron Microscopy (CTEM) and Analytical Electron Microscopy (AEM) studies of partially albitized plagioclase crystals taken from drill cores from the Stevens sandstone (Miocene), San Joaquin, California, and the Frio Formation (Oligocene), Gulf Coast, Texas, reveal that replacement of Ca-rich plagioclase cores by nearly pure albite (Ab96-Ab100) occurs along submicroscopic (approximately 15 nm wide) en echelon (001) and (110) cleavages. The cleavages are the result of changes in the localized stress regime created by dissolution of adjacent phases. Photomicrographs show albite-lined brittle cleavage crosscutting albitized semibrittle fractures. Such crosscutting relationships can be explained by a reduction in effective stress associated with the albitization process. On a macroscopic scale, this reduction in effective stress implies that the transition from hydrostatic to lithostatic pressure is discontinuous.