Mechanism of Plagioclase Albitization

Abstract

ABSTRACT Observations of closely spaced porous and cemented sandstones from the San Joaquin basin, the Los Angeles basin and the Gulf Coast reveal that albitization of plagioclase occurs during fracturing of the detrital plagioclase grains. Albitization is found to be a chemically selective hydrous reaction in which An-rich plagioclase is preferentially albitized first relative to An-poor plagioclase. The observed non-positive volume change during albitization indicates that aluminum mobility takes place on a scale at least as large as a thin section. These findings are supported by thermodynamic stability calculations for plagioclase with different compositions and degrees of aluminum-silicon ordering at 25°C in a representative sea water and at 100°C in an oil field water. The calculations predict that 1) An-rich plagioclase is less stable than An-poor plagioclase, and 2) plagioclase with a random Al/Si distribution is less stable than plagioclase with the same composition but an ordered Al/Si distribution. Any composition and structural state of plagioclase, including albite, is unstable in sea water at 25°C. The width of the albitization zone may be controlled by the degree of aluminum-silicon ordering. This agrees well with the observed smaller temperature interval of albitization in sandstones containing only volcanically derived plagioclase.