Abstract: Constraining Authigenic Quartz Precipitation from CL Microscopy and Trace Element Geochemistry

Patrick J. McKeever, Stuart D. Burley, Graham Walker

Quartz overgrowths are a significant authigenic cement in many sandstones worldwide. However, the source of silica, the transport mechanism and the causes of precipitation remain poorly constrained. An improved understanding of the origin of quartz cements can be achieved through a complete petrographic, spectroscopic and geochemical characterization of quartz overgrowths in sandstones.

Cathodoluminescence (CL) emission in alpha-quartz results from a combination of an intrinsic defect centre and a compensated Al³⁺ centre, both of which give rise to a broad blue emission. These two centres have overlapping but distinct emission peaks which can be resolved by comparison of room and low temperature spectra. Furthermore, CL microscopy reveals complex twin and growth zonation within most quartz overgrowths. This zonation has a strong correlation with the concentration of Al³⁺ in the quartz lattice, with areas of most intense emission corresponding to highest Al³⁺ uptake at room temperature.

The availability of aluminum for incorporation in quartz is limited by its ionic co-ordination as only tetrahedrally coordinated sites are present in quartz. Aluminum coordination in aqueous fluids passes from ochtahedral to tetrahedral over a narrow pH range (5.5-6.5 at 25°C; 4.5 at 100°C). Aluminum uptake in quartz varies with crystal growth direction, growth rate and the activity of aluminum in the pore water. The growth zonation in quartz as revealed by CL may therefore reflect changes in pore fluid pH or the temperature of precipitation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994