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Geology; May 2002; v. 30; no. 5; p. 431-434; DOI: 10.1130/0091-7613(2002)030<0431:OOTQIT>2.0.CO;2
© 2002 Geological Society of America
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Origin of "toasted" quartz in terrestrial impact structures

James Whitehead*,1, John G. Spray*,1 and Richard A.F. Grieve*,2

1 Planetary and Space Science Centre, Department of Geology, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
2 Earth Sciences Sector, Natural Resources Canada, Ottawa, Ontario K1A 0E4, Canada

Quartz present in the target rocks at impact sites and as clasts within impact melt rocks and some pseudotachylytes commonly exhibits a brown, nonpleochroic grainy appearance in plane- and cross-polarized light, known as "toasted" quartz. This variety of quartz also possesses a higher albedo in hand sample than colorless examples. Despite the intense coloration, no compositional origin for the browning is evident. A high proportion of exceptionally small (typically submicron scale) fluid inclusions, which are typically too small to be observed with the petrographic microscope, is considered to be the cause. These fluid inclusions are predominantly located along decorated planar deformation features, and they enhance scattering of transmitted light and reflect larger quantities of incident light. Statistically lower SiO2 and total oxide contents present in toasted quartz, relative to untoasted quartz, are consistent with increased surface electron scattering during microprobe analysis by fluid inclusions intersecting the surface.

Key Words: quartz • impact • shock • toasted • metamorphism




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