A New Interpretation of Round Embayments in Quartz Crystals

C. H. Donaldson and C. M. B. Henderson
Department of Geology, University of St Andrews, St Andrews, Fife KY16 9ST
Department of Geology, University of Manchester, Manchester M13 9PL

Abstract: The surfaces of quartz crystals that were partially dissolved in superheated, H2O-saturated rhyolite melt are covered with hemispherical embayments; each embayment is judged to have formed where a gas bubble in the melt approached the crystal. ‘Flux-line attack’ and ‘upward-drilling’ of the refractory lining of glass tanks are analogous processes. As a bubble nears a dissolving solid it enters a compositional boundary layer in the melt, resulting in melt of variable surface tension surrounding the bubble. This unstable situation results in small-scale convection of the melt about the bubble (Marangoni convection) which can cause locally enhanced dissolution rate of the solid. It is suggested that this mechanism could cause round embayments to form in quartz phenocrysts in acid volcanic and sub-volcanic rocks. Criteria by which embayed phenocrysts formed by dissolution can be distinguished from those formed by unstable growth are reviewed briefly.

Keywords: quartz • magmatic dissolution • embayment • crystal-liquid interface • surface tension • Marangoni convection

Mineralogical Magazine; March 1988 v. 52; no. 364; p. 27-33; DOI: 10.1180/minmag.1988.052.364.02
© 1988, The Mineralogical Society
Mineralogical Society (www.minersoc.org)