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Crack healing as a function of pOH- and fracture morphology

by Fallon, Jessica Anne

Abstract (Summary)
Crack healing in quartz has been investigated by optical microscopy and interferometry

of rhombohedral ( 1 1 10 ) cracks in polished Brazilian quartz prisms that were annealed

hydrothermally at temperatures of 250°C and 400°C for 2.4 to 240 hours, fluid pressure

Pf = Pc = 41 MPa, and varying pOH- (from 5.4 to 1.2 at 250°C for fluids consisting of

distilled water and NaOH solutions with molalities up to 1). Crack morphologies before

and after annealing were recorded for each sample in plane light digital images. Crack

apertures were determined from interference fringes recorded using transmitted

monochromatic light (l = 598 nm). As documented in previous studies, crack healing is

driven by reductions in surface energy and healing rates are governed by diffusional

transport; sharply defined crack tips become blunted and split into fluid- filled tubes and

inclusions. A rich variety of fluid inclusion geometries are also observed with nonequilibrium

shapes that depend on initial surface roughness.

Crack healing is significant at T=400°C. Crack healing is also observed at T=250°C for

smooth cracks with apertures <0.6 mm or cracks subject to low pOH-. The extent of

crack healing is sensitive to crack aperture and to hackles formed by fine-scale crack

branching during earlier crack growth. Crack apertures appear to be controlled by

hackles and debris, which prop the crack surfaces open. Upon annealing, crack

apertures are reduced, and these reduced crack apertures govern the kinetics of

diffusional crack healing that follows. Hackles are sites of either enhanced or reduced

loss of fluid-solid interface, depending on slight mismatches and sense of twist on

opposing crack surfaces. Hackles are replaced either by healed curvilinear quartz

bridges and river patterns surrounded by open fluid-filled crack, or by fluid- filled tubes

surrounded by regions of healed quartz. For a given temperature, aperture and anneal

time, crack healing is enhanced at low pOH- ( £ 1.2) either because of changes in the

hydroxylated quartz- fluid interface that enhance reaction rates or because of increased

rates of diffusional net transport of silica at high silica concentrations.

Bibliographical Information:

Advisor:Kronenberg, Andreas K.; Lamb, William; Russell, James; Popp, Robert K.

School:Texas A&M University

School Location:USA - Texas

Source Type:Master's Thesis

Keywords:crack healing fluid inclusions quartz fracture morphology

ISBN:

Date of Publication:12/01/2004

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