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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/1369 |
| Date | 17 February 2005 |
| Creators | Fallon, Jessica Anne |
| Contributors | Kronenberg, Andreas K., Lamb, William |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Thesis, text |
| Format | 3213184 bytes, electronic, application/pdf, born digital |
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