Effect of dislocations on the kinetics of aragonite/calcite transformations
It is often assumed that ductile deformation increases the kinetics of phase transformations by producing dislocations which serve as nucleation sites. Theoretical calculations for calcite/aragonite suggest that deformation can greatly enhance the kinetics of the transformation, and can also lead to the formation of either polymorph outside of its stability field. The authors have tested this prediction by measuring the relative kinetics of deformed and undeformed samples for both directions of the calcite/aragonite transformation under a variety of conditions. Aragonite was ductilly deformed in its stability field at 500/sup 0/C and 15 kb, then annealed at room P and at 320/sup 0/ to 400/sup 0/C. Undeformed aragonite was also annealed at these conditions. In a second set of experiments single crystals of aragonite were ductilly deformed in the calcite stability field at 500/sup 0/C, 12.5 kb and 500/sup 0/C, 9 kb, and compared with hydrostatic experiments under the same conditions. Deformed aragonite had a dislocation density of about 10/sup 9/cm/sup -2/. In all cases the undeformed aragonite transformed to calcite faster than the deformed aragonite. One possible explanation is that Sr concentrates on dislocations and inhibits calcite nucleation. High Sr concentrations are known to stabilize aragonite relative to calcite. In addition to these experiments, a calcite single crystal was deformed in the aragonite stability field at 500/sup 0/C, 15 kb and compared with a hydrostatic experiment at the same conditions. In this case the kinetics of the transformation of the deformed calcite are at least 20 times faster than for the undeformed calcite.
- Research Organization:
- Brown Univ., Providence, RI (USA)
- OSTI ID:
- 6273091
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Vol. 17; ISSN GAAPB
- Country of Publication:
- United States
- Language:
- English
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ALKALINE EARTH METAL COMPOUNDS
ANNEALING
ARAGONITE
CALCITE
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CARBON COMPOUNDS
CARBONATE MINERALS
CARBONATES
CRYSTAL DEFECTS
CRYSTAL STRUCTURE
CRYSTALLIZATION
CRYSTALLOGRAPHY
DEFORMATION
DISLOCATIONS
DUCTILITY
HEAT TREATMENTS
KINETICS
LINE DEFECTS
MECHANICAL PROPERTIES
MINERALS
NUCLEATION
OXYGEN COMPOUNDS
PHASE STUDIES
PHASE TRANSFORMATIONS
REACTION KINETICS
TENSILE PROPERTIES