Electron microscope study of the stability field and degree of nonstoichiometry in olivine

Journal Article · Thu Apr 10 00:00:00 EST 1986 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JB091iB05p04711· OSTI ID:5876710

Boland, J N; Duba, A G

The stability of San Carlos olivine (Fo 90.5) relative to changes in oxygen fugacity at 1300 /sup 0/C has been determined by studying the microstructural changes in single crystals using transmission and analytical electron microscopy. The first detectable metallic phase, heterogeneously nucleated along dislocations within (100) and (001) subboundaries, had a composition Ni/sub 65/Fe/sub 35/ equilibrated with a CO/CO/sub 2/ gas mixture having an oxygen fugacity f/sub 02/ of 10/sup -5.6/ Pa. Although the (100) subboundaries contained a high density of dislocations with Burgers vector (100), the nucleation sites were restricted to (010) and (001) dislocations: a result consistent with the theory of heterogeneous nucleation along dislocations. Under the reducing conditions of an f2 of 10/sup -12/ Pa, a forsteritic olivine is produced at the gas-mineral interface with a high density of metallic precipitates (Fe/sub 96/Ni/sub 4/) homogeneously nucleated in the olivine. Nonstoichiometry of the olivine was inferred from the absence of any additional silica-rich phases. The nonstoichiometry was analyzed in terms of possible crystalline defect structures, especially vacancies, assuming no evaporative losses from the surface except an oxygen exchange reaction. On the basis of the model proposed, the highly reduced olivine would contain an excess of SiO/sub 2/ (x2 = 0.355 compared with a stoichiometric value of 0.333) from the surface to a depth of several micrometers.

• Research Organization: Research School of Earth Sciences, Australian National University, Canberra, A. C. T., Australia
• OSTI ID: 5876710
• Journal Information: J. Geophys. Res.; (United States), Vol. 91:B5
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
OLIVINE
MICROSTRUCTURE
DISLOCATIONS
ELECTRON MICROSCOPY
MONOCRYSTALS
NUCLEATION
PHASE TRANSFORMATIONS
ALKALINE EARTH METAL COMPOUNDS
CRYSTAL DEFECTS
CRYSTAL STRUCTURE
CRYSTALS
IRON COMPOUNDS
IRON SILICATES
LINE DEFECTS
MAGNESIUM COMPOUNDS
MAGNESIUM SILICATES
MICROSCOPY
MINERALS
OXYGEN COMPOUNDS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
580300* - Mineralogy
Petrology
& Rock Mechanics- (-1989)