Oxygen isotope effects associated with the solid-state. alpha. -FeOOH to. alpha. -Fe sub 2 O sub 3 phase transformation

Journal Article · Mon Jan 01 00:00:00 EST 1990 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(90)90210-C· OSTI ID:6702061

Yapp, C J ^[1]

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1. Univ. of New Mexico, Albuquerque (USA)

Samples of synthetic and natural goethites were subjected to isothermal dehydration under both closed- and open-system conditions at various temperatures ranging from 160 to 300{degree}C. The oxygen isotope ratios of the dehydration product (hematite) were systematically different open- and closed-system dehydration. In every instance, open-system dehydration resulted in {sup 18}O enrichments of the product hematite relative to the starting goethite. For all but a few cases, closed-system dehydration produced an {sup 18}O depletion in the residual mineral relative to the starting goethite. These oxygen isotope effects indicate that significant mineral-vapor isotope exchange occurred during the solid-state, closed-system transformation of goethite to hematite. Calculated values of the apparent closed-system oxygen isotope fractionation factor between hematite and the corresponding liquid water suggest that equilibrium isotope exchange may have been approached in those samples for which the extent of closed-system dehydration was at least 95%. The rapidity of the mineral-vapor isotope exchange associated with the solid-state goethite to hematite transformation is indicated by the fact that the dehydration times ranged from only 24 to 68 hours. These results suggest that the oxygen isotope ratios of many hematites in low temperature geological systems reflect the environmental conditions associated with their formation from some hydrous precursor. Information about the {sup 18}O/{sup 16}O ratio in the hydrous precursor itself is not likely to be directly preserved in the hematite.

• OSTI ID: 6702061
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Vol. 54:1; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
GOETHITE
DEHYDRATION
ISOTOPE RATIO
PHASE TRANSFORMATIONS
HEMATITE
GEOCHEMISTRY
ISOTHERMAL PROCESSES
OXYGEN ISOTOPES
PHASE STUDIES
REACTION KINETICS
CHALCOGENIDES
CHEMISTRY
IRON COMPOUNDS
IRON ORES
IRON OXIDES
ISOTOPES
KINETICS
MINERALS
ORES
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
580000* - Geosciences