Structural environment around Th sup 4+ in silicate glasses: Implications for the geochemistry of incompatible Me sup 4+ elements

Journal Article · Fri Nov 01 00:00:00 EST 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90490-V· OSTI ID:5171735

Farges, F ^[1]

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1. Univ. Paris (France)

The structural environment around Th{sup 4+} in several silicate glasses containing 1-3 wt% Th{sup 4+} was investigated as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. One set of glasses had the bulk chemical composition of monticellite (MO), diopside (DI), anorthite (AN), albite (AL), olivine basalt (O-BAS), tholeiitic basalt (T-BAS), calc-alkaline dacite (DAC), and calc-alkaline rhyolite (RHY). A second set of Th-bearing glasses had the bulk Al composition but also contained either 5.5 wt% F or 2,000 ppm Cl. The presence of {sup VI}Th in silicate glasses, an unusual low coordination for Th, suggests that the ionic radius of Th in magmatic systems is lower than previously reported (r{sub Th} = 0.94 {angstrom}). The resulting high Th-O bond strength ({approx} 0.7 vu), unknown in minerals, may explain the observed low crystal-melt partition coefficients of Th{sup 4+}, i.e., its incompatible character during magmatic differentiation. Increasing polymerization is predicted to favor higher coordination around Th (VII, IX) as a result of a shortage of NBO to which Th preferentially bonds. Hence, dramatic changes in the crystal chemistry of Th are predicted, i.e., the incorporation of Th in minerals. Comparison with U{sup 4+} suggests that {sup VI}Th is less abundant in similar glass/melt systems, compared with {sup VI}U. This may explain the U/Th fractionation often observed in zircon, as well as the preferential incorporation of Th in accessory minerals containing large sites in which Th{sup 4+} can be substituted (e.g., the Ca{sup 2+} site in titanite, apatite, or zirkelite).

• OSTI ID: 5171735
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 55:11; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
ALBITE
STRUCTURAL CHEMICAL ANALYSIS
ANORTHITE
DIOPSIDE
AMORPHOUS STATE
BASALT
CHEMICAL BONDS
CHEMICAL COMPOSITION
GEOCHEMISTRY
MOLECULAR STRUCTURE
OLIVINE
RHYOLITES
THORIUM
THORIUM IONS
ACTINIDES
CHARGED PARTICLES
CHEMISTRY
ELEMENTS
FELDSPARS
IGNEOUS ROCKS
IONS
METALS
MINERALS
OXYGEN COMPOUNDS
PYROXENES
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
VOLCANIC ROCKS
580000* - Geosciences