Use of thin films in silicate mineral diffusion experiments
- Sandia National Labs., Albuquerque, NM (United States)
The diffusion of chemical species through silicate minerals is considered an important contributor to disequilibrium behavior in geological materials. Evaluation and modeling of petrogenetic processes, as well as geochemical processes related to nuclear waste, energy, and materials problems, require accurate cation diffusion data in a variety of silicate minerals. To evaluate the relatively slow diffusion of Mg[sup 2+], Ca[sup 2+], and Mn[sup 2+] in phases like garnet, the authors have developed a new technique for the preparation of diffusion couples using thin film deposition. Thin films of elemental Mg can be applied to polished mineral surfaces by evaporating 150 mg of MgO powder under high vacuum with a thermal-resistance evaporator and a tungsten crucible. Film thickness is monitored with an oscillating quartz crystal and verified by diamond-stylus profilometry. The elemental Mg film can be partially oxidized by heating in air at 450 to 500 C for a few hours with no disruption of the diffusion interface. Thermal-resistance evaporation is efficient, as only milligram quantities of source material are required, and is especially important when expensive enriched stable-isotopes are to be the source material for self-diffusion experiments. Samples of single crystal grossular and pyrope garnets with thin films of MgO, as created by these techniques, were annealed for 40 hours at 900 C, log fO[sub 2] = [minus]12, and 1 atm total pressure. Optical and SEM examination reveal no disruption of the interface. Depth profiling analysis with secondary ion mass spectrometry (SIMS) confirms the lack of interface disruption. Profiles of elemental counts versus depth exhibit predictable and expected patterns as the analysis proceeds through the thin film and into the garnet substrate. Analytically-induced zoning can be corrected by using measured mass profiles obtained from blank and zero-time diffusion couples.
- DOE Contract Number:
- AC04-76DP00789
- OSTI ID:
- 5750133
- Report Number(s):
- CONF-921058--
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Journal Name: Geological Society of America, Abstracts with Programs; (United States) Vol. 24:7; ISSN GAAPBC; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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ALKALINE EARTH METAL COMPOUNDS
ALKALINE EARTH METALS
CALCIUM
CHALCOGENIDES
DIFFUSION
ELEMENTS
ENVIRONMENTAL TRANSPORT
FILMS
GARNETS
INDUSTRIAL WASTES
MAGNESIUM
MAGNESIUM COMPOUNDS
MAGNESIUM OXIDES
MANGANESE
MASS TRANSFER
MATERIALS
METALS
MINERALS
OXIDES
OXYGEN COMPOUNDS
PERMEABILITY
RADIOACTIVE MATERIALS
RADIOACTIVE WASTES
SAMPLE PREPARATION
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
THIN FILMS
TRANSITION ELEMENTS
WASTES