A Method for the Flux Growth of Intermediate Composition Olivine
Abstract
Though solid solution of iron and magnesium between forsterite (Mg2SiO4) and fayalite (Fe2SiO4) is possible in the olivine crystal structure, the high oxygen fugacity condition of the terrestrial mantle inhibits the widespread crystallization of intermediate (Fo40-Fo60) composition olivine. This limitation is not the same for some other inner solar system bodies (e.g. the Moon and Mars), where conditions are reducing and olivine compositions are wide ranging. Unfortunately, the amount of samples from the Moon and Mars is extremely limited; with only Apollo and Luna mission samples, lunar meteorites, and Martian meteorites available for direct mineralogic and petrologic characterization. These characterizations have provided a useful basis for many spectroscopic and modeling interpretations, but many fundamental questions remain and may only be answerable through either direct observation of rocks or by analog experimentation. The motivation for our work on growth of intermediate olivine crystals, therefore, is to create realistic starting material for use in Mars and Moon analog experiments. A variety of crystal growth methods have been previously used to synthesize olivine, including: the Czochralski-pulling (CZ) method, the floating-zone image furnace (FZ) method, and sol-gel processing techniques. Both the CZ and FZ methods have the advantage of producing large crystals, but the growth apparatus and regulation of reduced atmospheric conditions during growth can make these techniques both time and cost intensive. Sol-gel processing to produces olivine fibers is a useful chemical technique, but obtaining larger grain sizes can be difficult. An alternative method for crystal growth is through the use a flux, which can grow crystals relatively quickly and inexpensively. We have grown synthetic crystals of intermediate composition (Fo30-Fo70) olivine using a lithium borate (B5Li3O9) flux. The starting material was a mixture of magnesite (MgCO3), siderite (FeCO3), and quartz (SiO2) powder in a 1:1:1 ratio. The advantage of using siderite is that the iron is already present in the ferrous form. Upon heating and decarbonation, this mixture represents a bulk composition of Fo50 (FeMgSiO4) olivine. Flux was then added to the starting material mixture so that the final mixture was 50% starting material and 50% flux by weight. This final mix was then placed in a platinum crucible that was heated to 1100 °C in a vacuum furnace for three days. The use of a vacuum furnace ensured that conditions remained reducing during crystal growth. The result was growth of olivine crystals that are generally small (< 1 mm in length) and have euhedral crystal form. These crystals have been analyzed by electron microprobe, and are systematically zoned from core to rim with Mg-rich cores (∼Fo70) transitioning to Fe-rich rims (∼Fo30). This zoning represents an expected heterogeneity due to olivine growth from a finite reservoir of starting material. The flux growth of this intermediate composition olivine was primarily a 'proof of concept' experiment, and showed that olivine crystals can be grown using a flux under sub-solidus conditions. Additional crystal growth experiments would be useful to gauge the response of olivine to changes in temperature, duration, and composition of the flux + starting material mixture.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.V13D..04D
- Keywords:
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- 1060 Planetary geochemistry (5405;
- 5410;
- 5704;
- 5709;
- 6005;
- 6008);
- 3611 Thermodynamics (0766;
- 1011;
- 8411);
- 3630 Experimental mineralogy and petrology;
- 3672 Planetary mineralogy and petrology (5410);
- 5455 Origin and evolution