Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p43a1657s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P43A-1657
Physics
[5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Pyroxenes are among the most common minerals in the inner solar system, and their reflectance spectra are highly diagnostic of their crystal structure and composition. Previous work has revealed that the ordering of Fe2+ and Mg between the M1 and M2 cation sites affects the relative strengths of absorption bands in the near-infrared. We have performed a series of heating experiments to help constrain the relationship between cation ordering and reflectance spectra for orthopyroxenes. For the experiments, crushed up samples of two pyroxenes (a bronzite and an orthopyroxene) were heated in an alumina capsule sealed inside an evacuated high-purity silica tube. The crushed samples consisted of grains greater than 250 μm and were hand-picked for purity. 100 mg of the clean sample was put in the alumina capsule to go into the silica tube; beneath the capsule we placed a small piece of iron foil to buffer oxygen fugacity. We also put a small piece of the iron foil outside the seal to show the difference between the atmospheres inside and outside the tube. Subsets of each pyroxene were heated to four different temperatures: 500C, 600C, 700C and 800C and held at those temperatures for between one day to a month to ensure Mg-Fe2+ equilibration. Samples were then measured using near-infrared reflectance spectroscopy and Mossbauer spectroscopy to determine the cation ordering. Various spectra will be taken of the heated samples and compared to unheated samples to get a sense of the order/disorder. In pyroxenes, the degree of order/disorder relates to the cooling history of the mineral. Fe2+ prefers the M2 site, and Mg prefers the M1 site in the crystallographic structure, and the slower it cools, the more ordered it will be. Since Fe2+ is responsible for crystal field absorption bands, the placement of the Fe2+ in the crystal structure affects the strength and possibly the positions of the absorption bands. Many of the pyroxenes on Earth have experienced slower cooling or metamorphism, resulting in a more ordered sample. More rapid cooling, as extraterrestrial materials often experience, results in more disorder, or a random placement of Fe2+ and Mg throughout the M1 and M2 sites.
Darby Dyar M.
Klima Rachel L.
Shank E.
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