Characterizing Pyroxene Cooling Rate Using Reflectance Spectra

April 14th, 2012

Pyroxenes are among the most common minerals in the inner solar system. The degree of order/ disorder in their crystal structure relates to the cooling history of the mineral. In pyroxenes, Fe2+ prefers the M2 cation site, and Mg prefers the M1 cation site in the crystallographic structure; the slower the pyroxene cools, the more ordered it will be. Previous work revealed that the ordering of Fe2+ and Mg between the M1 and M2 sites affects the relative strengths of absorption bands in the near-infrared. We performed a series of heating experiments to help constrain the relationship between cation ordering and reflectance spectra for orthopyroxenes.

For the experiments, 100 mg of crushed orthopyroxene was heated in an alumina capsule sealed inside an evacuated high-purity silica tube. A small piece of iron foil was placed beneath the capsule to buffer oxygen fugacity; a small piece of the iron foil outside the seal showed the difference between the atmospheres inside and outside of the tube. The samples were heated to four different temperatures (500°C, 600°C, 700°C and 800°C) and were held at those temperatures for 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. The results show that pyroxenes on Earth are more ordered, having experienced slower cooling or metamorphism, while extraterrestrial materials are more disordered, due to rapid cooling. However, our research will need some tweaking before doing final analyses.

Ellen Shank, ’12                                                                                                                               
Dallas Center, IA
Majors: Geology, Classical Studies                                                                                                                                 

Rachel Klima
John Hopkins University, Laurel, MD

M. Darby Dyar
Mount Holyoke College, South Hadley, MA                                                                                    

Sponsor: Emily Walsh

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