¹Elizabeth C. Sklute, ¹Timothy D. Glotch, ²Jennifer L. Piatek, ¹William R. Woerner, ¹Alexis A. Martone, ¹Meredith L. Kraner
¹Department of Geosciences, Stony Brook University, Stony Brook, New York 11794, U.S.A.
²Department of Physics and Earth Sciences, Central Connecticut State University, New Britain, Connecticut 06050, U.S.A.

The hydroxy sulfate jarosite [(K,Na,H3O)Fe3(SO4)2(OH)6] has both been discovered on Mars, and is associated with areas of highly acidic runoff on Earth. Because jarosite is extremely sensitive to formation conditions, it is an important target mineral for remote sensing applications. Yet at visible and near infrared (VNIR) wavelengths, where many spacecraft spectrometers collect data, the spectral abundance of a mineral in a mixture is not linearly correlated with the surface abundance of that mineral. Radiative transfer modeling can be used to extract quantitative abundance estimates if the optical constants (the real and imaginary indices of refraction, n and k) for all minerals in the mixture are known. Unfortunately, optical constants for a wide variety of minerals, including sulfates like jarosite, are not available. This is due, in part, to the inherent difficulty in obtaining such data for minerals that tend to crystallize naturally as fine-grained (~10 μm) powders, like many sulfates including jarosite. However, the optical constants of powders can be obtained by inverting the equation of radiative transfer and using it to model laboratory spectra. In this paper, we provide robust n and k data for synthetic potassium, hydronium, and sodium jarosite in the VNIR. We also explicitly describe the calculation procedures (including access to our Matlab code) so that others may obtain optical constants of additional minerals. Expansion of the optical constants library in the VNIR will facilitate the extraction of quantitative mineral abundances, leading to more in-depth evaluations of remote sensing target locations.

Reference
Sklute EC, Glotch TD, Piatek JL, Woerner WR, Martone AA, Kraner ML (2015) Optical constants of synthetic potassium, sodium, and hydronium jarosite. American Mineralogist 100, 1110-1122
Link to Article [doi: 10.2138/am-2015-4824]

Copyright: The Mineralogical Society of America

¹N.G. Rudraswami, ¹M. Shyam Prasad, ^1,2K. Nagashima, ³R.H. Jones
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Dona Paula, Goa 403004, India
²Hawaii Institute of Geophysics and Planetology, Hawai‘i at Māanoa, 1680 East-West-Road, Honolulu, HI 96822, USA
³Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA

Most olivine relict grains in cosmic spherules selected for the present study are pristine and have not been disturbed during their atmospheric entry, thereby preserving their chemical, mineralogical and isotopic compositions. In order to understand the origin of the particles, oxygen isotope compositions of relict olivine grains in twelve cosmic spherules collected from deep sea sediments of the Indian Ocean were studied using secondary ion mass spectrometry. Most of the data lie close to the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line, with Δ17O ranging from –5 to 0‰. The data overlap oxygen isotopic compositions of chondrules from carbonaceous chondrites such as CV, CK, CR and CM, which suggests that chondrules from carbonaceous chondrites are the source of relict grains in cosmic spherules. Chemical compositions of olivine in cosmic spherules are also very similar to chondrule olivine from carbonaceous chondrites. Several olivine relict grains in three cosmic spherules are 16O-rich (Δ17O –21.9 to –18.7‰), similar to oxygen isotopic compositions observed in calcium aluminium rich inclusions (CAIs), amoeboid olivine aggregates (AOAs), and some porphyritic chondrules from carbonaceous chondrites. These grains appear to have recorded the initial oxygen isotopic composition of the inner solar nebula. Three olivine grains from two cosmic spherules have δ18O values > +20‰, which could be interpreted as mixing with stratospheric oxygen during atmospheric entry.

Reference
Rudraswami NG, Shyam Prasad M, Nagashima K, Jones RH (2015) Oxygen isotopic composition of relict olivine grains in cosmic spherules: Links to chondrules from carbonaceous chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.05.004]

Copyright Elsevier