¹Kelsey B. Williams, ²Colin R.M. Jackson, ³Leah C. Cheek, ⁴Kerri L. Donaldson-Hanna, ⁵Stephen W. Parman, ⁵Carle M. Pieters, ⁶M. Darby Dyar, ⁵Tabb C. Prissel
¹Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, Missouri 63130, U.S.A.
²Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A.
³Department of Earth and Environment, Boston University, 685 Commonwealth Avenue, Boston, Massachusetts 02215, U.S.A.
⁴Atmospheric, Oceanic and Planetary Physics, Oxford University, Clarendon Laboratory, Parks Road, Oxford, Oxfordshire OX1 3PU, U.K.
⁵Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook Street, Providence, Rhode Island 02912, U.S.A.
⁶Department of Astronomy, Mount Holyoke College, 217 Kendade Hall, 50 College Street, South Hadley, Massachusetts 01002, U.S.A.

Visible to near-infrared (V-NIR) remote sensing observations have identified spinel in various locations and lithologies on the Moon. Experimental studies have quantified the FeO content of these spinels (Jackson et al. 2014), however the chromite component is not well constrained. Here we present compositional and spectral analyses of spinel synthesized with varying chromium contents at lunar-like oxygen fugacity (fO2). Reflectance spectra of the chromium-bearing synthetic spinels (Cr# 1–29) have a narrow (~130 nm wide) absorption feature centered at ~550 nm. The 550 nm feature, attributed to octahedral Cr3+, is present over a wide range in iron content (Fe# 8–30) and its strength positively correlates with spinel chromium content [ln(reflectancemin) = −0.0295 Cr# – 0.3708]. Our results provide laboratory characterization for the V-NIR and mid-infrared (mid-IR) spectral properties of spinel synthesized at lunar-like fO2. The experimentally determined calibration constrains the Cr# of spinels in the lunar pink spinel anorthosites to low values, potentially Cr# < 1. Furthermore, the results suggest the absence of a 550 nm feature in remote spectra of the Dark Mantle Deposits at Sinus Aestuum precludes the presence of a significant chromite component. Combined, the observation of low chromium spinels across the lunar surface argues for large contributions of anorthositic materials in both plutonic and volcanic rocks on the Moon.

Reference
Williams KB, Jackson CRM, Cheek LC, Donaldson-Hanna KL, Parman SW, Pieters CM, Dyar MD, Prissel TC (2016) Reflectance spectroscopy of chromium-bearing spinel with application to recent orbital data from the Moon. American Mineralogist 101, 678-689
Link to Article [doi:10.2138/am-2016-5408CCBYNCND]
Copyright: The Mineralogical Society of America

¹Elizabeth B. Rampe, ²Richard V. Morris, ³P. Douglas Archer Jr, ⁴David G. Agresti, ²Douglas W. Ming
¹Aerodyne Industries, Jacobs JETS Contract at NASA Johnson Space Center, 2101 NASA Parkway, Mail Code XI3, Houston, Texas 77058, U.S.A.
²NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
³Jacobs, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
⁴Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294, U.S.A.

Orbital and in-situ data from the surface of Mars indicate that nanophase weathering products are important constituents of martian rocks and soils. Nanophase minerals have the capacity to chemisorb anions like sulfate and phosphate onto their surfaces, but it is not known whether chemisorption is an important or even detectable process via orbital and in-situ observations. The detection of chemisorbed sulfate and phosphate anions on nanophase minerals would constrain the speciation of these anions and past aqueous environmental conditions. Here, we synthesized two nanophase weathering products that are common in terrestrial volcanic soils and have been identified on the martian surface: allophane and nanophase ferric oxide as represented by ferrihydrite. We specifically adsorbed sulfate and phosphate separately onto the nanophase mineral surfaces (4.5 and 1.6 wt% SO42−, and 6.7 and 8.9 wt% PO43− on allophane and ferrihydrite, respectively) and analyzed the untreated and chemisorbed materials using instruments similar to those on orbital and landed Mars missions (including X-ray diffraction, evolved gas analysis, Mössbauer spectroscopy, and VNIR and thermal-IR spectroscopy). Evolved gas analysis is the optimum method to detect chemisorbed sulfate, with SO2(g) being released at >900 °C for allophane and 400–800 °C for ferrihydrite. Chemisorbed sulfate and phosphate anions affect the thermal-IR spectra of allophane and ferrihydrite in the S-O and P-O stretching region when present in abundances of only a few weight percent; S-O and P-O stretching bands are apparent as short-wavelength shoulders on Si-O stretching bands. Sulfate and phosphate anions chemisorbed to allophane have small but measurable effects on the position of the OH-H2O bands at 1.4 and 1.9 μm in near-IR spectra. Chemisorbed sulfate and phosphate anions did not affect the X-ray diffraction patterns, Mössbauer spectra, and visible/near-IR spectra of ferrihydrite. These data suggest that sulfate chemisorbed onto the surfaces of nanophase minerals can be detected with the Sample Analysis at Mars (SAM) instrument on the Mars science laboratory Curiosity rover, and subtle signatures of chemisorbed sulfate and phosphate may be detectable by IR spectrometers on landed missions. The combined use of SAM, the Chemistry and Mineralogy (CheMin) instrument, and the Alpha Particle X-ray Spectrometer (APXS) on Curiosity allows for the most detailed characterization to date of nanophase minerals in martian rocks and soils and the potential presence of chemisorbed anionic complexes.

References
Rampe EB, Morris RV, Archer Jr PD, Agresti DG, Ming DW (2016) Recognizing sulfate and phosphate complexes chemisorbed onto nanophase weathering products on Mars using in-situ and remote observations. American Mineralogist 101, 627-643
Link to Article [doi:10.2138/am-2016-5305]
Copyright: The Mineralogical Society of America