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REFLECTANCE SPECTRA of ANHYDROUS CARBONATE MINERALS: IMPLICATIONS for MARS. E. A. Cloutis1, D. M. Goltz2, J. Coombs2, B. Russell1, M

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REFLECTANCE SPECTRA of ANHYDROUS CARBONATE MINERALS: IMPLICATIONS for MARS. E. A. Cloutis1, D. M. Goltz2, J. Coombs2, B. Russell1, M Lunar and Planetary Science XXXI 1155.pdf REFLECTANCE SPECTRA OF ANHYDROUS CARBONATE MINERALS: IMPLICATIONS FOR MARS. E. A. Cloutis1, D. M. Goltz2, J. Coombs2, B. Russell1, M. Guertin1, and T. Mueller1. 1Department of Ge- ography, University of Winnipeg, 515 Portage Ave., Winnipeg, MB, Canada R3B 2E9 ([email protected]; [email protected]; [email protected]), 2Department of Chemistry, University of Winnipeg, 515 Portage Ave., Winnipeg, MB, Canada R3B 2E9 ([email protected]; [email protected]). Introduction: The spectral reflectance properties in the 2.8-4.3 µm region: two sets of multiple, over- of a range of anhydrous carbonate minerals have been lapped absorption features in the 3.25-3.5 µm and investigated. The purpose of this study was to deter- 3.75-4.0 µm regions. The bands in the 3.25-3.5 µm mine the range of spectral variability which this class region are probably attributable to overtones and com- of minerals exhibits and whether they are plausible binations of C-O stretches near 7 µm [6,7]. The bands candidates for the absorption features reported in in the 3.75-4.0 µm region are probably attributable to Earth-based Martian spectra by [1]. combinations of the C-O stretching bands near 7 µm A number of criteria suggest that carbonates may and the C-O bending band near 9 µm [6,7]. The pre- be present on the surface of Mars, including weather- cise positions of these bands are a function of both ing models [2], detection of preterrestrial carbonates structure and type of cation present [8]. The aragonite in some SNC meteorite [3], and models of the planet’s + calcite spectrum is the most complex, probably due climatic evolution [e.g., 4]. The detection of carbon- to spectral contributions from both minerals. ates has been tentative and abundances and composi- The reflectance spectra of Ca-Mg carbonates - tions are not well constrained. In order to assist in dolomite (CaMg(CO3 )2 ) and huntite (Mg3 Ca(CO3 )4 ) - resolving this issue, we have undertaken a compre- are shown in Figure 2. Overall the spectra exhibit the hensive study of the spectral reflectance properties of a same general characteristics as the single cation car- range of carbonate minerals. This paper focuses on bonates: two series of absorption bands in the 3.3 and the 2.8-4.3 µm region of anhydrous carbonates [1]. 4.0 µm regions. The absorption bands in the huntite spectrum are not as apparent as in the dolomite. Experimental Procedure: A range of anhydrous Figure 3 shows the reflectance spectra of transition carbonate minerals including ankerite, aragonite, series element-bearing carbonates: siderite (FeCO3 ), dolomite, gaspeite, huntite, magnesite, siderite, cal- manganocalcite ((Mn,Ca)CO3 ), and gaspeite ((Ni,Mg, 2+ cite, and manganocalcite, as well as dehydrated hy- Fe )CO3 ). Once again all the spectra exhibit the two dromagnesite have been characterized by reflectance series of absorption features. No significant well- spectroscopy, atomic absorption/emission spectros- defined absorption bands attributable to transition copy, ICP-MS, wet chemistry, and X ray diffraction. series elements appear in the 2.8-4.3 µm region. Reflectance spectra were measured at the RELAB fa- Figure 4 shows the reflectance spectra of two de- cility at Brown University. The spectra were meas- hydrated hydromagnesites. The amount of water liber- ured from 0.3-26 µm relative to halon (0.3-2.6 µm; 5 ated from these two samples upon heating were differ- nm spectral resolution; i=30°, e=0°) and brushed gold ent (1.6% vs. 21.2%). The expected water content was (2.5-26 µm; 4 cm-1 spectral resolution; i=30°, e=30°) 15%. The upper spectrum is the low water content [5]. A variety of grain sizes were measured: <45 µm sample. The spectral differences between the two may (dry sieved), and 45-90 µm (dry sieved). Dehydration be due to some structural differences although we have of the hydromagnesite samples was undertaken in air not yet undertaken detailed analysis of the XRD data. at 375°C. Absorption band minimum wavelength po- Both spectra show the same types of features as the sitions were determined by fitting a third order poly- other anhydrous carbonates. nomial to 10-20 data points on both sides of a visually determined minimum. Discussion: The different samples show differ- ences in terms of absorption band positions and inten- Results: The reflectance spectra of the <45 µm sities. Table 1 is a compilation of band minima wave- size fractions of the carbonates are shown in Figures length positions for the samples. This table only lists 1-4. Figure 1 shows the reflectance spectra of single- those bands for which well-defined minima are deriv- cation anhydrous carbonates: calcite (CaCO3 ), magne- able. Some of the absorption features are complex and site (MgCO3 ), and an intimate mixture of subequal suggest the presence of multiple overlapped bands amounts of calcite and aragonite (CaCO3 ). All 3 sam- which our simple curve fitting does not resolve. ple spectra display roughly the same types of features Lunar and Planetary Science XXXI 1155.pdf ANHYDROUS CARBONATE SPECTRA: E. A. Cloutis, D.M. Goltz, J. Coombs, B. Russell, M. Guertin and T. Mueller Table 1. Wavelength positions of resolvable ab- huntite (Fig. 2); siderite, manganocalcite, and gaspeite sorption bands (in µm). (Fig. 3); and, dehydrated hydromagnesite _________________________________________ (+0,+0.2)(Fig. 4). Numbers in brackets denote vertical Calcite: 3.35 3.48 3.98 offsets of some spectra for clarity. Magnesite: 3.28 3.42 3.94 Aragonite: 3.35 3.43 3.97 1.6 Figure 1 Dolomite: 3.32 3.46 3.81 3.96 1.4 Huntite: 3.41 3.70 3.87 3.94 1.2 Calcite Siderite: 3.39 3.49 4.00 1 Mn-calcite: 3.36 3.48 3.98 0.8 Gaspeite: 3.33 3.46 3.97 0.6 Magnesite Dehydrated hydromagnesites: Absolute reflectance 0.4 3.28 3.42 3.94 0.2 Aragonite 3.94 0 _______________________________________ 2800 3300 3800 4300 From these spectra it can be seen that there are Wavelength (nm) measurable shifts in band positions, but that all of the samples exhibit absorption features in the 3.3 and 4.0 1 µm region of approximately equal intensity. Figure 2 0.8 Do lomite Implications for Mars: Blaney and McCord [1] 0.6 noted that the absorption feature seen in their reflec- tance spectra of Mars from ~3.8-4.0 µm was at shorter 0.4 Huntite wavelengths than for laboratory spectra of calcite and Absolute reflectance magnesite. Our magnesite spectrum is similar to that 0.2 measured by [1]. The huntite and dolomite spectra 0 have absorption bands which begin at shorter wave- 2800 3300 3800 4300 lengths than the magnesite, suggesting that they may Wavelength (nm) be plausible candidates for the absorption features identified by [1]. 0.6 Figure 3 0.5 Siderite References: [1] Blaney, D.L. and McCord, T.B. (1989) JGR, 94, 10159-10166. [2] Gooding, J.L. 0.4 Manganocalcite (1978) Icarus, 33, 483-513. [3] McSween, Jr., H.Y. 0.3 and Treiman, A.H. (1998) in Planetary Materials, 0.2 Min. Soc. Amer., vol. 36, Chap. 6. [4] Carr, M.H. Absolute reflectance (1996) Water on Mars, Oxford U. Press, NY. [5] 0.1 Gaspeite RELAB (1999) Keck/NASA Reflectance Experiment 0 Laboratory 1999; http://porter.geo.brown.edu/relab. 2800 3300 3800 4300 [6] Adler, H.H. and Kerr, P.F. (1962) Amer. Mineral., Wavelength (nm) 47, 700-717. [7] Adler, H.H. and Kerr, P.F. (1963) Amer. Mineral., 48, 839-853. [8] White, W.B. (1974) 0.9 Figure 4 Infrared Spectra of Minerals, Min. Soc., pp. 227-284. 0.8 Acknowledgments: This study was supported by 0.7 a University of Winnipeg discretionary grant (to 0.6 EAC), an Imperial Oil Ltd. University Research Grant 0.5 (to EAC), and a University of Winnipeg Major Re- 0.4 search Grant (to DMG). We wish to thank Drs. Carlé Absolute reflectance 0.3 Pieters and Takahiro Hiroi for providing access to the 0.2 RELAB facility for the spectral measurements. 0.1 Figures: Reflectance spectra (2.8-4.3 µm) of <45- 2800 3300 3800 4300 Wavelength (nm) µm size fractions of: calcite (+0.8), magnesite (+0.2) and aragonite/calcite (Fig. 1); dolomite (+0.2) and.
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