Thermal infrared emission spectroscopy of anhydrous carbonates

The objective of this study is to present the thermal emission spectra of various anhydrous, calcite- and dolomite-series carbonate minerals to illustrate the effect of the structural cation (Ca²⁺, Mg²⁺, Fe²⁺, Mn²⁺, Zn²⁺) on shifting the positions of the carbonate absorption bands. All of the carbonate mineral emission spectra included in this study exhibit three absorption features related to three specific vibrational modes of the carbonate anion (CO₃^2-). These anion vibrations include the out-of-plane bend, the asymmetric stretch, and the in-plane bend (i.e., the ν₂, ν₃, and ν₄ modes, respectively). The positions of the absorption-band emissivity minima are unique for each carbonate chemistry and are thus diagnostic of mineralogy. The average ν₂, ν₃, and ν₄ wavenumber positions for the various carbonate minerals are as follows: calcite (CaCO₃) 883, 1523, and 712 cm^-1; magnesite (MgCO₃) 901, 1572, and 748 cm^-1; siderite (FeCO₃) 876, 1523, and 736 cm^-1; rhodochrosite (MnCO₃) 877, 1535, and 726 cm^-1; smithsonite (ZnCO₃) 882, 1509, and 742 cm^-1; dolomite (CaMg(CO₃)₂) 894, 1547, and 728 cm^-1; and kutnahorite (CaMn(CO₃)₂) 882, 1526, and 716 cm^-1. Carbonates as a general mineral class crystallize in a variety of geological environments; however, each specific carbonate mineralogy typically is limited to a narrow range of geologic settings in which it forms. Thermal infrared emission data to be received from the Mars Global Surveyor thermal emission spectrometer will contain the spectral signature of carbonates if they are present above the detectibility limit on the surface of Mars. This study presents the spectral information necessary to recognize carbonate as a mineral class as well as identify the specific type of carbonate from thermal emissivity data. Knowledge of distinct carbonate mineralogy will be useful for interpreting the environmental conditions that were present on Mars during the carbonate formation. The result of this study is that the major carbonate mineral species (calcite, dolomite, magnesite, siderite, and smithsonite) can be identified from thermal infrared emission data, provided moderate (10 cm^-1) spectral sampling. Because of the similarity of absorption band positions between kutnahorite and calcite, high (2 cm^-1) spectral sampling is required to distinguish kutnahorite. Moderate spectral sampling data are also sufficient to determine the amount of Mg and Fe in Mg-Fe solid solution minerals to within 3-5% of the cation abundance.