Potential Existence of Al - Bearing Sulfates on Mars and Their Spectral Characteristics

2289 for XXXXIII Lunar and Planetary Science Conference (2012, Houston, TX) POTENTIAL EXISTENCE OF AL-BEARING SULFATES ON MARS AND THEIR SPECTRAL CHARACTERISTICS. Yuhang Zhou and Alian Wang, Dept of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University in St. Louis, St. Louis, MO, 63130, USA ([email protected], [email protected] ) Potential existence of Al-bearing sulfates on Mars: Hydrous Mg-, Ca-, and Fe-sulfates are one of the two main types of sedimentary minerals found on Mars by recent missions. The data from OMEGA instrument on Mars Express and CRISM instrument on MRO revealed the existences of kieserite (MgSO4·H2O), gypsum (CaSO4·2H2O) and polyhydrated sulfates in layered terrains on Mars [1]. Calcium-rich sulfates, mostly like Gypsum are observed in the north polar region of Mars [2]. Mg-, Casulfates, and especially ferric sulfates were found by two Mars Exploration rovers at Gusev and Meridiani Planum [3, 4]. On the other hand, data from orbiters, rovers, and landers all suggest that basalt is the major igneous rock type on Mars, composed mainly olivine, pyroxene and feldspar of various compositions [5]. When these minerals react with SO4-bearing fluids, the process with the lowest Gibbs free energy will be preferred. Thus, the weathering of olivine (Gf 0 = -6.58 to -4.0 Kcal/atom) and pyroxene (Gf 0 = -2.72 to -2.98 Kcal/atom) would happen earlier than the weathering of feldspar (Gf 0 = 1.32 to -0.32 Kcal/atom) [6]. The cations release by olivine and pyroxene weathering are mainly Mg, Fe, and Ca, as evidenced by the finding of Mg-, Fe-, and Ca-sulfates on Mars [1-4]. On Earth, Al-bearing sulfates are commonly formed in natural acidic sulfate-rich environments such as active crater lakes, fumaroles, and acidic hot springs, etc. Al-bearing sulfates were not yet reported on Mars but cannot be excluded, because the evidence of extensive weathering was observed on Mars [7-9]. During the late stage of weathering when feldspar altered, Al, K, Na and Ca can be released and can form (under suitable conditions) Al-sulfates and multi-cation-sulfates. Al-sulfates can also form from the acidic leaching of clay minerals [10], e.g., the OMEGA observation of sulfates in stratigraphic layer (in an acidic environment) formed post the formation of a layer of phyllosilicates [7]. In addition, the Inner Basin of Columbia Hills contains several features that likely have formed by volcanic processes [8]. The weathering process of volcanic ash can happen before the feldspar alteration, thus to release Al, K, Na and Ca and to form Alsulfates and multi-cation sulfates. Spectroscopic study of Al-bearing sulfates: In sorting of 101 species of Al-bearing sulfates in Dana’s New Sulfates Classification [Web 1], we selected 6 groups of hydrated and anhydrous Al-bearing sulfates for detailed structure and properties study. They are Alunogen group, Felsobanyaite group, Halotrichite group, Voltaite group, Alum group and Alunite group. From the 6 groups of Al-bearing sulfates commonly seen on Earth, we selected two for a preliminary spectroscopic study. The purpose is to learn their major spectral features and the differences from that of Mg-, Fe-, and Ca-sulfates. The two selected Al-bearing sulfates are alum-(K) (KAl(SO4)2·12(H2O)) and alunogen (Al2(SO4)3· 17(H2O)). Laboratory chemicals were used as the samples of measurements. XRD analysis was first made to confirm their structures, then spectroscopic measurements were made using laser Raman, MIR-Attenuated Total Reflectance (ATR) and NIR-Diffuse Reflectance (DR). XRD measurement was made using a Rigaku Geigerflex X-ray diffractometer with Cu K radiation. Raman measurements were made on single crystal grain under the microscope of KOSI HoloLab5000 Raman spectrometer, with ~ 9mW of 532 nm laser power at the sample. MIR-ATR measurements were made on crashed grains using a GoldenGate ATR accessory of Nicolet Nexus 670 FTIR spectrometer. NIRDR measurements were made on ground powder using Crocket accessory of the same FTIR spectrometer. Results and discussion: Peaks in laser Raman and MIR-ATR spectra provide the information on the fundamental vibration modes of these Al-bearing sulfates, thus can be directly linked to the crystal structural character of the samples. The spectral peaks of NIR-DR are the overtones and combinational modes that have the highest application potentials for interpreting the data from planetary missions. Figure 1 compares the Raman spectra of alum-(K) and alunogen with that of three typical hydrous Mg-, Fe-, and Ca-sulfates: epsomite (MgSO4·7(H2O)), melanterite (FeSO4·7(H2O)) and gypsum (CaSO4·7(H2O)). Those spectra were obtained by laborotorial measurements under similar conditions. There are two most distinguished differences between Al-sulfates and others, the first one is in spectral range 2600-3600 cm -1 (Fig. 1a). The H2O Raman peak of alunogen and alum(K) is much broad than other three sulfates, extends from 3600 to 2800 cm -1 . This feature is probably due to the multi-sites occupied by structural water in Albearing sulfates and the large site differences. In alunogen [Al(H2O)6]2(SO4)3(H20)5 unit cell, (SO4) tetrahedra and [Al(H2O)6] octahedra are linked together by hydrogen bonding. There are two kinds of