Preliminary Raman Spectroscopic Survey on a Martian Meteorite - Los Angeles

1431 for XXXII Lunar and Planetary Science Conference, Houston, March, 2001 Preliminary Raman spectroscopic survey on a martian meteorite – Los Angeles, Alian Wang, Karla E. Kuebler, John Freeman, Bradley L. Jolliff, Dept. Earth & Planetary Sciences, Washington University, St. Louis, MO, 63130 (alianw@levee.wustl.edu) We report a Raman spectroscopic survey on a rock slab of the Los Angeles martian meteorite [1-4]. The primary task of this survey is to identify the main mineral phases of the meteorite as well as accessory phases, even though they might be terrestrial alteration or contaminants. We observe Raman spectra of crystalline quartz and feldspar in addition to their glassy equivalents. Spectra of hematite and goethite suggest alteration of Fe-Ti oxides. Accessory or contaminant phases include hydroxyapatite, hydroxychlorapatite, graphitic carbon, calcite, SiC, and hydrocarbon compounds, including metalloporphyrin and paraffin. The rock slab (1.74 g from stone 2) we examined has two flat sawn surfaces, designated A and B. Microscopic observations reveal clusters of mostly dark pyroxene grains of (0.5-1.5 mm) size on the A side and abundant fractures filled with colorless glass. The B side has similar texture but depressed portions near the edges are covered by yellowish-brown fusion crust. On the cut surface of the B side, in some grains near the fusion crust, abundant tiny red spots (1-20 μm) are observed. Figure 1. Photos of rock chip of Los Angeles Martian meteorite studied by Raman spectroscopy. Most of the Raman spectra were obtained on the A side of the rock chip, mainly by spot reconnaissance [5], but also by linear and grid point-counting measurements [6]. Table 1 lists the major and minor mineral phases identified by the Raman survey, as well as chemical characteristics determined from a limited number of spectra. The results of a petrologic and mineralogical study of this meteorite reported previously [1,4] are listed in table 1 for comparison. A good correspondence exists between this Raman survey on a rough sawn rock slab and the results of [1] from thin sections in identification of the main mineral phases. Phases not reported previously but identified by the Raman survey are listed in red in table 1. Raman spectra of these phases are shown in Figures 2 and 3. In addition to the glassy forms of quartz and feldspar commonly observed in this meteorite, crystalline quartz and feldspar are also identified through their readily distinguishable Raman spectra (Fig.2). They could be either the relicts of these two minerals that survived the shock event or terrestrial contaminants. Merrillite and apatite are both identified by their Raman spectra. From subtle differences in their Raman spectra, hydroxylapatite and chlorapatite can be distinguished [7]. In a particular case, they were found co-exist. Figure 2 shows a spectrum having features of chlorapatite in fundamental vibration region and also a peak reflecting the OH stretching vibration. A carbonate grain was identified in an area where the main mineral phases are phosphates (merrillite and apatite). The grain itself is in tight contact with a merrillite grain. Figure 2 shows the Raman spectrum of this carbonate. Its peak positions suggest a composition of CaCO3. Calcite was observed filling the vesicles of another rock slab. Since these vesicles probably formed during the launch off Mars, the carbonates within them are probably terrestrial [8]. This Raman study Rubin et. al., 2000 [1,4] pyroxene Mg/(Mg+Fe+Ca) 0.481-0.302 pyroxene Ca/(Mg+Fe+Ca) 0.161-0.221 hedenbergite hedenbergite olivine Mg/(Mg+Fe) <0.06 fayalite glasses SiO2 glass silica glass & related feldspar glass K-rich feldpathic glass crystalline quartz maskelynite crystalline feldspar opaques ilmenite Ilmenite ulvospinel Variable comp. ulvospinel pyrrhotite Variable comp. pyrrhotite hematite baddeleyite goethite hercynitic spinel phosphate merrillite merrillite chlorapatite chlorapatite