lunarcarbonate


Identification of siderite: The Raman spectra taken from some grains and veins show a strong peak near 1084cm-1 indicates the existence of (CO3)2- as major anionic group; the spectral pattern suggests a calcite-like structure (R3-c); the positions of major peaks (near 1084, 735, 287, 188cm-1) distinguish this carbonate to be mainly siderite composition (FeCO3), different from calcite, magnesite, and other carbonates of similar structure. Identification of goethite: The Raman spectra obtained from the bright rims around inner siderite grains and along the out edges of some siderite vein show a characteristic spectral pattern of goethite (alpha-FeOOH), with a major peak near 395cm-1 and several minor peaks match well with the spectrum of standard goethite. No OH stretching vibration peak in 3500-4000cm-1 spectral range were observed in both standard goethite and studied lunar sample, caused by the non-symmetric locations of protons in goethite structure. Another two polymorphes lepidocrocite (gama-FeOOH) and akaganeite (beta-FeOOH) have completely different spectral patterns.
Structural features of siderite and goethite -- poor & variable crystallinity Comparing with the Raman spectra of standard siderite and goethite, the major peak of siderite and goethite (~1084cm-1 and 395cm-1) in studied sample are weak relative to other minor peaks. The Raman peaks of goethite also show an obvious broadening. These two features are sensitive to the degree of crystallinity of analyzed sample. A general trend observed is that the siderite grains at the outside of the rock chip have better crystallinity, the siderite identified from the "inner grains" and from the veins have much poor crystallinity, which could be the result of a fast solidification in a much confined space. Compositional variation of siderite The exact Raman peak positions of siderite vary among the spectra taken from the different locations in the same grain or from different grains, which indicates the cation substitutions in these siderites.