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Abstract-Raman spectra were acquired on a series of natural and synthetic sulfide minerals, commonly found in enstatite meteorites: oldhamite (CaS), niningerite or keilite ((Mg,Fe)S), alabandite (MnS), troilite (FeS), and daubreelite (Cr 2 FeS 4 ). Natural samples come from three enstatite chondrites, three aubrites, and one anomalous ungrouped enstatite meteorite. Synthetic samples range from pure endmembers (CaS, FeS, MgS) for the Cr-rich sulfides; and at 290 and 335 cm À1 for troilite with, occasionally, an extra peak at 240 cm À1 . A peak at 160 cm À1 is present in all Raman spectra and cannot be used to discriminate between the different sulfide compositions. According to group theory, none of the cubic monosulfides oldhamite, niningerite, or alabandite should present first-order Raman spectra because of their ideal rocksalt structure. The occurrence of broad Raman peaks is tentatively explained by local breaking of symmetry rules. Measurements compare well with the infrared frequencies calculated from first-principles calculations. Raman spectra arise from activation of certain vibrational modes due to clustering in the solid solutions or to coupling with electronic transitions in semiconductor sulfides.

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The presence of magnesium in glasses of geological, medical, and technological interests influences their physicochemical and durability properties. However, the understanding of the role of magnesium is dependent on the combined knowledge of the structural environment of magnesium in the glass or melt and of the silicate network connectivity of the studied systems. In this article, we present a Raman spectroscopic study of the network connectivity of 10 ternary silicate glasses in the system Na 2 O-MgO-SiO 2 and one Mg-free binary silicate glass Na 2 O-SiO 2 . Results obtained at constant polymerization suggest the existence of various Q n units according to the nature of the modifying cation. As polymerization decreases for Na 2 O-MgO-αSiO 2 glasses (labeled as NMSα with α decreasing from 10 to 2), the band associated with Si-O-Si bending in fully polymerized region disappears being gradually replaced by a band attributed to Si-O-Si bending in region containing mainly Q 2 and Q 3 species. For highly polymerized glasses (NMS10-NMS4), the coexistence of these two bands suggests the presence of two interconnected networks. Concomitantly, the signal associated with Q 3 species first increases. For a further decrease of the polymerization, the high wavenumber part of the signal associated with Q 3 species decreases, while the intensity of the high wavenumber part of the band related to Q 2 species increases. This result strongly suggests that magnesium charge-balances preferentially Q 2 species rather than Q 3 species.