Micro-Raman studies of hydrous ferrous sulfates and jarosites.

Ferrous sulfates of various hydration states (FeSO(4) X xH(2)O; x=7, 4, 1) and jarosites (MFe(3)(SO(4))(2)(OH)(6); M=Na or K) were synthesized and studied by micro-Raman spectroscopy between 295 and 8K. Spectral analyses of the sulfate and water/hydroxyl vibrational modes are presented. Fingerprint regions attributed to the symmetric (nu(1)) and antisymmetric (nu(3)) stretching vibrations of the sulfate group are found to vary with the degree of hydration in hydrous ferrous sulfate. In jarosites, the Raman shift of the OH stretching mode is related to the type of alkali metal present between the tetrahedral and octahedral layers. The Raman technique can thus unambiguously identify ferrous sulfate of various hydration states and jarosites bearing different alkali metal ions.

Pulsed remote Raman system for daytime measurements of mineral spectra.

A remote Raman system has been developed utilizing a 532nm pulsed laser and gated intensified charged couple device (ICCD) detector in the oblique geometry. When the system is set for 50m sample distance it is capable of measuring Raman spectra of minerals located at distances in the range of 10-65m from the telescope. Both daytime and nighttime operations are feasible and the spectra of minerals can be measured in a short period of time, of the order of a few seconds. In oblique geometry, measured sampling depth is more than 30m, during which the system maintains very high performance without any adjustments. Much longer sampling depth (0.1-120m) has been observed when the system is configured in the coaxial geometry. Clear advantages of using a gated detection mode over the continuous (CW) mode of operation in reducing the background signal and eliminating long-lived fluorescence signals from the Raman spectra are presented. The performance of the pulsed Raman system is demonstrated by measuring spectra of Raman standards including benzene (C(6)H(6)) and naphthalene (C(10)H(8)), a low Raman cross section silicate mineral muscovite (KAl(2)(Si(3)Al)O(10)(OH)(2)), and a medium Raman cross section mineral calcite (CaCO(3)).

Effects of particle size and laser-induced heating on the Raman spectra of alpha quartz grains.

Raman spectra of alpha-quartz (Qz) grains of various size (250 microm to < 11 microm) and arrangement (individual and aggregated) have been investigated with a combination of confocal Raman and micro-Raman systems. Frequency downshift and line broadening of the 464 cm(-1), v,(Si-O-Si) band are observed in the smallest size group (< 11 microm, both individual grains and aggregates) because of laser-induced heating and are used to estimate the temperature of the sampled region. The intensity ratio of the anti-Stokes to Stokes Raman lines is also used to estimate the vibrational temperature of the samples under different excitation power. The degree of laser-induced heating is more noticeable in the aggregates than in the individual grains with the use of medium-level laser excitation (< or = 150 mW). Heating diminishes with increasing grain size, and it can only be detected in grain aggregates between 11 and 20 microm in diameter using 150 mW excitation. Intensity studies of the v(s)(Si-O-Si) band using individual grains show no noticeable signs of grain size effects. However, grain size effects become an important factor in the study of aggregates in which spectral intensity diminishes with respect to decreasing grain size.

Raman spectroscopic investigation on Jarosite-Yavapaiite stability.

The stability of synthetic Jarosite (KFe(3)(SO(4))(2)(OH)(6)) at low temperature and reduced atmospheric pressure has been studied by Raman spectroscopy. Jarosite remains stable between 8 and 295 K, provided that the sample is not exposed to reduced atmospheric pressure. When exposed to reduced atmospheric pressure (2.0x10(-2) Torr), however, the conversion of Jarosite into a different mineral is readily detected at room temperature by the appearance of a new Raman peak. The Raman shift of this peak (1032 cm(-1)) matches with that of Yavapaiite (KFe(SO(4))(2)), which can be obtained by thermal decomposition of Jarosite above 473 K. These studies provide a better understanding of the stability of Jarosite subjected to conditions similar to that on the surface of Mars.