Results of reflectance spectral, mоssbauer, X-rey and electron microprobe investigations of terrestrial serpentine samples

RESULTS OF REFLECTANCE SPECTRAL, MЦSSBAUER, X-REY AND ELECTRON MICROPROBE INVESTIGATIONS: V. V. Busarev et al.

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Results of reflectance spectral, mоssbauer, x-rey and electron microprobe investigations of terrestrial serpentine samples

RESULTS OF REFLECTANCE SPECTRAL, MЦSSBAUER, X-REY AND electron microprobe INVESTIGATIONS OF TERRESTRIAL SERPENTINE SAMPLES. V. V. Busarev1, M. V. Volovetskij2, M. N. Taran3, V. I. Fel'dman4, T. Hiroi5, and G. K. Krivokoneva6, 1Sternberg State Astronomical Institute, Moscow University, 119992 Moscow, Russia Federation (RF), e-mail: busarev@sai.msu.ru; 2Division of Mossbauer Spectroscopy, Physical Department of Moscow State University, 119992 Moscow, RF; 3 Institute of Geochemistry, Mineralogy and Ore Formation, Academy of Sciences of Ukraine, 03142 Kiev, Ukraine; 4Division of Petrology, Geological Department of Moscow State University, 119992 Moscow, RF; 5Department of Geological Sciences, Brown University, Providence, Rhode Island 02912; 6All-Russia Research Institute of Mineral Resources (VIMS), 119017 Moscow, RF.

Abstract: Previously, we have reported on reflectance spectral measurements of arbitrary selected terrestrial serpentine and chlorite samples which may be considered as analogues of hydrated solid celestial bodies [1]. Then, we performed electron microprobe, Mцssbauer and X-ray investigations of the samples and made a previous correlation analysis of the results for Mg-serpentine samples. We discovered a considerable magnitude (up to ~25% in relative intensity) of an absorption band centered at 440 nm in reflectance spectra of lizardites and chrysotiles. It was found a strong correlation of the equivalent width of the absorption band in reflectance spectra of the samples with octahedral total Fe3+.

Reflectance spectral measurements: The visible and near-IR reflectance spectra of seven magnesial serpentine samples (powders of ?0.15 mm grain size) were measured in the 400-1000-nm range. A description of the facilities is given in [1]. Normalized reflectance spectra of 1a, 4a, 4b, 10a, 28, 30 and 2540 serpentine samples are shown in Fig. 1.

Fig. 1.

We have found a pronounced absorption band at 440 nm in reflectance spectra of the samples (except for 30 one). The curves are presented in the order of increasing of the band strength from top to down (Fig. 1). The equivalent width of the band was calculated for each of the reflectance spectra (after the continuum slope was calculated, withdrawn and the spectrum normalized) according to the following formula: spectral terrestrial serpentine celestial

,

where W440 is the equivalent width, is the spectral step, r(i) are the residual intensities in the spectrum, and N is the number of points in the band.

Reflectance spectra of 1a, 28, 30 and 2540 powdered serpentine samples were measured in a more wide spectral range (400-3600 nm). For that, it was used the Keck/NASA RELAB spectral facilities in Brown University [2]. The results are shown in Figs 2 and 3.

Fig. 2.

Fig. 3.

As seen from the spectra (Figs 2 and 3), unlike other serpentine samples having an absorption band at 440 nm, sample 30 has a strong and wide absorption band at 900 nm and a less intense absorption band assigned to vibrations involving the OH-stretching modes with lattice modes or to fundamental vibrations (e. g., [3]).

We have investigated an effect of magnetite content in sample 28 (3.5 wt.%) on its reflectance spectrum (Fig. 4).

Fig. 4.

For the purpose, magnetite was extracted from sample 28 (by a magnetic method and consecutive crushing of the sample), and reflectance spectra of sample 28nm (without magnetite) and magnetite from it were measured .

Results of electronic microprobe, X-ray and Mцssbauer investigations of the samples are given in tables 1-3. We have found a strong linear correlation of the equivalent width of 440-nm-absorption band in reflectance spectra of the samples with their contents of octahedral Fe3+ (coefficient R=0.97, table 3) and total (octahedral + tetrahedral) Fe3+ (R=0.90).

Conclusions: Thus, we suppose that a 440-nm-

absorption band in reflectance spectra of solid celestial

bodies may be used as an indicator of Fe3+ and, hence, hydrated and/or oxidized states of their matter.

Table 1. Microprobe results for serpentines.

Table 2. Results of X-ray investigation of mineral content of the samples.

Table 3. Results of correlation analysis for the equivalent width of a 440-nm-absorption band and iron content (according to measurements) for serpentine samples.

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