SPECTRAL-COMPOSITIONAL PROPERTIES OF IRON-BEARING SHEET SILICATES: REMOTE SENSING IMPLICATIONS

The spectral reflectance properties of a number of iron-bearing sheet silicate species were examined to develop quantitative relationships between composition and structure, and reflectance spectra. Because such minerals are potentially a volatile sink on Mars, the results of our study will yield information on the geologic history of surface minerals on Mars and certain groups of asteroids, through the identification of types, abundance, and composition of iron-bearing sheet silicate minerals.

A range of iron-bearing sheet silicate minerals have been analyzed via reflectance spectroscopy, XRF, wet chemistry, atomic absorption spectroscopy, X-ray diffraction, and thermal analysis. The reflectance spectra of some of these minerals exhibit a number of absorption features in the 0.3-1.2 µm region, including berthierine, chamosite, cronstedtite, glauconite, saponite, serpentine, and vermiculite.

Berthierine exhibits a band at 0.35µm, which is probably a metal-O charge transfer. The serpentine spectra show a narrow absorption band near 0.38µm, which is attributed to ferric iron. Serpentine, cronstedtite and glauconite exhibit an absorption feature at 0.43µm attributed to ⁶ A1₁ ->⁴T ₁,⁴E(G) Fe ³⁺ spin-forbidden crystal field transition, which becomes increasingly intense with increasing Mn content. Berthierine, chamosite, saponite, and glauconite share an inflection point near 0.48µm, which is due to a ferric-ferrous intervalence charge transfer transition (IVCT). In the saponite spectra, the slight inflection found at 0.54 µm is also credited to IVCT. The feature located at 0.65µm in the serpentine spectra is caused by transitions within Fe²⁺ ions. The absorptions at 0.7µm in berthierine, 0.75µm in cronstedtite, and 0.7 and 0.75µm in the glauconite spectra, are all attributed to a Fe²⁺->Fe³⁺ charge transfer transition. At 0.67µm in the saponite spectra, there is an inflection which has been attributed to a ⁶ A1₁ ->⁴T ₂,⁴E(G) Fe³⁺ charge transfer transition. Absorption features in chamosite at 0.72µm and cronstedtite at 0.82µm are both attributed to the ferric-ferrous intervalence charge transfers.

The 0.43 and 0.7µm bands seen in many dark asteroid spectra (Jarvis et al., 2000) are also seen in the reflectance spectra of a number of our samples, including berthierine, cronstedtite, glauconite, and serpentine. It should be noted that no single sheet silicate in this study exhibits every feature found in the asteroid spectra. The wavelength position of specific absorption bands due to various electronic transitions varies between different mineral species. Therefore we should be able to narrow down the range of possible sheet silicate minerals present on the surface of asteroids on the basis of absorption band wavelength positions. Iron-bearing sheet silicates exhibit a large number of absorption features attributable to both ferrous and ferric iron. As the wavelength positions of these bands vary from species to species, they are potentially diagnostic for remote sensing detection of specific, iron-bearing sheet silicate minerals on Mars and asteroids.