Spectral-compositional variations in the constituent minerals of mafic and ultramafic assemblages and remote sensing implications

Abstract

The 0.3-2.6pm reflectance spectra of most mafic and ultramafic assemblages can best be interpreted by considering the spectra as being composed of mafic silicate spectra modified by the presence of opaques, such as ilmenite or magnetite, and plagioclase feldspar. The systematic spectralcompositional relationships for olivine, orthopyroxene, and clinopyroxene have been examined and it has been determined that absorption band wavelength positions are correlated with ferrous iron content. Binary mafic silicate mixtures are generally less well understood, but certain spectral features such as reflectance maxima and minima wavelength positions and absorption band areas can be used to quantify or at least constrain end member abundances and compositions. The addition of opaques to a mafic silicate assemblage lowers overall reflectance and band depths. This differs from the effects of increasing grain size which are to lower overall reflectance but increase band depths. Plagioclase is relatively transparent compared to mafic silicates and must be present in appreciable amounts (tens of percent) to be spectrally detectable. The reflectance spectra of most mafic and ultramafic assemblages are dominated by mafic silicate absorption features and analysis of their spectra on this basis allows constraints to be placed on properties such as end member abundances and compositions.

Reflectance Spectra of Olivine–Orthopyroxene-Bearing Assemblages at Decreased Temperatures: Implications for Remote Sensing of Asteroids

“…8). A straight line continuum removal was performed to determine the wavelength positions of the band I and band II centers (see, e.g., Cloutis and Gaffey 1991b). Band centers were calculated by third-order polynomial fitting to several data points on either side of the visually determined minimum after dividing out the linear continuum.…”

Optical Effects of Regolith Processes on S-Asteroids as Simulated by Laser Shots on Ordinary Chondrite and Other Mafic Materials

“…The values of spectral parameters measured in this study are tabulated in Table III. Allende altered samples also contain metal/sulfide beads enes in Elenovka does not exceed 5% (McSween et al 1991) and should not affect the positions of band I and II (Cloutis and Gaffey 1991). So, the meteorite Elenovka may be roughly described as a two-component mixture of olivine Fa 25-26 (mole%) and orthopyroxene Fs [19][20][21][22][23] (mole%) (Baryshnikova and Lavrukhina 1979).…”

Spectroscopy of synthetic Mg‐Fe pyroxenes I: Spin‐allowed and spin‐forbidden crystal field bands in the visible and near‐infrared

“…This makes pyroxene a very useful mineral for remote geochemical analysis. Near-infrared studies of natural pyroxenes have revealed that changes in the positions of absorption bands are directly related to the primary octahedral pyroxene cations (Ca, Mg, and Fe), although substitutions of minor cations such as Ti 4+ , Cr 3+ , Fe 3+ , and Al 3+ also affect absorption features and can lead to misclassification of pyroxene compositions (Adams 1974;Sunshine et al 1990;Cloutis and Gaffey 1991b;Sunshine and Pieters 1993;Hamilton 2000;Cloutis 2002). The challenge for remote analyses has thus been to reduce the level of ambiguity in order to allow a quantitative assessment of mineral chemistry.…”