Identification of iron in Earth analogues of Martian phyllosilicates using visible reflectance spectroscopy: Spectral derivatives and color parameters
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Sánchez Marañón, Manuel; Cuadros, Javier; Michalski, Joseph R.; Melgosa Latorre, Manuel; Dekov, VesselinEditorial
Elsevier
Date
2018Referencia bibliográfica
Published version: Applied Clay Science 165 (2018) 264–276. https://doi.org/10.1016/j.clay.2018.08.016
Abstract
A range of phyllosilicate compositions have been detected spectroscopically on Mars, but the largest fraction by
far corresponds to clay minerals rich in Fe and Mg. Given that most of our understanding of Martian clays comes
from remote sensing data, it is critically important to explore the details of how compositional variation affects
spectral features of phyllosilicates. The greatest efforts have focused so far on near-infrared (NIR) spectroscopy.
Recently, ambiguities have been detected in the NIR spectra of 2:1 phyllosilicates with intermediate FeeMg
content that preclude mineral and chemical discrimination. Such ambiguities highlight the relevance of exploring
the visible spectral range as a complementary tool to characterize Martian phyllosilicates precisely. This
article reports the investigation of laboratory reflectance spectra (330–800 nm) from 34 Earth analogues of
Martian phyllosilicates with a wide range of MgeFe composition, including nontronite, celadonite and saponite
end-members, as well as interstratified glauconite-nontronite, talc-nontronite, and talc-saponite. The spectra
indicated the presence of Fe(III) by absorption modulations and a decrease in total reflectance, especially in
samples with tetrahedral Fe(III). Absorption bands at 370 and 420 nm were diagnostic of octahedrically and
tetrahedrally coordinated Fe(III), respectively. Band amplitudes in the second derivative of the Kubelka-Munk
function correlated positively with Fe(III) content (R2 > 0.8). Standard color analyses of the visible reflectance
spectra under the CIE illuminant D65 indicated that the CIELAB color parameter a*10 was positively correlated
with tetrahedral Fe(III), b*10 was positively correlated with octahedral Fe(III), and L*10 was negatively correlated
with Fe(III) in both structural sites. Because Fe(II) was in relatively low amount, it did not provide clear
spectral evidence. Multiple regression models using the amplitude of the diagnostic absorption bands predicted
well absolute Fe content in the phyllosilicates (R2=0.89) and the ratio Fe/(Fe+Mg+Al) (R2=0.84). CIELAB
color parameters improved the prediction of total Fe (R2=0.92) and the ratio Fe/(Fe+Mg+Al) (R2=0.93).
Application of these analyses to Martian data has challenges set by Fe oxide dust coating and spatial and spectral
resolution. However, these results mark an avenue to develop testable tools using visible-wavelength spectral
data from both satellite and lander probes to help establishing Fe content and mineral identification of Martian
phyllosilicates.