Mineralogy, Mineral Chemistry and SWIR Spectral Reflectance of Chlorite and White Mica

Abstract

Hyperspectral reflectance has the potential to provide rapid and low-cost mineralogical and chemical information that can be used to vector in mineral systems. However, the spectral signature of white mica and chlorite, despite numerous studies, is not fully understood. In this study, we review the mineralogy and chemistry of different white mica and chlorite types and investigate what mineralogical and chemical changes are responsible for the apparent shifts in the shortwave infrared (SWIR) spectroscopic absorption features. We demonstrate that the spectral signature of white mica is more complex than previously documented and is influenced by the Tschermak substitution, as well as the sum of interlayer cations. We show that an increase in the interlayer deficiencies towards illite is associated with a change from steep to shallow slopes between the wavelength position of the 2200 nm feature (2200 W) and Mg, Al(VI) and Si. These changes in slope imply that white micas with different elemental chemistry may be associated with the same 2200 W values and vice versa, contrary to traditional interpretation. We recommend that traditional interpretations should only be used in true white mica with sum interlayer cations (I) > 0.95. The spectral signature of trioctahedral chlorite (clinochlore, sheridanite, chamosite and ripidolite) record similar spectral relationships to those observed in previous studies. However, dioctahedral Al-rich chlorite (sudoite, cookeite and donbassite) has a different spectral response with Mg increasing with 2250 W, which is the opposite of traditional trioctahedral chlorite spectral interpretation. In addition, it was shown that dioctahedral chlorite has a 2200 W absorption feature that may introduce erroneous spectral interpretations of white mica and chlorite mixtures. Therefore, care should be used when interpreting the spectral signature of chlorite. We recommend that spectral studies should be complemented with electron microprobe analyses on a subset of at least 30 samples to identify the type of muscovite and chlorite. This will allow the sum I of white mica to be obtained, as well as estimate the slope of 2200 W absorption trends with Mg, Al(vi), and Si. Preliminary probe data will allow more accurate spectral interpretations and allow the user to understand the limitations in their hyperspectral datasets.

Analysis of Salts and Clays for Conservation of Porous Cultural Heritage

“…There are small differences in the calibration for each clay sample, but using a general calibration is likely to provide results of sufficient accuracy to be use-ful. The NIR also showed a good calibration with the peak at 2217 nm [30]. The peak position is indicative of a lower Al(iv) content, with higher contents shifting the peak towards 2199 nm [31].…”

Lithological discrimination and mineralogical mapping using Landsat-8 OLI and ASTER remote sensing data: Igoudrane region, jbel saghro, Anti Atlas, Morocco

“…The minerals were detected on the basis of their wavelength absorptions in the VNIR-SWIR regions. The muscovite has an absorption ranging between 2195 and 2215 nm, while phengite is ranging between 2215 and 2225 nm [ 85 ]. The illite is known for its deeper absorption at 1900 nm [ 45 ].…”

Eski̇şehi̇r'de Muttali̇p Ki̇li̇ İçi̇n Uygulamali Araştirma

“…Chlorites are also a group of phyllosilicate minerals. Chlorites are defined by the elements Mg, Fe, Ni and Mn in the silicate lattice [14]. Biotite is rich in iron, consisting of aluminum silicate and potassium [15].…”