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\begin{document}

\title{Mineralogy of Natural Dust Samples from LWIR Reflectance and
Transmission Spectroscopy}



\author[1]{Mohammad Reza Sadrian}%
\author[1]{Wendy Calvin}%
\affil[1]{Department of Geological Sciences and Engineering, University of Nevada, Reno, Reno, 89557, USA}%


\vspace{-1em}



  
  \date{May 25, 2024}


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\begin{abstract}
Mineral dust particles are ubiquitous in the atmosphere and can be
transported vast distances affecting climate, air quality, and human
health on a global scale. Mineralogical composition has a substantial
impact on dust properties and their effects. Natural dust samples are
both fine-grained and composed of many different minerals. Most
commonly, X-ray diffraction (XRD) has been used to characterize dust
mineralogy; however, this technique is less effective for identifying
poorly crystalline or amorphous phases. We used Fourier Transform
Infrared (FTIR) spectroscopy as a complementary method to identify
minerals and their abundances. Long wave infrared (LWIR) spectra (2.5 to
25 \selectlanguage{greek}μ\selectlanguage{english}m) are sensitive to molecular bonds rather than crystallography
providing additional details. We performed both XRD and reflectance
spectroscopy to characterize 37 atmospheric dust samples collected in
Ilam City, Iran. The dominant minerals in these samples are quartz,
feldspar (albite), calcite and clays (illite, montmorillonite,
kaolinite). LWIR reflectance is strongly dependent on particle size but
published data of pure silicate minerals in the size range of the Ilam
samples (0-63 \selectlanguage{greek}μ\selectlanguage{english}m) still show characteristic signatures between 8 and 10
\selectlanguage{greek}µ\selectlanguage{english}m (Salisbury et al. 1991; Wenrich and Christensen, 1996). Surprisingly,
diagnostic silicate features were not observed in any of the samples
although carbonate and OH bonds in the clay minerals were readily
identified. Past studies have shown that porosity, grain size and
packing can reduce the spectral contrast in the LWIR and additional
effects include grain coatings or the interaction of multiple minerals.
We also identify a peak at 7.8 \selectlanguage{greek}µ\selectlanguage{english}m which may be attributed to anomalous
dispersion or the interaction of quartz and calcite in this spectral
range. In order to understand the absence of Si-O features we made
transmission measurements of representative samples in KBr pellets.
Transmission is not influenced by multiple scattering and should clearly
detect fundamental Si-O absorptions. Transmission spectra show broad
features that include contributions from all silicate minerals (quartz,
feldspar and clays) both near 10 \selectlanguage{greek}μ\selectlanguage{english}m and at longer wavelengths. We are
using various spectral modeling techniques and will compare abundances
derived from reflectance and transmission measurements.%
\end{abstract}%



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