Icarus 300 (2018) 392–410
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Icarus
journal homepage: www.elsevier.com/locate/icarus
Spectral reflectance (0.35–2.5 μm) properties of garnets: Implications
for remote sensing detection and characterization
M.R.M. Izawa
a,b,*
, E.A. Cloutis
a
, T. Rhind
a
, S.A. Mertzman
c
, Jordan Poitras
a
,
Daniel M. Applin
a
, P. Mann
a
a
Department of Geography, University of Winnipeg, Winnipeg, MB R3B 2E9, Canada
b
Institute for Planetary Materials, Okayama University—Misasa, 827 Yamada, Misasa, Tottori 682-0193, Japan
c
Department of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-2615, USA
a r t i c l e i n f o
Article history:
Received 15 April 2017
Revised 9 August 2017
Accepted 5 September 2017
Available online 21 September 2017
Keywords:
Reflectance spectroscopy
Garnet
Remote sensing
a b s t r a c t
The utility of spectral reflectance for identification of the main end-member garnets: almandine
(Fe
2+
3
Al
2
Si
3
O
12
), andradite (Ca
3
Fe
3+
2
Si
3
O
12
), grossular (Ca
3
Al
2
Si
3
O
12
), pyrope (Mg
3
Al
2
Si
3
O
12
), spessartine
(Mn
2+
3
Al
2
Si
3
O
12
), and uvarovite (Ca
3
Cr
3+
2
Si
3
O
12
) was studied using a suite of 60 garnet samples. Com-
positional and structural data for the samples, along with previous studies, were used to elucidate the
mechanisms that control their spectral reflectance properties. Various cation substitutions result in dif-
ferent spectral properties that can be determine the presence of various optically-active cations and help
differentiate between garnet types. It was found that different wavelength regions are sensitive to dif-
ferent compositional and structural properties of garnets. Crystal-field absorptions involving Fe
2+
and/or
Fe
3+
are responsible for the majority of spectral features in the garnet minerals examined here. There can
also be spectral features associated with other cations and mechanisms, such as Fe
2+
–Fe
3+
and Fe
2+
–Ti
4+
intervalence charge transfers. The visible wavelength region is useful for identifying the presence of var-
ious cations, in particular, Fe (and its oxidation state), Ti
4+
, Mn
2+
, and Cr
3+
. In the case of andradite,
spessartine and uvarovite, the visible region absorption bands are characteristic of these garnets in the
sense that they are associated with the major cation that distinguishes each:
[6]
Fe
3+
for andradite,
[8]
Mn
2+
for spessartine, and
[6]
Cr
3+
for uvarovite. For grossular, the presence of small amounts of Fe
3+
leads to ab-
sorption bands near 0.370 and 0.435 μm. These bands are also seen in pyrope–almandine spectra, which
also commonly have additional absorption bands, due to the presence of Fe
2+
. The common presence of
Fe
2+
in the dodecahedral site of natural garnets gives rise to three Fe
2+
spin-allowed absorption bands in
the 1.3, 1.7, and 2.3 μm regions, providing a strong spectral fingerprint for all Fe
2+
-bearing garnets studied
here. Garnets containing Mn
2+
have additional visible (∼0.41 μm) spectral features due to
[8]
Mn
2+
. Gar-
nets containing Cr
3+
, exhibits two strong absorption bands near ∼0.7 μm due to spin-forbidden
[6]
Cr
3+
transitions, as well as
[6]
Cr
3+
spin-allowed features near 0.4–0.41 μm and 0.56–0.62 μm, and
[6]
Cr
3+
spin-
allowed transitions between 0.41 and 0.68 μm. Common silicate garnet spectra, in summary, are distinct
from many other rock-forming silicates and can be spectrally distinct from one garnet species to another.
Iron dominates the spectral properties of garnets, and the crystallographic site and oxidation state of the
iron both affect garnet reflectance spectra.
© 2017 Elsevier Inc. All rights reserved.
1. Introduction
Garnets are a widely distributed group of rock-forming min-
erals, and are important petrogenetic indicators. The presence of
garnets is commonly indicative of high-pressure formation con-
*
Corresponding author at: Institute for Planetary Materials, Okayama University—
Misasa, 827 Yamada, Misasa, Tottori, Japan.
E-mail addresses: matthew.izawa@gmail.com, matthew_izawa@okayama-u.ac.jp
(M.R.M. Izawa), e.cloutis@uwinnipeg.ca (E.A. Cloutis), stan.mertzman@fandm.edu
(S.A. Mertzman), poitras-j@webmail.uwinnipeg.ca (J. Poitras).
ditions (Juhin et al., 2010), though some garnets, especially cal-
cic varieties, also form at lower pressures e.g., in skarn deposits
(e.g., Meinert, 1992). Their crystal chemistry can provide informa-
tion concerning the composition and petrogenetic conditions of
their source region (e.g., Ghent, 1976; Holdaway, 2001). In addition
to high pressure environments, garnets may also form during the
crystallization of magma in some pegmatites and aluminous felsic
intrusive rocks, by regional or contact metamorphism, and in hy-
drothermal systems (Galoisy, 2013). The crystal structure of garnets
allows for a wide range of chemical substitutions, some of which
https://doi.org/10.1016/j.icarus.2017.09.005
0019-1035/© 2017 Elsevier Inc. All rights reserved.