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| Titel |
Reflectance Spectra of Synthetic Ortho- and Clinoenstatite in the UV, VIS, and IR for Comparison with Fe-poor Asteroids |
| VerfasserIn |
Kathrin Markus, Gabriele Arnold, Harald Hiesinger, Arno Rohrbach |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250133336
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| Publikation (Nr.) |
 EGU/EGU2016-13936.pdf |
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| Zusammenfassung |
| Major rock forming minerals like pyroxenes are very common in the solar system and show
characteristic absorption bands due to Fe2+ in the VIS and NIR [e.g., 1, 2]. The Fe-free
endmember enstatite is also a common mineral on planetary surfaces like asteroids and
probably Mercury [3] and a major constituent of meteorites like aubrites [4] and enstatite
chondrites [5]. Reflectance spectra of these meteorites as well as the enstatite-rich or
generally Fe-poor asteroids like the asteroidal targets of the Esa Rosetta mission (2867)
Steins [6] and (21) Lutetia [7] are often featureless in the VIS and NIR lacking the
absorption features associated with iron incorporated into the crystal structure of
silicates. Fe-bearing orthopyroxenes show diagnostic absorption bands at ∼1 μm and
∼2 μm. While systematic changes in positions and depths of these bands with
changes in Fe- and Ca-content of orthopyroxenes have been extensively studied
[e.g., 2, 8], almost Fe-free enstatite is so far only spectroscopically investigated by
[2].
For a better understanding of these Fe-poor bodies the availability of laboratory spectra of
Fe-free silicates as analog materials are crucial but terrestrial samples of enstatite usually
contain several mol% of FeO with pure enstatite being extremely rare. For easy availability of
larger amounts of pure enstatite we developed a technique for synthesis of enstatite. These
enstatite samples can be used as analog materials for laboratory studies for e.g. producing
mixtures with other mineral samples.
Enstatite has 3 stable polymorphs with clinoenstatite, orthoenstatite, and protoenstatite
being stable at low (<700˚ C), intermediate (>600˚ C), and high (>1000˚ C) temperatures
[9]. Orthoenstatite and protoenstatite are orthorhombic, while clinoenstatite is monoclinic.
Orthoenstatite is abundant in terrestrial rocks and in meteorites. Clinoenstatite is known from
meteorites [5, 9]. Both polymorphs of enstatite therefore exist on the parent bodies of aubrites
and enstatite chondrites. Clinoenstatite in enstatite chondrites and aubrites formed
presumably by crystallization from a melt and subsequent quenching and mechanical
deformation (brecciation) [5].
We synthesized powders of orthoenstatite and clinoenstatite. Following the synthesis we
used XRPD to discriminate between the polymorphs. The grain sizes of the samples
were determined using SEM pictures of the samples and are comparable to the
<25 μm sieving fractions of our terrestrial samples with some additional larger
grains. The orthoenstatite sample is slightly coarser than the clinoenstatite sample.
We collected reflectance spectra of both enstatite samples ranging from 0.25 μm
to 17 μm using the Vertex 70v and Vertex80v at IR/IS facility at the Institut für
Planetologie at the University Münster and the Institute of Planetary Research at DLR in
Berlin.
In the VIS and NIR both samples show weak absorption bands. The clinoenstatite shows
absorption bands at 1.75 μm and 0.90 μm. Both absorptions bands can be attributed to
minor amounts of Fe2+ in M2 positions of the clinoenstatite. The orthopyroxene shows
several weaker absorptions bands between 0.4 μm and 1 μm which are due to Fe3+ and
possibly Ti. Both samples show a steep red slope in the UV while spectral slopes
in the VIS and NIR are almost neutral with only a slightly reddish slope in the
VIS.
[1] Burns (1993) Mineralogical Applications of Crystal Field Theory, 2nd ed. [2] Klima
et al. (2007) Met. Planet. Sci., 42, 235-253. [3] Izenberg et al. (2014) Icarus, 228, 364-374.
[4] Keil (2010) Chem. Erde, 70, 295-317. [5] Mason (1968) Lithos, 1, 1-11. [6] Markus et
al. (2014) EGU 2014, #13341. [7] Coradini et al. (2011) Science, 334, 492-494. [8] Klima et
al. (2011) Met. Planet. Sci., 46, 379-395. [9] Lee and Heuer (1987) J. Am. Ceram. Soc., 70,
349-360. |
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