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Raman study of a Martian meteorite-- Zagami

 

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Zagami fell on Oct. 3, 1962 about 0.75 miles from Zagami Rock, Katsina province, Nigeria. It was later identified as a Martian meteorite of the SNC type.

Photo of the Zagami sample, which has a rough-sawn surface, 1.5 x 1.0 x 0.5 cm in size.  Linear traverses, along which several hundred Raman spectra were taken, are shown in this photo.

 

The major and accessory mineral phases identified by the Raman measurements include pyroxene, maskelynite (shocked plagioclase), merrillite, apatite, pyrrhotite, and magnetite. These identifications are  consistent with previous petrologic studies of Zagami. A few hematite spectra were also obtained, although this phase had not been observed before. Typical Raman spectra are shown in following figures.

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major minerals -- two types of pyroxene and shocked plagioclase (maskelynite) accessory phosphate minerals merrillite (whitlockite) and apatite

minor Fe oxides and sulfides

 

From over 300 Raman mesurements along five linear traverses, we got the mineral proportions in the Zagami matrix . The results agree with literature values.

Stolper & McSween

(1979)

McCoy et al. (1992)

Fine grained

Coarse grained
zagami_8.gif (22599 bytes) Pyroxene 75.5 76.3 69.7 77.7 74.3 76.0 80.4
Maskelynite 20.2 18.8 24.7 17.6 18.8 18.6 10.3
Mesostasis 1.7 2.6 1.8 3.0 2.1 3.7
Oxides 2.0 2.7 2.8 1.5 1.8 2.0 2.6
Sulfides 0.5 0.5 0.2 0.6 0.4 0.4 0.6
Phosphates 1.7 0.5 0.6 0.5 1.3
Shock melt 0.1 0.9 0.3 0.9

Pyroxene is the most abundant mineral in Zagami.  Some 245 pyroxene spectra were obtained, and nearly all have the spectral pattern of monoclinic pyroxene, but a few have the spectral pattern of orthopyroxene. The Raman peaks of pyroxene vary systematically over a  range of tens of wavenumbers, which suggests these pyroxene are strongly zoned compositionally over a short distance.

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Raman peak positions of Zagami pyroxenes are plotted on the field of lunar pyroxenes, for which the correlation of Raman peak positions with cation ratios were established. The points for Zagami pyroxene lie along the same broad curve observed for lunar pyroxenes and are cover a range in Mg# from ~0.3 to ~0.6, which is consistent with literature studies. Compositional variation of Zagami pyroxenes along two traverse paths

Conclusion: This study demonstrates that basic, first order information on the mineralogy and petrology of a Martian rock (Zagami) could be obtained using a Raman measurement procedure of the type anticipated for in situ analysis on planetary surfaces.  From this set of data, we were able to recognize that the rock had been shocked but was otherwise unaltered.   It is a pyroxene-rich igneous rock with plagioclase as its second most abundant mineral. The pyroxenes are strongly zoned, and the texture of the rock is basaltic. 

Reference: Wang A., Jolliff B. L., Haskin L. A., “Raman spectroscopic characterization of Martian meteorite Zagami”, Twenty-ninth Lunar and Planetary Science Conference, 1998.
                       
Wang A., Jolliff B.L., and Haskin L.A. (1999) Raman spectroscopic characterization of a Martian SNC meteorite: Zagami. J. Geophys. Res., 104, 8509–8519.

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