LABORATORY INFRARED SPECTRA OF AND INTERPLANETARY DUST FROM 2.5 TO 25 MICRONS. S.A. Sandford, McDonnell Center for the Space Sci- ences, Washington University, St. Louis, MO 63130

Infrared transmission spectra from 2.5 to 25 microns of 53 meteorites show that some classes of meteorites can be identified by their infrared spec- trum alone. Spectra were taken from 14 carbonaceous , 5 LL ordinary chondrites, 6 L ordinary chondrites, 10 H ordinary chondrites, 1 enstatite , 4 , 3 , 4 , 1 , 1 , 2 , 1 shergottite, and the anomalous Angra dos Reis. The CM carbonaceous chondrites have distinctive spectra and can easily be separated from the other groups. The spectrum of the carbonaceous chon- dr it e Renazzo was unique among the carbonaceous chondr it es , con£ irming McSween's classification of this as an example of a new CR carbona- ceous chondrite (1) group. The one mesosiderite and the one CI examined both had distinctive spectra. The CO and CV carbonaceous chondrites have spectra similar to each other and it is not possible to separate these two classes spectroscopically. In addition, their spectra can be confused with those from some of the ordinary chondrites. The enstatite meteorites also exhibit distinctive spectra and while it is possible to uniquely identify the enstatite meteorites, it is not possible to separate these meteorites into their chondrite and achondrite () subgroups. The ordinary chondrites, while being spectrally similar to each other, exhibit the greatest spectral variability of any meteorite class and they cannot be separated into their LL, L and H subgroups. The eucrites, howardites, diogen- ite, nakhlites, shergottite, and angrite that were examined all had similar spectra, Although these spectra were distinct from those of the other meteor- ites, they could not be separated into their constituent classes. Interpretation of the meteorite spectra are consistent with the known mineralogies of these meteorites. Major conclusions are: (i) The CM carbona- ceous chondrite spectra are dominated by hydrated silicates, as is the spec- trum of the single CI carbonaceous chondrite examined. The CM carbonaceous chondrites also show weak bands associated with the presence of carbonates and the CI carbonaceous chondrite spectrum contained several bands that could be produced by sulphates. (ii) The CO and CV carbonaceous chondrite spectra are dominated by . (iii) The carbonaceous chondrite Renazzo has a unique spectrum that can be interpreted as a mixture of hydrated silicates and olivine (or perhaps pyroxene). (iv) The spectra from the ordinary chondrites are consistent with mixtures of olivine, pyroxene, and perhaps plagioclase. (v) The enstatite meteorite spectra are dominated by the pyroxene enstatite. (vi) Pyroxene is the major component seen in the spectra taken from the eucrites, howardites, diogenite, nakhlites, shergottite, and angrite examined. (vii) The single mesosiderite spectrum obtained is also probably dominated by pyroxene but it contains an additional unidentified band. It has been found that the majority of the interplanetary dust particles examined in the laboratory have spectra falling into one of 3 major spectral groups (2). These spectral groups have been classified as pyroxenes, hydrated silicates and , respectively.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System INFRARED SPECTlU OF METEORITES

Sandford, S. A.

Although the pyroxene group has silicate absorption bands with positions and widths similar to those seen in terrestrial pyroxenes, no two particles in this group have identical 10 micron bands. Only one particle's spectrum has been clearly associated with a known pyroxene, enstatite. Particles falling into this spectral group tend to have fluffy, reentrant morphologies. Particles in the hydrated silicate group have spectra which are very similar to the spectra obtained from the CM carbonaceous chondrites, although the carbonate feature at 7 microns tends to be stronger in the dust particles. The olivine particles have spectra that are virtually identical with those obtained from terrestrial iron-rich olivines. In general, the olivine and hydrated silicate particles have more compact morphologies than the pyrox- ene particles. It is not believed that the dust represents fragments of the known meteorite classes, despite the fact that the infrared groups observed in the dust particles correspond to the three major types seen in the meteor- ites. A great deal of mineralogic morphologic, and isotopic evidence points to an independent source for the majority of interplanetary dust particles (3-6).

References

(1) H.Y. McSween Jr . (1979) Are carbonaceous chondrites primitive or processed? A review. Rev. Geophys. &. Phys. l7, p. 1059-1078. (2) S.A. Sandford, P. Fraundorf, R.I. Patel, and R.M. Walker (1982) Laboratory infrared spectra of interplanetary dust. ~etebritics17, p. 276-277. (3) P. Fraundorf (1982) Interplanetary dust in the transmission electron microscope: diverse materials from the early solar system. Geochim. Cosmochim. --Acta 45, p. 915-943. (4) P. Fraundorf, D.E. Brownlee and R.M. Walker (1982) Laboratory studies of interplanetary dust. In : Gases, Ices, Grains and Plasma (L. Wildening, ed.), Univ. Arizona Press, p. 383-409. (5) J.P. Bradley and D.E. Brownlee (1983) Heterogeneous catalysis - Its role in the formation of carbon in interplanetary dust (abstract). 46th Met. Soc. Conf., p. 17. (6) E. Zinner, K.D. McKeegan, and R.M. Walker (1983) Ion probe measurements of H and C isotopic ratios in interplanetary dust and meteorites (abstract). 46th Met. Soc. Conf., p. 221.

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