Spectral reflectance properties of carbonaceous chondrites: 1. CI chondrites
Research highlights
► Most CI reflectance spectra exhibit mineral-associated absorption bands. ► Magnetite seems the likeliest explanation for blue-sloped CI spectra. ► Larger grain size samples generally have the bluest and darkest spectra. ► Phase angle can cause CI spectra to become redder or bluer. ► Underdense CI samples have lower reflectance than regularly packed CI samples.
Introduction
Carbonaceous chondrites (CCs) are an important group of meteorites for understanding the origin and evolution of the solar system. They include a diversity of subgroups that exhibit different degrees of aqueous and/or thermal alteration, but are generally characterized by an overall dark appearance. They are important from an astrobiological perspective because some of their carbon-bearing phases are organic in nature and include many biological precursor molecules (e.g., Nagy, 1975). They may also have been an important source of water for the proto-Earth (e.g., Morbidelli et al., 2000). Their importance extends to understanding the origin and evolution of the solar system, because they have been linked, largely through spectroscopic comparisons, to various classes of asteroids (e.g., Gaffey and McCord, 1977, Gradie and Tedesco, 1982, Vilas and Gaffey, 1989, Gaffey et al., 1993, Burbine and Binzel, 1995, Burbine, 1998, Burbine et al., 2001).
CC-asteroid links have often been made on the basis of similarities in overall spectral shapes, which has often been necessitated by the fact that many low albedo asteroid (and cometary nuclei) spectra are devoid of resolvable and/or diagnostic absorption bands (e.g., Hiroi et al., 1993, Abell et al., 2005). The carbonaceous chondrite class consists of a number of groups, many of which have not been spectrally characterized in a comprehensive way.
This paper is the first in a series of CC papers, and discusses the spectral reflectance properties of the CI carbonaceous chondrites. We have undertaken a spectral reflectance study of CCs for a number of reasons: (1) to determine the range of spectral variability within CC classes; (2) to determine the spectral similarities or differences that exist between CC classes; (3) to search for absorption bands that are diagnostic of constituent phases; (4) and to better elucidate the relationships between CC mineralogy, structure, and spectra. In our analysis we include both existing CC reflectance spectra, new measurements made for this study, and new and existing reflectance spectra of constituent minerals and phases and mixtures. The goal of this study is to determine the range of mineralogic and petrologic information that can be derived from analysis of CC reflectance spectra. This analysis can also inform the search for CC parent bodies among the asteroids.
Section snippets
Carbonaceous chondrite classifications
The carbonaceous chondrite class has been defined by a number of criteria. They were defined by Mason (1962) as “stony meteorites characterized by the presence of an appreciable amount of carbonaceous material other than free carbon (diamond and graphite)” – but “appreciable” was not defined. In his classification, petrologic type 1 CCs are made up predominantly of largely amorphous hydrated magnesium–iron silicate and hydrated magnesium sulfate and magnetite. Type 2 CCs consist largely of a
Mineralogy of CI chondrites
Interpreting reflectance spectra of CCs requires knowledge of the spectral properties of the constituent phases, their composition, and physical disposition. Previous studies (e.g., Cloutis et al., 1990) have shown how even small amounts of dark fine-grained materials can strongly affect reflectance spectra when the opaque phases are fine-grained and effectively dispersed.
As mentioned, the known CI chondrites are confined to petrologic type 1, and perhaps grading into type 2. Meteorites that
Mineralogy and petrology of individual CIs
In order to assist the interpretation of the reflectance spectra of the three CIs included in this study (Alais, Ivuna, Orgueil), we summarize their mineralogy and petrology below. The state of knowledge is variable for each of these CIs but they appear to possess mineralogic and petrologic differences and similarities that may have implications for spectral analysis.
Weathering effects
Terrestrial weathering is a factor which must be considered when analyzing reflectance spectra of CCs. Terrestrial weathering can include the formation of brown (iron oxide) staining of silicates, cavities due to the loss of material by leaching, formation of sulfate veins, loss of sulfides, and oxidation of magnetite and Fe–Ni metal (e.g., Kallemeyn et al., 1991). Terrestrial weathering is often characterized by alteration of any pre-existing metal and sulfides to iron oxides and hydroxides,
Experimental procedure
This study focuses on analysis of reflectance spectra of powdered samples of various CIs. It includes existing spectra from the RELAB data base (http://relab.brown.edu), many of which have not been compared or analyzed in detail (e.g., Hiroi et al., 1993, Hiroi et al., 1994, Hiroi et al., 1997), CI reflectance spectra from other sources (Johnson and Fanale, 1973, Gaffey, 1974, Salisbury et al., 1975), new spectra of Alais and Orgueil (Table 3), and reflectance spectra of most of the constituent
Anhydrous silicates
Olivine is the only anhydrous mafic silicate identified in CIs. However it is rare (< few wt.%) and, when coupled with the low albedo of CIs and more abundant Fe-bearing phyllosilicates, is not expected to contribute to the spectral signature of CI chondrites.
Phyllosilicates
The nature of phyllosilicates in CIs is still imperfectly understood (Roy-Poulsen et al., 1981, McSween, 1987, Tomeoka, 1990, Brearley and Jones, 1998). This arises from the fact that they are commonly finely intergrown with additional
CI reflectance spectra
Three CIs have been spectrally characterized by multiple investigators, in some cases involving multiple grain sizes. These results, the spectral properties of CI constituent phases, phyllosilicate + opaque mixtures, and analysis of CI mineralogy and petrology, can collectively provide insights into the spectral properties of CI chondrites, as the known CI chondrite group includes only seven members, all of petrologic type 1 (e.g., Weisberg et al., 2006), and the three included CIs exhibit
Discussion
CI spectra exhibit a variety of spectral shapes, and their most common characteristics are low overall reflectance (<10%), a slope change or peak near 0.6 μm, and weak (<4% deep) absorption bands in the 0.8–1.2-μm region. Absorption bands that do appear are of variable utility for detecting the presence of specific components, and in many cases confident detections of absorption features are hampered by noise. Here we examine the utility of the various absorption bands that have been detected,
Phase angle effects
While the RELAB and PSF reflectance spectra that form the bulk of this analysis were acquired at i = 30° and e = 0°, asteroidal observations are often acquired at different phase angles. Coupled to this, disk-unresolved asteroid spectra will include contributions from the subsolar point to the limb, thus spanning a wide range of incidence, emission, and phase angles. This difference in observing conditions between the laboratory and telescopes may translate into differences in spectral slope and
Identification of CI parent bodies
A number of spectroscopic studies have forged tentative links between various asteroids and CCs. Here we review some of these results in light of the CI laboratory spectra. In spite of the fact that CI chondrites contain abundant Fe2+-Fe3+-bearing phyllosilicates, their spectra are largely devoid of an expected Fe3+–Fe2+ charge transfer absorption band in the 0.7-μm region. This is likely attributable to suppression by the various opaque phases. Consequently, the presence of such an absorption
Summary and conclusions
Individual CI chondrites possess differences in mineralogy and petrology that are significant enough to result in wide variations in spectral properties, particularly in terms of spectral slopes. Small differences in composition can cause significant changes in spectral slope, as well as the appearance of phyllosilicate-associated absorption bands. Significant spectral differences are seen even in duplicate spectra of the same sample after repacking.
Most of the CI spectra exhibit weak but
Acknowledgments
We wish to thank the invaluable and generous assistance provided by many individuals which made this study possible. We particularly thank Dr. Jeffrey Post of the Smithsonian Institution National Museum of Natural History and Dr. Linda Reinen of Pomona College for providing a number of the mineral samples used in this study; Mr. Neil Ball and Dr. Frank Hawthorne of the University of Manitoba for acquisition of XRD data for the mineral samples, Dr. Stanley Mertzman for XRF analysis of the
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