Visible/NIR Bidirectional Reflectance
The reflectance spectroscopy results for this wavelength region are presented in Fig. 3 (coarse particles) and Fig. 4 (fine particles). The positions of the 1 and 2 μm pyroxene absorption features are consistent with pyroxene compositions for the 2-pyroxene separates prepared from each sample, as the higher-Ca pyroxenes have absorption features at longer wavelengths (e.g., Adams 1974; Cloutis and Gaffey 1991). The 15058 augite spectra appear to have slightly stronger 1.2 μm absorption features relative to the Apollo 15 pigeonite spectra, which could be suggestive of a faster cooling history (Klima et al. 2008), although a stronger 1.2 μm absorption could also be caused by the higher-Ca content of the augites (e.g., Burns 1993; Klima et al. 2007, 2008). However, such conclusions cannot be drawn without quantitative deconvolutions of individual pyroxene bands in the spectra (e.g., Sunshine et al. 1990; Klima et al. 2008) such as those presented below.
The pyroxene separates from both Apollo 17 samples are more compositionally distinct than those prepared from the Apollo 15 samples (see Fig. 2). Their reflectance spectra reflect this compositional diversity, as they exhibit clear differences in the position of the 1 and 2 μm absorptions. The Apollo 17 pyroxene spectra appear to have weaker 1.2 μm absorptions than those observed in the Apollo 15 sample spectra. On the whole, the Apollo 17 pyroxene separates have higher abundances of CaO than do the Apollo 15 separates, suggesting that in this case, the comparatively weak 1.2 μm bands are due to a slightly slower cooling history. We make this interpretation because the weak 1.2 μm absorptions cannot be attributed to less iron partitioning into the M1 site, as the high-Ca content would tend to cause more iron to partition into the M1 site, not less (Klima et al. 2008). The slow cooling interpretation is consistent with the results of petrographic analyses of the paired thin section samples and previous analyses of these samples, which found them to have cooled relatively slowly (Brown et al. 1975). However, confirmation of the relative strengths of the 1.2 μm absorption features in these spectra requires quantitative deconvolutions.
Several interpretations are possible for the broad visible wavelength absorption observed in the Apollo 17 augite separate spectra (observed most prominently in Fig. 3). These pyroxenes have relatively high TiO2 contents, roughly twice those observed for pyroxenes in the Apollo 15 samples. It has been suggested that the presence of Ti3+ cations in the pyroxene structure can cause such absorptions in the visible region (Burns et al. 1976; Burns 1993), although a Fe2+-Ti4+ metal–metal charge transfer is also a possible explanation (Loeffler et al. 1975; Burns et al. 1976). Ti in these pyroxenes is likely to exist in both +3 and +4 valence states, so multiple transitions could contribute to the observed absorption feature. The feature is slightly stronger in the augite separate spectra relative to the pigeonite spectra, and augites in the Apollo 17 high-Ti samples have elevated Ti contents because they appear to have coprecipitated with ilmenite found in a more Ti-rich environment than pigeonites (this is discussed in the Supporting Information). These pyroxenes exhibit sector zoning, and the augites typically have small inclusions of ilmenite (illustrated in Figs. 1f and 1h). An alternative interpretation is that the ilmenite inclusions produce the broad feature across the visible in the Ti-rich pyroxene separate spectra. Similar features have been observed in analyses of microcrystalline ilmenite-bearing glassy phases, and have been attributed to the microcrystalline ilmenite (Pieters and Taylor 1989; Tompkins and Pieters 2010). If ilmenite were responsible for this feature, the transition responsible would likely be the Fe2+-Ti4+ metal–metal charge transfer in the ilmenite structure, in which these are the two dominant cations (Loeffler et al. 1975). We favor the structural Ti interpretation due to the apparent high abundance of TiO2 in the pyroxene and lack of direct visual evidence for microcrystalline ilmenite, although resolution of the structural Ti in pyroxene versus microcrystalline ilmenite interpretation is beyond the scope of this article and likely requires additional analytical techniques.
The broad multicomponent absorption centered near 1.05 μm in the olivine separate spectra (Figs. 3 and 4) is consistent with the olivine’s composition (approximately Fo50) based on the methods of Sunshine and Pieters (1998) and Isaacson and Pieters (2008). As is commonly observed in lunar olivine spectra, the spectra also have slightly asymmetric absorption features near 2 μm. Such features are not typically observed in spectra of synthetic or terrestrial olivines. We attribute this feature primarily to the presence of small chromite inclusions in the olivine (Pieters et al. 1990; Isaacson and Pieters 2010), as spinels have characteristic, strong absorption features near 2 μm (Cloutis et al. 2004). The chromite is observed as small crystalline growths within olivine crystals, a common feature of lunar olivine (Dymek et al. 1975; Papike et al. 1998). Photons interacting with olivine also interact with the chromite grains without having to pass through void spaces between mineral grains. Common nonlinear models for radiative transfer in particulate media (e.g., Hapke 1993, 2008) do not account for this situation quantitatively, but can be used to indicate that such a situation has major effects on the overall reflectance properties of a material. Because olivine in 15555 typically intergrows with plagioclase and pyroxene, as illustrated in Fig. 1, it is possible that trace amounts of pyroxene and plagioclase exist in our olivine separate. While visual inspection of the olivine separate shows it to be relatively pure, olivine is not optically active at wavelengths around 2 μm, and it is possible that minor quantities of pyroxene contamination could contribute to such features. However, the character of the absorption and the lack of any pyroxene features near 1 μm, confirmed by the quantitative deconvolutions performed by Isaacson and Pieters (2010), point to chromite being responsible for the 2 μm feature in the olivine spectra.
The Apollo 15 plagioclase spectra (Figs. 3 and 4) have strong absorption features relative to other pure plagioclase separate spectra, likely due to the high Fe content (Bell and Mao 1973) of these separates (about 0.5 wt% FeO). The Apollo 17 plagioclase separate spectra have slightly weaker 1.3 μm features, likely due to their lower Fe content (about 0.3 wt%). All plagioclase spectra presented here exhibit additional absorptions at longer wavelengths. The long wavelength edge of the 1.3 μm absorption feature has lower reflectance than its counterpart on the short wavelength side for the plagioclase spectra presented here (most apparent in the coarse particle spectra presented in Fig. 3). Although the 1.3 μm feature clearly dominates, the plagioclase absorption feature is not simply a single crystal field transition in Fe2+. While most absorption properties of plagioclase are understood based on mineral physics (e.g., Bell and Mao 1973; Adams and Goullaud 1978; Pieters 1996), the long wavelength behavior may be related to Fe content and a separate absorption feature (Hofmeister and Rossman 1984; Cheek et al. 2009). Lastly, the 70035 plagioclase is a somewhat less pure separate than the 70017 plagioclase, as evidenced by a weak absorption at 1 μm superimposed on the wing of the plagioclase 1.3 μm absorption (most apparent in the fine particle spectrum in Fig. 4). We interpret this feature as a pyroxene absorption likely caused by minor pyroxene contamination of the plagioclase separate. The feature is stronger in the finer particle separate spectrum, likely because the strength of the plagioclase absorption decreases, and the more optically active pyroxene absorption becomes proportionally stronger.
Ilmenite acts as an opaque across visible wavelength when the grain size is significantly greater than the wavelength of the light being measured (i.e., grain size much greater than 1 μm). Thus, the ilmenite separate spectra are dark and relatively featureless through the visible and part of the NIR before showing increased reflectance beyond approximately 1.7 μm (seen more prominently in Fig. 4, but also apparent in Fig. 3). The magnitude of features in the ilmenite spectra is strongly dependent on particle size (they become more prominent with decreasing particle size, as evidenced by the greater spectral contrast in the ilmenite spectra in Fig. 4 relative to those in Fig. 3), but the overall spectral properties of these lunar ilmenites are consistent with measurements of synthetic ilmenite (Riner et al. 2009).
The bulk sample spectra exhibit largely mirrors the pyroxene spectra. Pyroxene is the dominant optical phase in these samples, so this is unsurprising. The effect of the range of pyroxene compositions on the bulk sample is observed in the broader 1 and 2 μm absorptions in the bulk sample spectra as compared to the individual pyroxene separate spectra. The prominent inflection near 1.2 μm seen in all bulk basalt spectra (Figs. 3 and 4) has many potential contributions. Pyroxene, olivine, and plagioclase all have absorption features in this region, although not all play equal roles in bulk sample spectra. Plagioclase has an absorption feature in this region, although it is quickly overwhelmed in mixtures with more strongly absorbing phases like pyroxene and olivine. Careful quantitative analysis (i.e., spectral deconvolution and quantification of the properties of individual absorption features) is required before drawing conclusions about the detectability of plagioclase in such spectra (Klima et al. 2008). The pyroxene absorption is the most dominant component, as the inflection is observed in olivine-rich and olivine-poor samples alike. The effect of olivine is apparent in a comparison of the 15555 bulk sample spectra with other bulk sample spectra; the proportionally stronger 1.2 μm feature in 15555 relative to the 1.2 μm features exhibited by the other bulk sample spectra is attributed to the contribution of the olivine absorption.
The Apollo 17 bulk sample spectra differ markedly from the Apollo 15 bulk sample spectra. The most prominent differences are decreased albedo, significantly weakened absorption features at 1 and 2 μm, and pronounced long wavelength (greater than 1.7 μm) reflectance upturns in the Apollo 17 bulk sample spectra. The curvature of the bulk sample spectra in the 2 μm region is caused by the reflectance upturn, and mimics an absorption feature. These properties are attributed to the abundant ilmenite, which exhibits a reflectance maximum near 1 μm, is generally opaque, and exhibits an upturn in reflectance beyond approximately 1.7 μm. These effects are observed in both coarse particle and fine particle spectra, although they are exhibited more clearly by the fine particle spectra in Fig. 4. The difference between spectra of the coarse and fine particle splits of the Apollo 17 bulk samples is one of the most prominent differences between particle size separates, so we have illustrated this difference explicitly in a separate figure. Figure 9 shows visible/NIR reflectance spectra of coarse and fine particle sizes for the 15058 bulk sample, 70017 bulk sample, and 70017 ilmenite separate. These spectra are also plotted in Figs. 3 and 4, but are replotted here to illustrate clearly some prominent effects of particle size differences. This figure illustrates the dramatic effect of ilmenite on reflectance spectra of basalts, as the principal difference between the low-Ti (15058) and high-Ti (70017) samples is the abundant ilmenite in 70017. The spectral effects of ilmenite described above are exhibited quite clearly in the 70017 bulk sample spectra, as is the much more pronounced effect of ilmenite at finer particle size (compare the heavy black lines; the only difference between the two is particle size, assuming no preferential sorting during the crushing process). The effects of ilmenite on reflectance spectra are similar to some of the optical effects of space weathering, which include decreased albedo, increased spectral slope, and weakened absorption features. All of these effects are observed when comparing the low-Ti (Apollo 15) to the high-Ti (Apollo 17) bulk sample spectra. These samples are optically immature, so these results hint at the complexity of addressing space weathering and maturity in ilmenite-rich regions of the Moon, and generally indicate that ilmenite is a major complication in interpretation of reflectance spectra (e.g., Hiroi et al. 2009).
Figure 9. Selected coarse and fine particle visible to near-infrared reflectance spectra. Sample spectra plotted include 15058 bulk (low-Ti, ilmenite-poor), 70017 bulk (high-Ti, ilmenite-rich), and 70017 ilmenite. The abundant ilmenite in the 70017 bulk sample causes suppressed absorption features, reduced overall albedo, and a pronounced “red” spectral slope at longer wavelengths. These effects are far more prominent at the fine particle size, suggesting that the effect of ilmenite on reflectance spectra of basalts is strongly dependent on texture and particle size, as discussed in the text.
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NIR/Short Wavelength Mid-IR 2.5–8 μm (Volume Scattering)
Many spectra presented in Figs. 5 and 6 have broad absorptions at 3 μm. The feature near 3 μm is characteristic of an O-H vibrational absorption (Farmer 1974). We attribute this feature to adsorbed terrestrial water, which persists despite measurement in a dry air purged environment (Gibson and Moore 1972; Epstein and Taylor 1973; Dyar et al. 2010; Hibbitts et al. 2010). This feature is apparently the strongest in the plagioclase separate spectra. The 3 μm feature also is generally more apparent in coarse particle sample spectra (Fig. 5). A number of explanations for this observation are possible. First, plagioclase may be more susceptible to adsorption of water, perhaps due to a surface chemistry process. Additionally, although plagioclase is a nominally anhydrous mineral (it has no OH or H2O in its chemical formula), it has been shown to contain structural water and OH (e.g., Bell and Rossman 1992; Seaman et al. 2006), in some cases up to approximately 1000 ppm (e.g., Johnson and Rossman 2003, 2004), so the 3 μm absorption may be caused partially or entirely by internal water in the plagioclase grains. Alternatively, the apparent stronger absorption feature in plagioclase may be simply an effect of the relative transparency of plagioclase relative to the other phases analyzed. The mean optical path length is greater in a transparent phase, which increases the number of times light interacts with grain surfaces. If water is adsorbed on grain surfaces, additional surface interactions due to this increased mean optical path length would produce a proportionally stronger 3 μm absorption feature. The strength of the O-H absorption may also be anticorrelated with the depolymerization state of the Si-O bond in silicates (strongest absorption in plagioclase). The 3 μm absorption is the weakest for ilmenite, although this observation would be consistent with both the mean optical path length explanation and the Si-O depolymerization explanations.
Spectra of all separates show sharp absorption features near 3.4 μm, although the strength of these features is variable between spectra. These features are attributed to the C-H fundamental stretch, which is diagnostic of organic hydrocarbons (e.g., Cruikshank and Brown 1987; Gaffey et al. 1993; Clark 1999). The features are much stronger in spectra of separates from 15058 and 70035, likely because these samples were subjected to a preparation step in which they were processed in ethanol (CH3CH2OH) which caused some ethanol to remain in the separates as a trace contaminant. The weaker features in the spectra of separates from 15555 and 70017 likely are due to small amounts of contamination during sample preparation, but these samples were not processed in ethanol, explaining the weaker 3.4 μm absorption. The features are also weaker in the bulk samples, which were not subjected to the preparation step involving ethanol.
The pyroxene separate spectra show similar absorption properties to one another at these wavelengths. The position and strength of their absorption features do not show significant variation with mineral composition.
Absorption features in the olivine spectra between 5 and 6 μm are weak (slightly more apparent in Fig. 6) but highly diagnostic. These features have potential value for remote detection, as they provide an independent indication of the presence of olivine beyond the 1 μm crystal field absorptions (which can be complicated by the presence of other absorbing phases).
Plagioclase spectra show a weak but important absorption feature near approximately 4.4 μm. Because the 4.4 μm feature occurs in a wavelength region unaffected by the mafic crystal field absorptions, the presence of this feature may allow plagioclase to be detected unambiguously. The Apollo 17 plagioclase separate spectra are somewhat brighter than their Apollo 15 counterparts in the 3 to 4 μm region. The Apollo 17 plagioclase separates have slightly lower iron contents than the Apollo 15 plagioclase separates, although we cannot attribute unequivocally this brightness difference to variable iron contents.
The bulk sample spectra generally follow the behavior of the pyroxene spectra. The presence of olivine is apparent in the fine particle olivine-bearing bulk sample spectrum (15555), as evidenced most notably by the feature near 5.7 μm. The plagioclase absorption at 4.4 μm is weak-to-absent in the bulk sample spectra, suggesting that the modal abundance of plagioclase in these basaltic samples may be below the detection limit for this feature.