If we pretend that the only data we have about the Martian meteorites are their thermal infrared spectra, and that we don't know what the rocks are made out of, we can compare the spectra of the meteorites to the spectra of common minerals and learn several things.

For example, we can compare the spectrum of the Nakhla meteorite to the spectra of several mineral types, such as silicates, carbonates, and oxides. In comparing the positions of the absorptions in the meteorite spectra to the absorptions in the mineral spectra, we can observe that the meteorite spectrum looks most like the silicate spectrum, suggesting that the rock is comprised dominantly of silicate minerals.

Next, we can compare the Nakhla spectrum to the spectra of various subgroups within the silicate mineral class, such as the framework, sheet, chain, and isolated silicate groups. These subgroups are defined by the basic arrangement of the Si-O tetrahedra in the mineral. Framework silicates have highly polymerized structures, while the Si-O tetrahedra of the isolated group are the least polymerized. Thus the tetrahedra of the isolated group are only connected to each other via the cations in the mineral, and are not directly linked as in the other silicates.

The positions of the absorptions in the Nakhla spectrum look most like the minerals of the chain and isolated groups, so let's look more closely at the minerals in one of these groups, the single chain silicates, otherwise known as pyroxenes:

The first pyroxene is enstatite. Enstatite is an orthopyroxene, meaning that its crystal structure is orthorhombic. This spectrum doesn't resemble the spectrum of Nakhla very much. Diopside is a clinopyroxene, which has a different structure (monoclinic) than enstatite. This spectrum looks a little bit more like the Nakhla spectrum, particularly at short wavelengths (large wavenumber), but is still not a great match. This suggests that the structure of diopside matches Nakhla better, but something is still not right. The next pyroxene, augite has the same structure as diopside, but a slightly different composition and is part of the solid solution series between diopside and hedenbergite. This spectrum looks a lot like the spectrum of Nakhla, suggesting that this composition is closer to that of Nakhla's than was diopside. Finally, the spectrum of jadeite looks totally different from Nakhla, and is a very poor match.

So, the mineral augite provided the best spectral match to the spectrum of the meteorite Nakhla. Now, let's recall that we were pretending not to know the composition of the meteorite. In fact, the composition of Nakhla is very well known, and it turns out that augite is the most abundant mineral in the meteorite, comprising ~75-85% (depending on which piece you look at) of the rock! The subtle differences between the spectrum of Nakhla and the augite spectrum are due to the presence of other minerals in the Nakhla meteorite, which have their own unique spectral features, such as olivine and plagioclase. These minerals can be seen in the silicates plot, labeled "labradorite" (plagioclase) and "forsterite" (olivine).

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