Subtle spectral differences in the kaolinite group minerals near 2. Spectral bandwidth is 1. Each spectrum was scaled to 0. Original reflectances were between 0. Figure Comparison of calcite CaCO 3 and dolomite CaMg CO 3 2 spectra in the mid-infrared showing small band shifts due to the change in composition between the two minerals.
The level change calcite higher in reflectance than dolomite is because the calcite has a smaller grain size. The numbers indicate the fundamental stretching positions of v 1 v 2 , v 3 , and v 4. The v 1 stretch is infrared innactive, but may be weakly present in carbonates.
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The v 3 fundamental is so strong, only a reflection peak is seen in these spectra. The gypsum curve is offset upward 1. Both samples have very low reflectance because of the water content of the samples. Water is a strong infrared absorber. The montmorillonite also has a small grain size, which also tends to produce low mid-infrared reflectance because of the strong absorption in the mid-infrared.
Figure 13a. Spectral signature diagram from Hunt, The widths of the black bars indicate the relative widths of absorption bands. Figure 13b. Illustration of the locations and causes of absorptions in mid-infrared spectra of silicates, from Hunt Lattice modes are sometimes denoted by v T and v R and also couple with other fundamentals, resulting in finer structure seen in some spectra.
The causes of vibrational absorptions in mid-IR spectra are summarized in Figure 13b from Hunt Mid-infrared reflectance spectra of quartz are shown in Figure 4b.
Olivine spectra in the mid-infrared are shown in Figure 5b. The absorptions shift with composition as shown in Figure 5b, and discussed in more detail in Farmer , pp. Pyroxene mid-infrared spectra are shown in Figure 6b. The Si-O fundamentals are at similar to other silicates, as indicated in Figure 13b.
Grain size effects will be discussed below in section 6. Iron oxide and iron hydroxide mid-infrared spectra are shown in Figure 7b. Because iron is more massive than silicon, Fe-O fundamentals will be at longer wavelengths than Si-O stretching modes. Water and OH hydroxyl produce particularly diagnostic absorptions in minerals. In the isolated molecule vapor phase they occur at 2.
The overtones of water are seen in reflectance spectra of H 2 O-bearing minerals Figure The first overtones of the OH stretches occur at about 1. Thus, a mineral whose spectrum has a 1. The hydroxyl ion has only one stretching mode and its wavelength position is dependent on the ion to which it is attached. In spectra of OH-bearing minerals, the absorption is typically near 2. The OH commonly occurs in multiple crystallographic sites of a specific mineral and is typically attached to metal ions.
Thus, there may be more than one OH feature. The combination metal-OH bend plus OH stretch occurs near 2. Carbonates also show diagnostic vibrational absorption bands Figure 10a, The observed absorptions are due to the planar CO 3 -2 ion. There are four vibrational modes in the free CO 3 -2 ion: the symmetric stretch, v 1 : cm -1 9. The v 1 band is not infrared active in minerals. There are actually six modes in the CO 3 -2 ion, but 2 are degenerate with the v 3 and 4 modes.
In carbonate minerals, the v 3 and v 4 bands often appear as a doublet. The doubling has been explained in terms of the lifting of the degeneracy e. Combination and overtone bands of the CO 3 fundamentals occur in the near IR. Three weaker bands occur near 2.
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Figure 10a e. Hunt and Salisbury, The band positions in carbonates vary with composition Hunt and Salisbury, ; Gaffey, , Gaffey et al. An example of such a band shift is seen in Figures 10a, 11 and, in more detail, Figure 24a showing the shift in absorption position from calcite to dolomite.
Phosphates, borates, arsenates, and vanadates also have diagnostic vibrational spectra.
Theory of Reflectance and Emittance Spectroscopy
Space precludes inclusion of spectra here. See Hunt et al.
In general, the primary absorptions e. P-O stretch occurs at mid-infrared wavelengths. However, many of these minerals contain OH or H 2 0 and have absorptions in the near-infrared. In the mid-infrared, minerals with H 2 O, or those that are fine grained, like clays, have very low reflectance, and show only weak spectral structure e. Therefore, in emittance, spectral features will also be weak and thus difficult to detect.
Grain size effects will be discussed further below.
Typical spectra of minerals with vibrational bands are shown in Figure 4b, 5b, 6b, 7b, and A summary of absorption band positions and causes is shown in Figure 13a, b. Organic materials are found all over the Earth, and in the solar system. Organics can be important compounds in some environmental problems. The C-H stretch fundamental occurs near 3. Figure 14a the first overtone is near 1. The combinations near 2. Figure 10 , especially at low spectral resolution. Figure 14a. Transmittance spectra of organics and mixtures showing the complex absorptions in the CH-stretch fundamental spectral region.
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Reflectance spectra of montmorillonite, and montmorillonite mixed with super unleaded gasoline, benzene, toluene, and trichlorethylene. Montmorillonite has an absorption feature at 2. The first overtone of the CH stretch can be seen at 1. From King and Clark b. Just like water in minerals shows diagnostic absorption bands, ice crystalline H 2 O which is formally a mineral, also shows strong absorption bands. In the planetary literature it is referred as water ice, so as not to confuse it with other ices.
The spectral features in Figure 15 are all due to vibrational combinations and overtones, whose fundamentals have previously been discussed in general. Note the H 2 O spectra show broad absorptions compared to the others. The reason is that while ice is normally a hexagonal structure, the hydrogen bonds are orientationally disordered e. Hobbs, , and the disorder broadens the absorptions. There are many ices in the solar system e. Ice, being ubiquitous in the solar system is found mixed with other minerals, on the Earth, as well as elsewhere in the solar system e.
Clark et al. The spectral properties if ice and ice-mineral mixtures have been studied by Clark a, b , Clark and Lucey , Lucey and Clark and references therein. Spectra of vegetation come in two general forms: green and wet photosynthetic , and dry non-photosynthetic but there is a seemingly continuous range between these two end members. The spectra of these two forms are compared to a soil spectrum in Figure Because all plants are made of the same basic components, their spectra appear generally similar.
However, in the spectral analysis section we will see methods for distinguishing subtle spectral details.