Minerals, reflectance spectra

Figure 10.1 Comparisons of visible to near-infrared spectra of calcic pyroxene in transmitted and reflected light. Polarized absorption spectra of single crystals are correlated with the reflectance spectrum of a powdered sample of the same mineral (cf. fig. 5.14).

The contrasting temperature-induced shifts of the pyroxene 1 and 2 pm bands could lead to erroneous estimates of the composition and, to a lesser extent, structure-type of a pyroxene-bearing mineral assemblage deduced from the remote-sensed reflectance spectrum of a hot or cold planetary surface if room-temperature determinative curves, such as that shown in fig. 10.5, are used uncritically. For example, remote-sensed spectra of planets with hot surfaces, such as Mercury and the Moon, would lead to overestimates of Fe2+ contents of the orthopyroxenes and underestimated Fe2+ contents of the clinopyroxenes (Singer and Roush, 1985). Planets with cold surfaces, such as Mars and the asteroids, could produce opposite results. On the other hand, the room-temperature data underlying the pyroxene determinative curve shown in fig. 10.5 may impose constraints on the compositions of pyroxenes deduced from telescopic spectra of a planet with very high surface temperatures, such as Mercury. 

For infrared measurements, cells are commonly constructed of NaCI or KBr. For the 400 to 50 cm 1 far-infrared region, polyethylene is a transparent window. Solid samples are commonly ground to a fine powder, which can be added to mineral oil (a viscous hydrocarbon also called Nujol) to give a dispersion that is called a mull and is pressed between two KBr plates. The analyte spectrum is obscured in a few regions in which the mineral oil absorbs infrared radiation. Alternatively, a 1 wt% mixture of solid sample with KBr can be ground to a fine powder and pressed into a translucent pellet at a pressure of —60 MPa (600 bar). Solids and powders can also be examined by diffuse reflectance, in which reflected infrared radiation, instead of transmitted infrared radiation, is observed. Wavelengths absorbed by the sample are not reflected as well as other wavelengths. This technique is sensitive only to the surface of the sample. 

Crystallinity is a metric related to mineral maturity and is a measure of mineral crystallite size, mineral maturity, and the amount of substitution into the apatitic lattice. Crystallinity increases when crystals are larger and more perfect (i.e. less substitution). It is directly proportional to the inverse width of the 002 reflection (c-axis reflection) in the powder x-ray diffraction pattern of bone mineral. Several features in the infrared spectra of bone correlate with mineral crystallinity, most of which are components of the phosphate Vi,V3 envelope [8]. Any of these correlations should be usable in the Raman spectrum provided there are no other overlapping Raman peaks. However, there has been less emphasis on crystallinity in the bone Raman literature and only the inverse width of the phosphate Vi band has been used as a measure of crystallinity [9-12]. 

Electromagnetic radiation in the visible and near-ir and uv regions of the spectrum, particularly with energies between approximately 33,000 and 4,000 cm (3,000-25,000 A, 4-0.5 eV) interacts with the eleetrons in solids (or liquids and gases). These interactions give rise to the processes of absorption and reflection qualitatively observed in minerals as the properties of color and luster. The quantitative measurement of such absorption and reflection processes forms the basis of eleetronic (optieal) absorption spectroscopy. The absorption and reflection phenomena arise from electronic excitation processes involving the valence electrons, excitation processes that may be of several kinds ... 

Light (or near-ir and uv radiation) that is incident on opaque minerals is partly absorbed and partly reflected by them. There are two kinds of reflection processes that occurring when light is reflected from a flat polished surface of the mineral (specular reflectance) and that occurring when the light is reflected from the mineral after it has been finely powdered (diffuse reflectance). The latter arises from radiation that has penetrated the crystals (as in an electronic absorption spectrum) and reappeared at the surface after multiple scatterings in this case there will also be a specular component to the reflectance from light that is reflected from the surfaces of the particles. The specular reflectance of a flat polished surface of an opaque mineral measured at normal incidence can be related to the n and k terms of the complex refractive index (N) in which ... 

The acquisition of solid-state FTIR spectra suitable for use in the characterization of polymorphic impurities is performed using either the Nujol mull technique, diffuse reflectance (DRIFT), or attenuated total reflectance (ATR). One should avoid the use of pelleting techniques to eliminate any spurious effects associated with compaction of the KBr pellet. The simplest approach is to prepare a mull of the sample in mineral oil, sandwich this between salt plates, and measure the spectrum using ordinary transmission techniques. The main drawback of the mull technique is that regions in the IR spectrum overlapping with carbon-hydrogen vibrational modes will be obliterated because of absorbance from the oil. 

The solid-state MSi NMR spectrum of talc displays a single resonance at -98.5 ppm [24], Upon, VA1 substitution, the wSi chemical shift value is shielded (more positive ppm value), whereas further distortion of the tetrahedral sheet is reflected in a decreased chemical shift value (deshielded, increased negative ppm value). The authors of this report suggest that solid-state wSi NMR may be used to determine the compositional nature of phyllosilicate minerals. In each of these studies, the spectra were obtained at a frequency of 71.5 MHz on a Fourier transform spectrometer interfaced to a 8.45-T magnet. Tetramethylsilane (TMS) was used as an external chemical-shift standard. 

The object of this study was to apply mid-infrared spectroscopy to zeolite structural problems with the ultimate hope of using infrared, a relatively rapid and readily available analytical method, as a tool to characterize the framework structure and perhaps to detect the presence of the polyhedral building units present in zeolite frameworks. The mid-infrared region of the spectrum was used (1300 to 200 cm"1) since that region contains the fundamental vibrations of the framework (Si,Al) 04 tetrahedra and should reflect the framework structure. Infrared data in similar spectral regions have been published for many mineral zeolites (30) and a few synthetic zeolites (23, 49, 50). There is an extensive literature on infrared spectra of silica, silicates, and aluminosilicates (17). However, no systematic study of the infrared characteristics of zeolite frameworks as related to their crystal structure has appeared. 

Like their ultimate parent base, the 5-methyl-1,2,4-thiadiazoles (392) are colorless mobile thermally stable liquids of characteristic odor the 3,5-dimethyl homolog is miscible with water in all proportions. Their spectral properties reflect their structural features the H NMR spectrum of 392 (R = Me) includes signals attributable to the 3- and 5-methyl groups at 52.56 and 2.72, respectively. 5-Methyl-l,2,4-thiadiazoles form salts with mineral acids and 1 1-adducts with boron trifluoride and antimony penta-chloride.287... 

White asbestos (12001-29-5) refers to the degree to which the mineral surface achieves complete reflectance over the visible spectrum rather than to the actual colour or hue of the mineral consequently, white describes multiple asbestos types, although it is most commonly applied to actinolite, chrysotile, and sometimes anthophyllite and tremolite. Similarly, while brown asbestos principally refers to amosite, it actually describes the degree to which the mineral deviates from colourlessness toward yellow, tan, or brown. 

This class of solids is an extension of the sample types already discussed, and many of the procedures already highlighted may be used here. If the material dissolves in a suitable solvent, then a cast film may be prepared on an IR transmitting window or on the surface of an ATR element. Moldable materials, such as polymer pellets, may be prepared as hot-pressed films, with care taken to ensure that material does not thermally degrade. Grind-able materials can be handled as previously discussed for powders using the compressed halide pellet, mineral oil mull, or diffuse reflectance methods to acquire the spectrum. 

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