Spectra calcite

The phyllic alteration zone coincides with a subtle but consistent shift in the dominant AlOH peak in the short-wave infrared spectrum ( 2210 nm) to slightly lower wavelengths, consistent with an inner white mica-ferroan carbonate mineral assemblage. A preliminary analysis of hyperspectral data over the visible to near infrared range suggests that ferroan carbonates may be detected but not reliably quantified. However, TIR data allow calcite and ferroan carbonate to be distinguished, and may also detect increasing Fe content in ferroan dolomite as mineralized structures are approached. 

The problem with limited selectivity includes some of the minerals which are problems for XRD illite, muscovite, smectites and mixed-layer clays. Poor crystallinity creates problems with both XRD and FTIR. The IR spectrum of an amorphous material lacks sharp distinguishing features but retains spectral intensity in the regions typical of its composition. The X-ray diffraction pattern shows low intensity relative to well-defined crystalline structures. The major problem for IR is selectivity for XRD it is sensitivity. In an interlaboratory FTIR comparison (7), two laboratories gave similar results for kaolinite, calcite, and illite, but substantially different results for montmorillonite and quartz. 

Two types of Ce centers in calcite were detected by steady-state spectroscopy (Kasyanenko and Matveeva 1987). The first one has two bands at 340 and 370 nm and is connected with electron-hole pair Ce -COj". The second one has a maximum at 380 nm and was ascribed to a complex center with Ce and OH or H2O as charge compensators. Such a center becomes stronger after ionizing irradiation and disappears after thermal treatment. The typical example of Ce luminescence in the time-resolved liuninescence of calcite consists of a narrow band at 357 nm with very short decay time of 30 ns, which is very characteristic for Ce " (Fig. 4.13a). It was found that Ce " excitation bands occurs also in the Mn " " excitation spectrum, demonstrating that energy transfer from Ce to Mn " occurs (Blasse and Aguilar 1984). 

The violet emission of the radiation-induced center (COs) " is well known in steady-state luminescence spectra of calcite (Tarashchan 1978 Kasyanenko, Matveeva 1987). The problem is that Ce also has emission in the UV part of the spectrum. In time-resolved luminescence spectroscopy it is possible to differentiate between these two centers because of the longer decay time of the radiation-induced center. Its luminescence peaking at 405 nm becomes dominant after a delay time of 100-200 ns while emission of Ce is already quenched (Fig. 4.14f). 

Fig, 5.66. Laser-induced time-resolved luminescence spectrum (a) and excitation spectrum (b) of radiation induced center in calcite... 

Under short-waved UV lamp excitation (254 nm) visually observed luminescence of calcite is violet-blue with very long phosphorescence time of several seconds. Under long-waved UV lamp excitation (365 nm) calcite exhibits visually the same violet-blue luminescence as under 254 nm excitation, but long phosphorescence is not detected. Under short laser excitations, such as 266 and 355 nm, at 300 K calcite demonstrates intensive UV-violet emission band peaking at 415 nm with half-width of 55 nm (Fig. 5.76a). Excitation spectrum of this band is composed of short waved tail in the spectral range less... 

The LIBS technique may be extremely useful for sorting of fluorite ores. Figure 8.8 clearly demonstrates the opportunities of time-resolved LIBS in comparison with the steady-state method in the case of fluorite-carbonate ores. Fluorite and calcite both has Ca as a major element and its emission lines dominate in the steady-state spectra making sorting impossible. After a delay of several ps the intensity of Ca lines is strongly diminished and a F line with a longer decay becomes visible in the fluorite spectrum. 

Dolomite is an alternative mineral that is used in some regions in place of cal-cite for certain applications. Dolomite is a calcium magnesium carbonate (CaCOj.MgCOj) and occurs widely in nature. Although generally similar to cal-cite in properties, it is shghtly harder (3.5), denser (2.85) and more acid resistant. Production is similar to that for calcite,but miUing is more costly and it tends to only be available at the coarser end of the size spectrum. 

Since the polarizers discussed above involve light reflection combined with the real part of the refractive index tensor, they can be used effectively over a broad spectral range about a central wavelength. Calcite Glan-Thompson polarizers, for example, operate successfully over the entire visible spectrum. When fabricated of crystalline quartz, these polarizers can be used to polarize ultraviolet light as well as visible light. 

Figure 1. Spectra of non-biogenic calcite (Iceland spar) and liquid H2O. Calcite spectrum offset vertically by 0.2.

Mineral fractions from all sources have been examined in a preliminary way by x-ray diflFraction. Most helpful has been x-ray spectrometry before and after treatment with acid. Beforehand, the main constituents were calcite and quartz with some dolomite afterwards, they were largely quartz with some feldspar and occasional traces of gypsum and clay. Because of its high crystallinity, quartz tends to dominate all spectra, and methods of subtracting out its contributions are being studied. The remaining difference spectrum would be an adequate fingerprint of the minerals for comparison purposes. Mineral analyses that are more accurate than those in the tables as well as far more repeatable can be expected eventually. 

Figure 8.18. Effect of HNF angle on Raman spectrum of calcite. Increasing the angle shifts the rejection band to lower Raman shift but also decreases the optical density at the laser wavelength. (Adapted from Reference 11. with permission.)...

The 2 polymorphs of CaCOs, aragonite and calcite, can readily be distinguished on the basis of their " Ca MAS spectra (Figure 8.28B). Calcite shows a characteristic second-order quadrupolar lineshape from which the NMR parameters can he extracted by spectral simulation. The narrower Ca MAS resonance from aragonite shows no discernible structure, but the corresponding static aragonite spectrum is about 20 times broader than under MAS conditions. This suggests that CSA is a major contributor to the static linewidth, which is confirmed by satisfactory simulation of the spectrum... 

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