1. Introduction
Spectral recognition of hyperspectral is the main criterion for distinguishing hyperspectral from multispectral, and it is also the advantage of hyperspectral. At present, the methods for identifying and extracting hyperspectral rock and mineral alteration information are mainly based on spectral waveform parameters, spectral similarity measures, and mixed spectral models. These methods are highly adaptable when there are a large number of known object spectra, and image object recognition is more useful. However, the obvious deficiency is due to the spectral variation of actual ground objects, the influence of the observation angle and particle size of the acquired data, and the spectral change is difficult, resulting in confusion and error in the identification and analysis of rocks and minerals.
2. Mineral spectral characteristics
In the current spectrum range commonly used in optical remote sensing photography (04~2.Within 5 m), the spectral characteristics of rock and mineral standard types are related to anionic groups such as carbonate, water, hydroxyl and iron ions, or some cations. The difference in the absorption characteristics of minerals mainly depends on the physicochemical properties of mineral crystals, that is, the valency of anions and cations, the number of cations, the mass of major cations, the covalentity of bonds, the distance between anions and cations, and the difference in crystal symmetry. These characteristic absorptions refer to the unique and relatively stable absorption bands of minerals. These bands have more stable wavelength positions in different minerals, and have more stable and unique waveforms, indicating the presence of a certain mineral. Generally speaking, cations (such as Fe2+, Fe3+ and Mn3+) have characteristic spectral bands in the visible region or near-infrared due to electronic transitions, and anionic groups (such as CO32-, H2O, OH- and metal-OH groups) produce frequency doubling and combined frequency light in short-wave infrared due to bending vibration.
2.1 Mineral spectral characteristics
1) MN ion: The spectral characteristics of MN in water manganese ore, hard manganese, rhodochrosite, spessartite and manganese olivine are mainly analyzed. Mn2+ is a typical D5 electronic architecture, and its bands are mainly generated by electronic transitions. In the visible region (Figure 1), the characteristic bands of manganese are predominantly 045 m with 055 m as well as in 036~0.Absorption characteristics at 41 m near 3, or at 04~0.A steep semi-absorption feature is produced in the 6 m zone. 
Figure 1: Spectrum of manganese-bearing minerals
The image on the right is a partial enlargement of the image on the left
2) Fe ions: In the composition of minerals, Fe is a very important element. According to the crystal field theory, the ground state D of Fe2+ is a higher quintet energy level eg and a lower quintet energy level T2g in the tetrahedral field. Since there is only one spin allowing transition, thus at 10~1.A common strong and wide band is produced around 1 m; Fe3+ has a symmetrical ground state s, which is not ** in any crystal field, and the transition to the higher energy level formed by the 4g state is spin forbidden, so the spectrum is relatively weak, but at 06~0.Strong absorption bands were generated between 9 m. For different types of iron compounds, their characteristic spectra are quite different due to the differences in molecular structure, crystal structure and transparency. Figure 2 shows the spectral curves of some iron-bearing minerals.
From the spectral curve, it can be seen that the characteristic band of iron silicate minerals is located at 09 m, symmetrically spread at 08~1.0 m spectral region. The hydroxide characteristic bands of iron are located at 1Around 0 m, it spreads asymmetrically at 07~1.40 m. The absorption bands of carbonate minerals of iron are wide and straight, and spread at 08~1.5 m. However, due to the influence of mineral color, iron oxides and sulfides belong to the opaque mineral series, and there are spectral absorption limits, spectral characteristics are masked, and slight oxidation or alteration may make some characteristic spectra of iron visible.
Figure 2: Spectrum of iron compoundsThe ordinate of the figure below is shifted so that the spectral characteristics of different iron compounds can be clearly displayed
Left: Al oxides and hydroxides; Right: Silicate minerals of AL
3) Spectrum of aluminium ion (Al3+) minerals: Aluminum ion can be found in most minerals, especially silicate minerals. Aluminium plays a dual role in the silicate mineral structure. First, it is a four-order coordination, which replaces part of the Si4+ and enters the complex anion to form aluminosilicate; the second is a six-order coordination, which exists outside the silicon-oxygen backbone, plays a general cationic role, and forms the silicate of aluminum; Or both. Figure 3 shows the spectral curves of these minerals.
The oxide spectrum of aluminum is 1The reflective peak around 4 m is stronger. In the hydroxide of aluminum, gibbsite due to the combined or doubling effect of the telescopic vibration containing strong al-o-h bonds, in 14~1.Between 6 m produces a sharp one by the main one435 m with minor 1"Binary" absorption characteristics of the composition of 558 m. Diaspore is due to the Al3+ filling of the 1 2 octahedral void formed by O2- and (OH)-, and the Al-O-H bond is weaker at 1408 m produces a weaker absorption signature.
Figure 4: Feldspar-epidote spectrum
Figure 5: Kaolinite-silanite spectrum
The characteristic spectra of the silicate minerals of aluminum (Figs. 3,4,5) are not correlated with [AlO4] and are not significantly correlated with [AlSi3O8]. The spectral behavior of aluminum in silicate is mainly due to the electronic transition of aluminum cations and the frequency combination and frequency doubling of the stretching and contractile vibrations of al-OH groups, while the characteristic absorption spectrum is generated in the short-wave infrared spectrum. With the enhancement of Al-OH bond strength and the increase of its quantity, the characteristics of silicate minerals highlight the sharp and narrow absorption characteristics of OH groups. Due to the differences in the energy levels of Al-OH groups in different crystal structures, the morphology and position of the absorption bands and the morphology of the secondary absorption peaks are different. As can be seen from the figure below in Figure 3, the spectral characteristics of albite are not obvious; Both muscovite and kaolinite exhibit more consistent 142 m with 215 m characteristic absorption peak, but at 20~2.However, there is a large difference in the morphology of the absorption peaks between 5 m, which is a sign of identification and differentiation between the two, and is the external spectral manifestation of different occupancy of Al in the crystal structure.
At low temperatures, the volume of [AlO4] tetrahedron is slightly larger than that of [SiO4] tetrahedron, so it is easy to transform into a sixth-coordination form. For example, calexald feldspar Ca[Al2Si2O8] is transformed into epidote Ca2(Al,Fe)3O(OH)[SiO4][Si2O7], so that A is all in octahedral coordination and becomes the most stable form. The spectral characteristics of epidote (Fig. 4) are more complex than those of afeldspar and have the same spectral shape as kaolinite or muscovite, but the position of the characteristic bands relative to the latter two shifts in the direction of long waves. This is mainly due to the fact that the epidote crystal has a Fe-like coincidence in the crystal, which affects the bond force and bond length of Al in the crystal, so that the size of the energy level ** or the direction of the OH group stretching and bending vibration changes.
In a high-temperature Al-rich environment, [ALO4] can change from sixth-coordinate to fourth-coordinate of partial AL. For example, kaolinite Al4[Si4O10](OH)8 can be partially transformed into schillite Al[Alsio5] at 1000 1500°C. Comparing the spectral characteristics of the two (Figure 5) can be seen in 2There is little difference in the absorption of the main characteristics around 2 m, the main difference is that the absorption peak generated by the vibration of the OH group in kaolinite is sharper than that of the silanite, and the secondary absorption peak is more obvious. In general, there is no characteristic absorption of Al ions and silicon-aluminum tetrahedrals, and the spectral characteristics of aluminosilicate or aluminum-containing aluminosilicate minerals are more mainly the result of the frequency combination or frequency doubling of Al-OH bond vibrations, which will be further discussed in the spectral characteristics of metal-OH.
2.2 Anion spectral characteristics
1) Metal-OH: Due to the difference in crystal symmetry caused by the covalent degree of bonds between cations and cations, the distance between cations and cations causes certain differences in the spectrum of metal-OH groups.
Fig.6 Spectral characteristics of Mg-OH characteristic ions
Fig. 6 shows the spectra of minerals containing Mg-OH bonds. This group of minerals is most pronounced in 2There is a strong absorption characteristic peak near 30 m. Actinolite strong absorption bands are located at 2315 nm, biotite 2335 nm, brucite 2315 nm, leaf serpentine 2325 nm, serpentine 2325 nm, heptolite 2305 nm, serpentine 2315 2325 nm, phlogopite 2325 nm, talc 2315 nm, tremolite 2315 nm. The absorption bands of 2315 and 2335 nm are characteristic of the Mg-OH containing minerals, which is also a significant indicator for the identification of minerals containing Mg-OH bonds.
Fig.7 Al-OH characteristic ion spectra
Figure 7 focuses on the spectrum of Al-OH bond-containing minerals. The most significant absorption characteristic of this class of minerals is determined by the number located in 2The absorption peak around 20 m and some secondary absorption peaks on both sides of it constitute a "binary structure". The characteristic absorption bands of alum stone are located at 2165 nm and 2325 nm; 2175nm and 2365nm for lithium chlorite; Halloysite at 2205nm and 2165nm; illite at 2215nm and 2355nm; kaolinite at 2205nm and 2165nm; montmorillonite is 2205nm and 2215nm; Muscovite 2195, 2225nm and 2355nm; Phyllophyllite 2165nm and 2315nm; Letolite 2195nm; Topaz 2085nm and 2155nm, 2215nm. It can be seen that the identification spectrum of minerals containing Al-OH bonds is between 2165 nm and 2215 nm.
2) CO32-: In carbonate CO3 complex anions, C and O are covalent bonds with strong bond strength, while CO3 and metal cations are dominated by ionic bonds. In 1Before 3 m, the spectral characteristics of carbonate minerals were mainly determined by metal ion transitions. And 1After 3 m, the spectral characteristics are generated by group vibrations, i.e., CO32 or the frequency doubling or congenial mode vibrations of some water molecules themselves (Figure 8). In the near-infrared region, carbonates typically exhibit five distinct characteristic bands:255μm，2.35μm，2.16μm，2.0 m and 19 m, the first and first are the clearer double bands, and the intensity is stronger than the other three; The most striking identifying feature of carbonate minerals is 23~2.It has a single characteristic absorption between 4 m, and the symmetry is wide on the left and narrow on the right, which is different from the spectral characteristics of other minerals. At the same time, the vast majority of minerals in this class also have 21~2.2 m as well as 2Absorption of secondary features around 5 m. The cations contained are different and at 04~1.The spectral characteristics of cation transitions in the 2 m interval can be used to identify different types of carbonate minerals.
Fig.8 Spectral characteristics of carbonate rocks
The picture on the right is a partial enlargement of the image on the left; The ordinates of both plots have been offset
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