| 我国四种锡矿床中的锡石的谱学研究 |
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| 引用本文：.1985.A spectroscopic study of cassiterites from four types of tin deposits in China[J].Mineral Deposits,4(2):69~76 |
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| 基金项目:中国科学院科学基金 |
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| A spectroscopic study of cassiterites from four types of tin deposits in China |
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| Abstract:Spectroscopic study on the genesis of cassiterite is a new aspect of modern genetic mineralogy and is of absolute importance in quantum mineralogy. Based upon the crystal field theory and the energy band theory, this article deals with the spectra and geneses of cassiterites of different origins by means of absorption spectra(AS), electron paramagnetic resonance spectra (EPR)and infrared spectra(IR). Following are some results obtained. 1．Cassiterite is a semi-conductive mineral with wide energy gap. Isomorphic substitution of Nb5+, Ta5+ and Fe3+ ions increases the donor level or acceptor level and thus deepens its color and decreases Eg. High temperature is more favorable for heterovalent isomorphic substitution of Nb5+,Ta5+ and Fe3+ for Sn4+. Therefore, in the same deposit, early-formed cassiterite is darker than late-formed one, and Eg(1ate Eg) is greater than Ege (early Eg). For exam pie in Danchang, cassiterite of the A association is darker than that of the B association, and from early to late stages, the cassiterite changes from black brown through brown and light brown to colorless, accompanied by a gradual increase in Eg.2．Heat-treatmentJ X-ray radiation and EPR spectra of eassiterite of different geneses show that there exists a color center in this mineral. The authors consider that this color center is predominantly caused by isomorphic substitution of heterovalent ions. If Sn4+ is replaced only by Fe3+, as in dachang, hole center is likely to formed, if Sn4+ is replaced by Nb5+ and/or Ta5+, as in Limu, electron center(it-center)is expected to be produced. Besides, color center can possibly result from vacancies of anions or cations during crystallization caused by the pulsation of mineralization. Ring structure is commonly seen in cassiterite and its AP spectra and EPR spectra indicate the presence of color center, with each color of the ring representing a pulsation structure. 3．In the case of color center resulting from heterovalent isomorphic Substitution, electron center is probably to Occur in acidic media, while hole center in alkaline media. 4．According to group theory analysis of IR absorption band in the spectrum of cassiterite, only four IR active vibration frequencies are seen, i. e.,A2u + 3Eu of more than 100IR spectra of cassiterites from four deposits show that the positions of the IR spectra are only slightly changed as a whole, but their shapes and intensities can supply some information on the crystal structure, as well as the composition and the origin of cassiterite. According to the change in the intensity and the shape of the IR spectra, the authors classify the IR spectra into three types: standard(Type I), deformed(Type II) and distorted spectra (Tpe Ⅲ). TypeⅠspectra are from pure SnO2 (mostly synthetic cassiterite with very few natural one)formed at comparatively low temperature; Type II spectra, mainly related to the substitution of Fe3+ for Sn4+, are independent of the variations in such factors as the content of Fe, color and temperature. The hydrothermal cassiterite-sulfide deposit and the contact metasomatic deposit serve as examples of this type; Type Ⅲ is mainly related to the substitution of Ta5+, Nb5+, or Ta5+ and/or Nb5+ lus Fe3+ for Sn4+. Its IR spectra vary in accordance with the Ta5+, Nb5+ and Fe3+ contents, the color and the temperature. Nb-, Ta-, Fe-bearing cassiterite oxides, such as Nb-, Ta-bearing granites, pegmatites and W, Sn quartz veins, belong to this type. Moreover, more absorption bands of OH or H2O are seen in the TypeⅢthan in the TypeⅡ. In some areas, both TypeⅡand can sometimes be seen in the same sample. Late stage cassiterite-sulfides in the Yuulong tin deposit, for instance, are superposed on the cassiterite oxides formed in the early stage. The mono-vibration of γ4(A2u)corresponds to the asymmetricl stretching vibration along c-axis of Sn4+ and O2. The spectral shape of the γ4 absorption band obviously changes with the increase in temperature and contents of Nb and Ta. Thus, direct information on the crystal chemistry and the genesis of cassiterite can be obtained fromγ4. |
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