Quantitative Determination of Calcium Rare Earth Fluoro-carbonate Minerals by Electron Probe Microanalyzer
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摘要:
稀土氟碳酸盐矿物是工业开发利用轻稀土元素的主要赋存矿物。钙稀土氟碳酸盐矿物化学成分的准确测定，是快速鉴别其种属的前提，也是研究稀土矿床的成因和高效开发利用稀土矿产的必然要求。通过测试条件优选、C含量直接测定并校正计算，完善了利用电子探针准确分析钙稀土氟碳酸盐矿物化学成分的定量分析方法。采用理论分子式法和电荷平衡法对样品中C含量进行校准计算，结果显示理论分子式法计算结果更合理。笔者对赋存在金川正长花岗岩中的钙稀土氟碳酸盐矿物化学成分进行电子探针分析和结构拉曼光谱分析。结果表明，该矿物组合存在多种物相，以新奇钙铈矿为主，氟碳钙铈矿和伦琴钙铈矿呈微细针状分布于新奇钙铈矿中。结合矿物共生关系，认为金川正长花岗岩中钙稀土氟碳酸盐矿物为岩浆作用晚期，富F−、CO2、REE3+流体与磷灰石交代反应生成。
Abstract:Rare earth fluoro-carbonate minerals are the most important LREE-bearing minerals in industrial development. Accurate determination of the chemical composition of calcium rare earth fluoro-carbonates is a prerequisite for rapid identification of their species and an essential requirement for studying the genesis of rare earth deposits and the efficient development and utilization of rare earth resources. In this study, by optimizing the experimental conditions, direct determination of carbon content with a subsequent calibration, we have improved the quantitative analytical method for accurately analyzing the chemical composition of calcium rare earth fluoro-carbonate minerals using an EPMA. The calibration calculation of carbon content in the samples was performed using both theoretical crystallochemical formula and charge balance methods, and the results showed that the theoretical crystallochemical formula method yielded more reasonable results. Based on compositional analysis by EPMA and structural analysis by Laser Raman Spectroscopy, Jinchuan syenogranite comprises at least two species of calcium rare earth fluoro-carbonate minerals. The predominant phase is synchysite, the minor phase that exhibited as radial needles is parasite or roentgenite-(Ce). Based on the mineral paragenesis, it is believed that the calcium rare earth fluoro-carbonate minerals in the Jinchuan syenogranite are formed during the post-magmatic stage due to the metasomatism of apatite by F−, CO2, and REE3+-rich fluids.
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表 1 4种常见钙稀土氟碳酸盐矿物的化学特征（据张培善，1998；Donnay等，1953修改）
Table 1. Chemical composition of four common calcium rare earth fluoro-carbonate Minerals
矿物名称 英文名称 代号 理想化学式 化学成分（%） REE2O3 CaO CO2 F 氟碳铈矿 Bastnaesite B （Ce,La） [CO3]F 74.77 − 20.17 8.73 氟碳钙铈矿 Parisite BS （Ce,La）2Ca[CO3]3F2 60.97 10.42 24.58 7.07 伦琴钙铈矿 Roentgenite-Ce BS2 （Ce,La）3Ca2[CO3]5F3 57.24 13.12 25.77 6.63 新奇钙铈矿 Synchysite S （Ce,La） Ca[CO3]2F 51.25 17.62 27.67 5.97 
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表 2 钙稀土氟碳酸盐矿物电子探针分析晶体选择及检出限
Table 2. Analytical crystal selection and element detection limits for calcium rare earth fluoro-carbonate in EPMA analysis
分析元素 选择晶体 检出限（10–6） 分析元素 选择晶体 检出限（10–6） F LDE1 344 Ce LiF 819 C LDE2 1654 Pr LiFH 913 Mg TAP 96 Nd LiFH 812 Fe LiF 303 Sm LiFH 871 Ca PETH 80 Eu LiFH 916 U PETH 226 Gd LiFH 972 Th PETH 206 Y TAP 272 La LiF 923 Zr PETH 233
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表 3 金川正长花岗岩中钙稀土氟碳酸盐矿物电子探针分析结果表（%）
Table 3. EPMA results of calcium rare earth fluoro-carbonates in Jinchuan syenogranite
化学成分 点1 点2 点3 点4 点5 点6 点7 点8 点9 点10 点11 点12 实测值 F 5.97 7.83 7.10 7.18 6.29 7.91 7.37 6.85 8.29 5.63 5.61 6.94 CO2 24.34 25.68 28.83 24.66 23.17 27.26 26.37 25.14 23.55 22.03 22.25 22.18 MgO − 0.06 0.01 0.03 0.02 0.10 0.02 − 0.02 − 0.02 0.02 Gd2O3 0.31 0.22 1.84 1.82 1.66 − 0.59 0.42 0.64 0.31 − − Eu2O3 − 0.26 − − − − − − − − − 0.11 FeO 3.02 8.23 0.35 1.95 1.48 1.87 1.17 0.62 2.86 0.55 3.03 3.03 Sm2O3 0.64 0.10 40.88 0.74 0.47 0.59 1.19 0.47 1.11 1.03 0.13 0.92 Pr2O3 2.04 2.09 3.11 2.19 2.74 2.03 2.23 2.17 2.58 2.57 2.28 2.52 Ce2O3 26.26 23.80 24.01 20.86 22.98 21.69 21.43 25.14 22.58 24.23 22.91 24.53 La2O3 15.21 11.75 9.07 15.31 13.10 11.48 10.09 12.87 7.91 11.51 11.97 10.97 CaO 12.19 8.98 15.62 15.83 15.91 18.59 18.75 18.55 17.53 19.04 16.32 17.17 UO2 0.04 0.07 − 0.04 0.08 0.15 − 0.02 0.10 0.05 0.02 0.09 ThO2 2.54 4.61 2.23 4.44 3.42 2.19 1.56 1.03 3.41 1.75 2.31 0.60 Nd2O3 7.22 6.85 12.74 5.98 7.47 6.61 9.62 6.33 10.37 8.23 7.92 8.24 ZrO2 − 0.06 − 0.06 − − − − 0.06 0.02 0.07 − Y2O3 0.63 0.24 1.03 0.45 0.39 0.55 1.55 0.43 0.81 0.73 0.48 1.12 Total 97.88 97.52 103.81 98.52 96.52 97.69 98.83 97.16 98.33 95.30 92.99 95.50 ∑REE2O3 52.29 45.30 52.66 47.35 48.81 42.95 46.70 47.84 46.00 48.61 45.74 48.41 理论分子式法 CO21 25.90 25.09 26.97 27.10 27.05 27.79 28.34 27.95 28.48 28.65 27.38 28.52 Total1 99.44 96.93 101.95 100.96 100.40 98.22 100.80 99.97 103.26 101.92 98.12 101.85 电荷平衡法 CO22 26.45 22.89 26.18 25.90 26.88 24.79 26.38 26.62 25.66 28.96 27.44 27.14 Total2 99.99 94.72 101.16 99.76 100.24 95.23 98.84 98.64 100.44 102.24 98.18 100.46 矿物种属判别 Par/Roe Par/Roe Syn Syn Syn Syn Syn Syn Syn Syn Syn Syn 注：−表示低于检出限； 上角标1与 2分别表示按照理论分子式法（方法1）与电荷平衡法（方法2）计算得到的值。
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