Quantitative analysis method of bulk mineralogy: Take the 9th Reynolds Cup samples for example
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摘要:
X射线衍射（XRD）法是矿物样品分析最常用的方法，被广泛用于矿物学定量分析研究。对于含有黏土矿物的样品进行准确的定量分析一直以来都充满挑战。本文以第9届雷诺杯（Reynolds）提供的3种未知混合样为例，使用Rietveld法对矿物进行定量分析。首先采用XRD法获得样品图谱，鉴定图谱中的矿物组成后，使用Rietveld法对全岩图谱进行拟合得出矿物的质量相对含量。最终分析结果与雷诺杯举办方赛后提供的准确值进行比较。研究结果表明，样品中的主要矿物比较容易鉴定，大部分鉴定出的矿物都可以通过Rietveld法准确计算。其中，非黏土类矿物（不含无定形相）的质量分数高于1.5%即可准确鉴定，而黏土类矿物由于较为复杂，鉴定和计算更为困难。且非黏土类矿物的定量偏差（平均质量偏差1.0%～1.3%）要低于黏土类矿物（平均质量偏差2.8%～5.8%）。经举办方比对定量分析结果，3个样品的质量偏差总和为156.7%（最终结果排名24位，本次获奖前三名的质量偏差总和分别为70.5%、89.8%、95.6%）。偏差主要来自矿物（特别是黏土类矿物）鉴定不准确。在矿物鉴定准确的条件下，如有合适的晶体结构卡片和拟合条件，其矿物的计算偏差可以大大降低（如RC9-3的偏差和可降至19.2%）。结果表明，Rietveld法能准确定量出样品中的大多数矿物含量，但也依赖研究人员对矿物组成做出准确的鉴定。总体而言，我分析室已初步建立了可靠的XRD法分析矿物组分，对于指示海洋沉积物的来源、成因及古环境等具有重要意义。
Abstract:X-ray diffractometry (XRD) is well accepted as a definitive tool for mineral identification and quantitative analysis. Accurate quantification of clay-rich minerals has always been challenging. Three samples from the 9th biennial Reynolds Cup competition for quantitative phase analysis (QPA) were taken for the example. Application of the Rietveld method using X-Ray powder-diffraction data requires that all components are correctly identified. The weight percent (wt%) of the Rietveld refinement will be submitted and compared with the actual content. Results show that most kinds of the mineral were precisely calculated by Rietveld method and high content mineral phases were generally easy to identify. Non-clay minerals were less problematic and were quantified with a typical bias of better than 1.5% (wt%), with the exception of amorphous phases. Samples with clay-rich minerals are difficult to identify and quantified because of their complex structures. The average bias per non-clay mineral was just 1.0%~1.3%, but the average bias per clay mineral was three to four times higher at 2.8%~5.8%. The sum total bias of three samples was 156.7% (on placing 24, the sum total bias of the top three winners was 70.5%, 89.8%, 95.6% respectively). The bias is mainly due to inaccurate identification of minerals, especially clay-rich minerals. Under the condition of accurate mineral identification, the calculation deviation of the mineral can be greatly reduced (e.g., the total bias of RC9-3 can be reduced to 19.2%) if there are suitable crystal structure and fitting conditions. The results show that the Rietveld method can accurately quantify the content of most minerals in the sample, but also relies on researchers to make accurate identification of the mineral composition. In general, the QPA is reliable by XRD in our laboratory, which could be an efficient tool on tracing the source of marine sediments.
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表 1 XRD仪测试参数
Table 1. Working parameters of the X-Ray diffraction
2θ/(º) 步长/(º) 每步时间 /s 总时长/h 黏土粒级扫描 2～15 0.0167 120 0.2 全岩扫描 3～80 0.0334 600 3.0 
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表 2 雷诺杯样品中矿物测量值与理论值的质量百分数
Table 2. Comparison between submitted and actual mineral contents of the 9th Reynolds Cup sample
样品RC9-1 样品RC9-2 样品RC9-3 RC9-3按实际
矿物拟合矿物 实际值
/%测量值
/%偏差 矿物 实际值
/%测量值
/%偏差 矿物 实际值
/%测量值
/%偏差 测量值 偏差 石英 25.4 21.8 3.6 石英 14.8 12.0 2.8 方解石 13.9 16.5 2.6 12.2 1.7 方解石 3.4 3.2 0.2 正长石 5.4 7.3 1.9 白云石 21.9 23.4 1.5 21.2 0.7 白云石 6.0 5.7 0.3 钠长石 2.1 2.5 0.4 碳钡矿 6.4 7.3 0.9 6.1 0.3 菱镁矿 0.5 0.5 石膏 3.4 3.8 0.4 萤石 3.9 4.1 0.2 3.5 0.4 石膏 3.7 4.8 1.1 针铁矿 3.4 2.2 1.2 磷灰石 3.4 5.1 1.7 3.3 0.1 硫 2.6 2.6 蓝铁矿 0.3 0.3 透辉石 6.7 9.9 3.2 8.0 1.3 石盐 4.3 3.6 0.7 金红石 1.0 1.3 0.3 镁橄榄石 7.0 7.0 5.1 1.9 钾盐 0.3 0.3 锐钛矿 3.2 3.5 0.3 方钠石 2.5 2.2 0.3 2.6 0.1 辉沸石 1.3 1.3 水铁矿* 7.3 7.3 重晶石 2.6 2.8 0.2 2.8 0.2 冰晶石 0.5 0.5 非黏土类矿物总和 40.9 32.6 14.9 钒铅矿 0.8 0.8 0.9 0.1 蛋白石* 7.0 7.0 高岭石 10.6 15.0 4.4 锆石 0.4 0.4 0.5 0.1 非黏土类矿物总和 55.0 39.1 18.1 埃洛石 7.2 7.2 电气石 1.0 1.0 1.2 0.2 伊蒙混层 6.3 8.5 2.2 云母 8.3 11.6 3.3 非黏土类矿物总和 70.5 71.3 19.8 67.4 7.0 蒙脱石(Ca,Na) 8.9 24.4 15.5 伊利石 3.6 9.0 5.4 云母 11.9 7.5 4.4 18.4 6.5 皂石 12.3 12.3 海绿石/蒙脱石混层 12.2 12.2 滑石 8.0 9.8 1.8 9.1 1.1 坡缕石 7.5 6.5 1.0 蒙脱石(Ca,Na) 13.8 24.9 11.1 蛭石 4.3 1.3 3.0 1.4 2.9 海泡石 10.0 12.2 2.2 绿泥石 3.4 4.0 0.6 绿泥石 5.3 7.4 2.1 3.6 1.7 黏土类矿物总和 45.0 51.6 33.2 黏土类矿物总和 59.1 64.5 44.2 黏土类矿物总和 29.5 26.0 11.3 32.6 12.2 定量总和 100.0 90.7 51.3 定量总和 100.0 97.1 59.1 定量总和 100.0 97.3 31.1 100.0 19.2 误判总和 9.6 误判总和 2.9 误判总和 2.7 注：*为无定形相。
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