Variation of Clay Mineral Contents in Completely Decomposed Granite
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摘要: 全风化花岗岩三大矿物组分——石英、长石和粘土矿物含量变化情况怎样?粘土矿物含量变化幅度多大?以香港九龙3个探坑中采到的18箱(边长30cm的立方体)样品为例, 将1箱样品的不同区分别取样后的XRD鉴定结果(包括全岩3种主要矿物成分变化、粘粒中高岭石和伊利石变化范围值)进行对比, 结果发现它们中矿物成分含量变化范围一般为10%~20%.高岭石相对含量变化幅度大于伊利石, 粗粒结构样品变化值大于细粒样品值.对同一样品不同分析人员得出的结果差别很大.分析后认为样品自身的非均质性和分析人员经验不同, 都可造成粘土矿物含量的明显差别.为进一步分析XRD鉴定样品间矿物成分差异值的可靠性, 还将XRD结果同薄片统计结果进行了对比.结果发现来自石硖尾样品中的石英含量普遍较高, 长石含量普遍较低, 而粘土矿物含量在石硖尾和观塘都有高有低.从两者差值看, 矿物含量相差0%~35%.这项研究说明全风化花岗岩一定范围内的非均质性程度同风化物中矿物是原矿物还是粘土矿物的认定一起对最终结果产生很大影响.Abstract: There are three major components in completely decomposed granite (CDG) : quartz, feldspar and clay mineral. How about the variation and the value range of their contents, especially the clay mineral? Taking samples from 18 blocks (each side 30 cm length) from 3 trial pits in Kowloon, Hong Kong, as an example, we try to solve this problem. The specimens for XRD test are cut from different zones in the blocks. The test results (involving the three components contents, percentages of kaolinite and of illite in clay size particles) are analyzed and compared. It is found that the mineral contents generally vary within a range of 10%-20%; the varied range of kaolinite content is bigger than that of illite; the change values of specimens with coarse-particle structures are larger than that of finer-particle structures; and the results from different analyzers vary to an extent. After analysis it is known that both the inhomogeneity of CDG and the experiences of analyzers mostly make the clay mineral contents change even in the same zone of one block. Furthermore, the XRD results are compared with those from statistic values via thin sections to check the variation of mineral contents. As a result, it is shown that for the mineral contents from XRD, the quartz is usually higher, the feldspar is smaller, and the clay mineral higher and smaller for different specimens. The mineral contents from XRD and thin section are different in a range of 0%-35%. Totally this research quantitatively describes the inhomogeneity of the CDG in block and trial pit sizes, which together with the identification of initial mineral or secondary clay mineral (e.g. feldspar or kaolinite, mica or vermiculite), causes the variations of the content results to a great extent.
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Key words:
- clay size particle /
- clay mineral /
- completely decomposed granite /
- Hong Kong
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图 1 香港九龙2处采样点及开箱后全风化花岗岩整块样品
a.石硖尾skm; b.观塘kt
Fig. 1. Block samples of completely decomposed granite (CDG)
图 2 整箱样品不同面结构区划分
a.各面结构区分; b.4个侧面图像
Fig. 2. Structural zonation in different faces of one blocky CDG sample
图 3 18箱中的47个样品石英-长石-粘土矿物含量变化
Fig. 3. Three mineral component (quartz-feldspar-clay minerals) contents in 47 specimens of the 18 blocks of CDG
○.skm1;□.skm2;△.kt; 1.skm1-1;2.skm1-2;3.skm1-4;4.skm1-6;5.skm1-7;6.skm2-1;7.skm2-2;8.skm2-3;9.skm2-5;10.kt1;11.kt2;12.kt2;13.kt4;14.kt5;15.kt6;16.kt7;17.kt8;18.kt10
图 4 18箱中的47个样品粘粒中主要粘土矿物相对含量变化对比
Fig. 4. Relative percentages of clay minerals in clay size portions from 47 specimens in 18 blocks
图 5 不同人员对同一箱中同一区样品粘粒矿物成分鉴定结果对比
Fig. 5. Comparison of XRD results by different analyzers for specimens in the same zone
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