Gemmological Characteristic and Genetic Mineralogy of Dark Green Jade
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摘要: 近期市场上出现一种名为“撒金花黑青玉”玉石，为了解该玉石的结构与矿物组成，对其基本宝石学特征进行了测试，并利用偏光显微镜、X射线粉末衍射仪、扫描电子显微镜、电子探针等测试手段进行了深入分析。“撒金花黑青玉”样品整体呈墨绿色，表面可观察到金属色片状反光，宝石学测试结果表明该玉石折射率为1.60~1.61，与软玉相比偏小；相对密度为3.11~3.21，与软玉相比偏大。X射线粉末衍射、偏光显微镜与扫描电子显微镜测试结果表明，“撒金花黑青玉”样品的主要矿物组成为阳起石(45%)、绿泥石(45%)、黄铁矿(5%)、其它矿物(5%)。其中，阳起石呈纤维状变晶结构，束状集合体，存在微定向；绿泥石呈鳞片状变晶结构，d060峰值为1.53~1.54 Ǻ，为三八面体结构，绿泥石与阳起石共同组成样品的基质成分，黄铁矿镶嵌于基质内。根据电子探针计算结果，对阳起石中Mg/(Mg+Fe2+)值与Si值进行投点定名，其Mg/(Mg+Fe2+)值介于0.88~0.89，投点均落在阳起石范围内；根据绿泥石的Fe-Mg-(Al+□)分类图解进行投点定名，投点均落在镁绿泥石范围内，同时也落在I型三八面体绿泥石范围内[2]，与XRD检测结论相符；根据绿泥石的Fe-Si分类图解进行投点定名，投点均落在斜绿泥石范围内(一种镁绿泥石)。两种绿泥石种属定名模式均显示出样品中绿泥石有富镁贫铁的特性。根据阳起石中的较低的Mg/(Mg+Fe2+)值(≈0.9)对其成因进行判定，结果显示其成因可能与蛇纹石化超基性岩有关；根据绿泥石中Al/(Al+ Mg+ Fe)与Mg/(Mg+ Fe)值进行投点，结果显示样品原岩系超基性岩且富镁不为含铁建造，与阳起石的判定结果相同。对绿泥石中Mg与Si、AlIV，AlVI，Fe离子之间的进行投点以显示其相关性，结果显示Mg与上述主要阳离子之间线性相关性较弱，指示样品可能经历多期变质作用。根据绿泥石化学成分对其形成环境进行估算，结果显示其形成温度介于219~252℃，氧逸度lgf(O2)介于-63.3~-59.4，硫逸度lgf(S2)介于-15.6~-9.1，指示样品的形成环境是相对还原条件的低温热液。因在GB/T 16552-2017《珠宝玉石名称》中并未对和田玉中主要组分的具体含量与结构有很好的界定，且样品中较高的绿泥石含量已经对其主要性质产生了影响。考虑到不同标本中矿物含量存在差异性，建议对阳起石含量大于50%的标本定名为和田玉，尽管有鉴定机构鉴定本样品为和田玉，但笔者建议本样品不能定名为和田玉。Abstract: Recently, a new kind of jade called "Sajinhuaheiqingyu" appeared in the market. A series of samples have been tested on basic gemmological characteristics and analyzed by using polarizing microscope, X-ray diffraction (XRD), scanning electron microscope (SEM), electron probe micro-analyzer (EPMA) and other test methods for the structure and mineral components of the samples. The samples are dark green and the metallic flake is reflected on the surface. The gemmological results showed that the refractive index of the samples are between 1.60—1.61, smaller than that of normal nephrite, and the relative density is between 3.11—3.21, larger than normal nephrite. The results of XRD, polarizing microscope and SEM showed that the main mineral components of the samples are actinolite (45%), chlorite (45%), pyrite (5%) and other minerals (5%). The actinolite was fibrous, crystalline, fascicular, aggregative and micro directional. Chlorite showed a scale-like crystalline structure, and the peak value of d060 is between 1.53-1.54 Ǻ, which indicates a trioctahedral structure. Chlorite and actinolite dominate the matrix components, and pyrite is embedded in the matrix. The actinolite is further named according to the Mg/(Mg+Fe2+) value (between 0.88 and 0.89) and the Mg/(Mg+Fe2+)-value vs Si-value diagram based on the results of EPMA. According to the classification diagram of Fe-Mg- (Al+□) of chlorite, the points are all plotted within the range of amesite, and within a trioctahedral structure of chlorite, which were consistent with the results of XRD analyses. According to the Fe-Si classification diagram of chlorite, the points dropped all within the range of clinochlore (one of the amesite). The naming models of two kinds of chlorite species showed that the chlorite in sample is rich in Mg and poor in Fe. It is indicated that the genesis of the samples should be related to the serpentinized ultrabasic rocks according to the lower values of the Mg/(Mg+ Fe2+)(≈0.9)in actinolite. According to the values of Al/(Al+ Mg+ Fe) and Mg/(Mg+ Fe) in chlorite, the results showed that the original rock series of the samples are likely ultrabasic and the magnesium-rich, but not iron-bearing, in good agreement with the result from actinolite. The correlations between Mg and Si, Mg and AlIV, Mg and AlVI, Mg and Fe in chlorite are studied, respectively. The results showed that the linear correlations between Mg and the above main cations were weak, indicating that the sample should have undergone multistage metamorphisms. Temperature estimates from chemical composition of chlorite indicated that the forming temperature should be between 219 ℃—252 ℃, the oxygen fugacity lgf(O2) should be between -63.3—-59.4, and the sulfur fugacity lgf(S2) should be between -15.6—-9.1. It is concluded that the forming environment of the sample is a low temperature hydrothermal deposit with relative reduction condition. Since the GB/T 16552-2017 Gems-Nomenclature do not well define the specific content and structure of the main components of nephrite, the high chlorite content in the sample has have a great impact on its main features. Considering the differences in mineral content in different samples, it could be named as nephrite with more than 50% actinolite. The authors suggested that the samples should not be named as nephrite, despite the accrediting institutes have named them as nephrite.
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Keywords:
- genetic mineralogy /
- chlorite /
- actinolite /
- low temperature hydrotherm /
- nephrite
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图 1 “撒金花黑青玉”样品图片(样品5是透射光下的图像)Figure 1. Pictures of "Sajinhuaheiqingyu" samples (sample No.5 under transmitted light)![]()
图 2 “撒金花黑青玉”样品的偏光显微镜下特征a.黄铁矿，正交偏光；b.蠕虫状绿泥石，单偏光；c.存在简单定向的绿泥石，正交偏光；d.简单定向的阳起石，正交偏光；e.阳起石的两组解理；正交偏光；f.磷灰石，正交偏光Py-黄铁矿；Chl-绿泥石；Act-黄铁矿；Ap-磷灰石Figure 2. Mineral characteristics under the polarizing microscope of "Sajinhuaheiqingyu" samples![]()
图 3 “撒金花黑青玉”样品的扫描电子显微镜下特征a.纤维状阳起石的微定向排列; b.鳞片状绿泥石Py-黄铁矿；Chl-绿泥石Figure 3. Mineral characteristics under SEM of "Sajinhuaheiqingyu" samples![]()
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图 5 绿泥石Al/(Al+ Mg+ Fe) - Mg/(Mg+ Fe)图解注：图件引自文献[4]，有修改Figure 5. Diagram of Al/(Al+Fe+ Mg)- Mg/(Fe+ Mg) for chlorites![]()
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图 1 “撒金花黑青玉”样品图片(样品5是透射光下的图像)
Figure 1. Pictures of "Sajinhuaheiqingyu" samples (sample No.5 under transmitted light)
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图 2 “撒金花黑青玉”样品的偏光显微镜下特征
a.黄铁矿，正交偏光；b.蠕虫状绿泥石，单偏光；c.存在简单定向的绿泥石，正交偏光；d.简单定向的阳起石，正交偏光；e.阳起石的两组解理；正交偏光；f.磷灰石，正交偏光Py-黄铁矿；Chl-绿泥石；Act-黄铁矿；Ap-磷灰石
Figure 2. Mineral characteristics under the polarizing microscope of "Sajinhuaheiqingyu" samples
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图 3 “撒金花黑青玉”样品的扫描电子显微镜下特征
a.纤维状阳起石的微定向排列; b.鳞片状绿泥石Py-黄铁矿；Chl-绿泥石
Figure 3. Mineral characteristics under SEM of "Sajinhuaheiqingyu" samples
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图 5 绿泥石Al/(Al+ Mg+ Fe) - Mg/(Mg+ Fe)图解
注：图件引自文献[4]，有修改
Figure 5. Diagram of Al/(Al+Fe+ Mg)- Mg/(Fe+ Mg) for chlorites
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[1] Hawthorne F C, Oberti R. Classification of the amphiboles[J]. Reviews in Mineralogy and Geochemistry, 2007, 67(1): 55-88. doi: 10.2138/rmg.2007.67.2
[2] Foster M D. Interpretation of the composition and a classification of the chlorites[J]. US Geology Survey Prof, 1962, 414: 33. http://doc736.meixbooks.com/interpretation-of-the-composition-and-a-classification-of-the-chlorites--foster-m-d--P-d59lz.pdf
[3] Deer W A, Howie R A, Zussman J. Rock-forming minerals: Sheet silicates[M]. London: Longman. 1962: 270.
[4] 张朱武, 干福熹, 承焕生. 不同成矿机理和地质环境下形成的软玉的化学成分特征[J]. 矿物学报, 2010, 30(3): 367-372. https://www.cnki.com.cn/Article/CJFDTOTAL-KWXB201003016.htm [5] Laird J. Chlorites: Metamorphic petrology[J]. Reviews in Mineralogy and Geochemistry, 1988, 19(1): 405-453.
[6] El-Sharkawy M F. Talc mineralization of ultramafic affinity in the eastern desert of Egypt[J]. Mineralium Deposita, 2000, 35(4): 346-363. doi: 10.1007/s001260050246
[7] Walshe J L. A six-component chlorite solid solution model and the conditions of chlorite formation in hydrothermal and geothermal systems[J]. Economic Geology, 1986, 81(3): 681-703. doi: 10.2113/gsecongeo.81.3.681
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