木变石的结构构造特征及其宝石学意义

Texture and Structure Characteristic of Silicified Crocidolite and its Gemmological Significance

木变石因交代假象作用而具有独特魅力,吸引了众多国内外学者对其矿床地质方向进行研究和探讨。然而,目前鲜有从宝石学角度研究木变石的成果,对木变石宝石学意义的研究相对较少。本文旨在分析不同种类(以颜色划分)的木变石之间的宝石学差异,并尝试通过结构与成分两点因素阐述其差异性。研究主要分为三个过程:运用宝石学基础仪器等测试及描述木变石的宝石学性质;运用偏光显微镜、扫描电镜等对比分析不同种类木变石之间的结构特征差异;运用扫描电子显微镜能谱仪、X射线荧光能谱仪等现代测试仪器对比分析不同种类木变石所含的微量元素种类及含量的差异。进而,综合分析结构与成分对木变石宝石学价值的影响。对金黄色、棕色、蓝色三种颜色的木变石样品进行研究,不同颜色木变石物性(密度、硬度)与石英相近,有略微差异,棕色木变石密度最高(2.65g/cm3),蓝色木变石密度次之(2.64g/cm3),金黄色木变石密度最低(2.62g/cm3),推断与其所含包裹体相关。显微硬度测试结果显示,黄色木变石硬度略高于蓝色木变石,推测是由青石棉硬度较低造成;木变石平行于纤维测量的结果略高于垂直于纤维的测量结果,这一结果与石英的晶体结构相关。使用傅里叶红外光谱仪进行测试,分别使用了镜面反射法和溴化钾压片法,木变石中的主要矿物为石英。镜面反射法显示1100cm-1附近有两个区分度良好的吸收峰,说明石英矿物结晶度较好。使用X射线衍射分析仪测出木变石的五指峰,运用公式CI=10×F×a/b(F为比例因子)进行计算,木变石的结晶度位于8.69-8.70之间,石英结晶度较高。木变石猫眼效应的主要成因为纤维状结构,为对其结构进行观察,分别沿垂直和平行于纤维结构的面切磨薄片,并在显微镜下进行观察,纤维状结构主要为石英,可见少量空管和充填的氧化铁包体(赤铁矿和针铁矿),直径约为20-50μm。相邻的石英纤维具有相近消光性。同时,氧化铁包体主要充填于纤维状石英颗粒间隙。在扫描电子显微镜下,对木变石中纤维状石英的间隙进行能谱分析,检测到Fe元素,进一步佐证了氧化铁包裹体的存在;对黄色、棕色木变石的相似区域进行能谱分析的结果对比,棕黄色木变石中氧化铁含量明显多于金黄色木变石,黄色系木变石的颜色随氧化铁包体含量的增加而加深。扫描电子显微镜下观察蓝色木变石的自然断裂面,可见纤维状矿物呈混杂状或放射状分布,使用能谱测试,与纤维状石英表面区域进行对比分析,检测出仅有的Na、Mg、K、Ca等元素,推断该纤维状物质为交代残余的青石棉。使用X射线能谱进行元素分析,检测出少量的Fe、Mg、Al、Na、Ca等元素,符合角闪石石棉的元素种类,进一步验证了以上结果。扫描电子显微镜下观察,木变石表面分布石英颗粒可呈略具自形粒状,推断形成于交代作用后期,对木变石的形成温压条件具有一定的指示意义。

Silicified crocidolite has an unique charm because of the metasomatic pseudomorph texture, and has attracted many domestic and foreign scholars to study and discuss the field of geological deposit. However, there are few studies from the perspective of the silicified crocidolite geology. The structure and composition of the silicified crocidolite are main fac-tors influencing the gemmological properties. In this paper, the research sample is silicified crocidolite of three colours, including golden, tan and blue. The gemmological conventional instrument, the optical microscope, scanning electron microscopy（SEM） , X-ray powder dif- fraction（XRD） and X-ray fluorescence spectrometer（XRF） were used to analyze all the sam-ples, microscopic structure, mineral composition and contents of elements.The physical properties （density, hardness） of these samples are closed to the quartz properties, but there is a little difference among different colours, which are due to the dif-ferent kinds and contents of inclusion probably. The main mineral in the silicified crocidolite is quartz by using Fourier infrared spectrometer （FTIR）. the crystallinity of quartz is fine, because two absorption peaks can be seen clearly close to 1 100 cm^-1. The quartz five fingers peaks diffraction pattern can be acquired by XRD. Calculating by the formula of CI= 10×F× a/b（F is Scaling factor） , the crystallinity of tan and blue silicified crocidolite is found to be similar to each other, both between 8. 69 - 8. 70.Through the optical microscope, the fibrous structures are formed by fibrous quartz mainly, a little vacant fibre and the vacant fibre filling with iron oxide. Fibrous quartz is a- bout 20 to 50 ｜Ltm in diameter,and the adjacent fibrous quartz have similar extinction position. Meanwhile, energy spectrum analysis for the clearance of fibrous quartz detected the element of Fe, and confirmed the point above. For the yellow kind of silicified crocidolite, colour difference is decided by the amount of iron oxide inclusions content. The more iron inclu-sion, the deeper the colour is.Observing the natural fracture of the hawk？s eye samples under scanning electron mi-croscopy （SEM） , the fibrous minerals can be seen in the shape of radiation. By the energy- spectrum testing, compared with fibrous quartz surface, the especial elements such as Na, Mg, K , Ca can be detected. Thus, the fibrous mineral can be concluded to be metasomatism residual blue crocidolite. By X-ray fluorescence spectrometer （XRF） , the especial elements were detected, such as Fe, Mg, Al,Na,Ca. These were consistent with the elements of blue crocidolite, and the former results were further verified. By scanning electron micro-scope （SEM）, silicified crocidolite^s surface all distribute a little granular quartz grain, and some slightly show automorphic granular structure？ which are concluded that formed in late metasomatism 5 and have certain significance to the formation temperature and pressure con-ditions of the silicified crocidolite.Inclusions filling in fiber particles clearance （that is,limonite,goethite, blue crocidolite）,caused the “ line” of the cat’ s eye of different colours,is the main reason forming “the cat＇s eye＂ of silicified crocidolite. These minerals are arranged parallel to the fibrous quartz. According to the direction of the fibrous quartz,the cat’ s eye effect of silicified crocidolite can be discussed.