一种新型南非紫色玉石的矿物学特征研究

为探究南非最近产出的一种新型紫色玉石的矿物学特征及颜色成因，对其进行了常规宝石学特征测试，并采用X射线粉末衍射仪（XRD）、扫描电子显微镜（SEM）及紫外-可见分光光度计（UV-Vis）等仪器对其矿物组成、结构特征、化学成分和致色元素进行了研究。常规宝石学测试结果表明，该玉石折射率为1．54（点测），相对密度为2．1-2．2，摩氏硬度为1-2。XRD分析结果表明样品主要矿物组成为鳞镁铁矿，呈紫色，次要矿物成分为利蛇纹石，呈褐绿色；SEM分析结果显示鳞镁铁矿大多为集合体，主要以鳞片状和纤维状两种集合体形式存在，能谱分析（EDS）结果显示紫色部分主要化学成分为Mg，O，C，Fe，Cr等，佐证了紫色部分为鳞镁铁矿；UV-Vis结合EDS分析结果显示，鳞镁铁矿的紫色可能由Cr致色引起。

Mineralogical Characteristics of A New Purple Jade from South Africa

A new purple jade has been found in South Africa recently. In the article, mineral assemblages, structural characteristics, chemical components and the main colour causing el- ements were investigated by means of X-ray diffraction （XRD）, scanning electron microscope （SEM）, and ultraviolet-visible spectrophotometer （UV-Vis） . The testing results show that the refractive index of the jade is 1.54 （point measure- ment） and the specific gravity is 2. 13-2. 14. The special gravity increases slightly along with the content increase of brownish green mineral. When nails swipe the surface of sam- ples, power easily fall off, which indicates its Moh＇s hardness is 1 to 2. The colour of the jade differs slightly from dark purple to light purple and the opaque jade has a rare silk luster. Mineral component was investigated by X-ray diffraction produced by the German Bruk- er, AXS DS-FOCUS models. Owning to the fact that the separation of the purple minerals and brownish green mineral was quite difficult, the two testing parts of the samples were visually relatively pure purple mineral and mixture of purple mineral and brownish green mineral. The samples were grided into powder of 200# screening and dried. Retrieving the X-ray diffraction pattern of the sample with the software of JADE, the authors found that dvalues at strong peaks equaled to 7. 842 5 A（003）, 3. 925 7 A（006）, 2. 618 9 A（012） and d values at some weak peaks equaled to 2. 332 6 ,A（015） ,1. 985 3 A（018）, 1. 521 5 A（113）. The characteristic peaks and intensities were corresponding with those of pyroaurite. The re- suits also showed the mineral component of the brownish green part is lizardite. Chemical composition and structural characteristics of samples were studied by scanning electron microscope（SEM） and its attached energy dispersive（EDS）. The results showed from three main aspects. Firstly, pyroaurite from the samples mainly occurs as mineral ag- gregates. Most are flaky texture, with only some fibrous texture of orientation arrangement. Secondly, the length of the lamellae in the flaky texture is about 100 μm and the thickness ranges from 1 to 2 μm. The fact that there exits Nano-sized pores between the lamellae proves the sample is of loose structure. Observing the surface of the lamellae under the con- dition of amplification to 11 000 times, the authors find that the crystal is not smooth and the surface has some irregular granular mineral, whose length is about 10 μm. Finally, the di- ameter of the fibrous crystal is about lt~m, which is arranged more closely than the flaky tex- ture. The result of the EDS shows that the chemical compositions of the purple part from the sample are Mg, O, C, Fe and Cr and the mineral component is pyroaurite. Meanwhile, the result of EDS indicates that the chemical composition of the particulate matter is pyroaurite, which is the same with the body of the purple part. The coloration theory was preliminarily studied by using Lambda 650S ultraviolet-visible spectrophotometer. The reflection method was adopted for testing the purple part of the sample and the results showed that the purple part had a broad absorption of purple light and orange light with a very low absorption of the red light and blue light. Accordingly, when ir- radiated by the visible light, the mineral would present the mixing colour, the red light with blue light, namely purple colour. The results of the UV-Vis and EDS showed that the color- ation might result from the chemical element Cr. Combining with the structure of pyroau- rite, Cr3＋ is in oetahedral crystal field and replaces Fe3＋. Its base spectral term 4F is divided into three levels of 4A2,4T2 and 4T1. The energy level transitions between 4A2 and 4T2, 4A2 and 4T1 lead to two general absorption centers at 550 and 400 nm, as a result the mineral presents the eolour of purple.