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 elements 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 measurement) 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 samples,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 Bruker,AXS D8-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 J ADE。the authors found that d values at strong peaks equaled to 7.842 5 Å(003),3.925 7 Å(006),2.618 9 Å(012)and d values at some weak peaks equaled to 2.332 6 Å(015),1.985 3 Å(018),1.521 5 Å(113). The characteristic peaks and intensities were corresponding with those of pyroaurite.The resuits 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 aggregates.Most are flaky texture,with only some fibrous texture of orientation arrangement. Secondly,the length of the lamellae in the flaky texture is about 1 00 um 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 condition of amplification tO 1 1 000 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 diameter of the fibrous crystal is about 1 μm,which is arranged more closely than the flaky texture.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 irradiated 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 coloration might result from the chemical element Cr.Combining with the structure of pyroaurite,Cr³⁺ is in octahedral crystal field and replaces Fe什.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 5 50 and 400 nm,as a result the mineral presents the colour of purple.