Study On Heat Treatment Experiment Of Red And Yellow Jadeite And The Cause Of Colour

Different methods including standard gemological testing,polarizing microscope,EPMA,DTA/TG analysis,infrared spectrum,X-ray diffraction are used in this paper,to study red and yellow jadeite,together with heat treatment experiments,the cause of color in red and yellow jadeite is discussed.The Jadeite has various colors from yellow,orange,red with different saturation,brown and gray can be mixed in the color,which can be appeared as color banding in a rhythmic pattern.It has refractive index of 1.66,with mobs hardness between 6-7 and specific gravity from 3.26 to 3.33.A variety of yellow-red hues can mix in a natural jadeite and transparency differ from each other,while heat treated ones usually has single color,trending to be intense red,generally their transparency is lower and luster poorer comparing with natural ones,consisting of quenching cracks.Observed by polarizing microscope,the texture of jadeite is usually hypidiomorphic grained,Microscopic to medium sized,Columnar or granular shaped crystalloblastic texture,sometimes cataclastic texture porphyritic texture or metasomatic structure.Iron which cause red and yellow mainly exist as cryptocrystal between grains or in cleavage,and a very small amount scattered crystalline limonite and hematite were found in samples.XRD analysis showed,the main component of the red and yellow jadeite is jadeite,with a small amount of omphacite,kalsilite or berthierine.Iron hydroxides is a minor component in each sample,and it exist in form of crystalline lepidocrocite in heat treated ones.the crystallinity of red jadeite>99%,compareing with the yellow<99%,and the heat treated samples has higher average crystallinity then natural ones.According to the result of EPMA,main ingredient in red and yellow jade are SiO2,Al2O3 and Na2O,each make up 59.791%,23.836%and 14.165%.The correlation coefficient reflect replacement of ions.Mg2+ and Ca2+ appear in pairs,respectively replace Al3+ and Na+ in M1 and M2 position.The possibility for Cr3+ to replace Si4+ in T position is greater than the replacement of Al3+ in M1 positon,while Fe may appear both in Ml and T.In DTA/TG analysis,yellow jadeite,shows endothermic valley near the 100 ?and 383 ?,showing characteristic absorption of limonite,while red jadeite has weakened or lost peak in the same positions,that implied the yellow and brown color is mainly caused by limonite.The heat treatmcolor ent results showed that,as the temperature increased from 200? to 500?,the with brown tone gradually turned into intense red.Jadeite with original white color and those secondary color has brown or gray tones is suitable for heat treatment,with ideal condition of oxidizing atmosphere,temperature range of 3 50-500?.Characteristic absorption of jadeite appeared in the fingerprint area(1500-400cm-1)of the IR spectrum,as well as Vibration absorption peak of hydroxy in 3800?3200cm-1,indicating the content of water.After heating,the samples' absorption rate in the area were reduced,with the shape changed from U into V.The phenomenon implied that water content is higher in the untreated samples.Heat treatment process made water lost from the red and yellow jadeite more easily.