Study on the effect of heat treatment on the color of zoisite and the cause of coloration:The example of yellow and blue Tanzanian zoisite
The natural Tanzanian zoisite(tanzanite),with its distinctive blue-violet tone,is a rare and valuable gemstone.Heat treatment can optimise the color of zoisite and make it more economically valuable.However,the exact cause of this captivating blue-violet of zoisite has been a subject of debate for years.This study presents an examination of natural yellow and blue zoisites.The color changes of stones at different thermal stages were ob-served by setting three different temperature conditions 300℃,400℃ and 500℃,and using the L*a*b*color pa-rameter.The research findings indicate that the gemological properties of different colored zoisites remain consistent before heat treatment.It was determined that a temperature of approximately 500℃ is optimal for achieving a light blue-violet tone in zoisite.Heat treatment results in a pronounced enhancement of the blue-violet hues,enabling the modulation of zoisites'yellow or brown undertones through temperature control.Notably,the yellow zoisite expe-riences a fascinating color transformation from yellow-brown to blue-violet,with the critical temperature range between 400℃ and 500℃.Using energy dispersive X-Ray spectroscopy(EDXRF),laser ablation inductively coupled plas-ma mass spectrometry(LA-ICP-MS)and ultraviolet and visible spectrophotometry(UV-VIS)techniques,the study identified that the blue color in natural zoisite is primarily caused by V3+and Ti3+ions working in tandem.On the other hand,the yellow color is mainly attributed to Ti3+.The V/Ti ratio plays a supporting role in coloration of nat-ural zoisite.Moreover,the development and intensification of blue-violet tones in heat-treated zoisite,as well as the transition from yellow to blue-violet,are intricately linked to the charge migration of Ti3++V4+→Ti4++V3+.
Tanzanian zoisiteheat treatmentcolorthe cause of coloration
国家科技期刊平台
NSTL
万方数据
