Colour Mechanism And Treatment Study On Amethyst

Amethyst and ametrine are valuable gemstones.Due to the low Fe concentration and complex occupations sites in crystal structure,its colour mechanism is still need further study and discussion.Also,the treatments are effected.By the treatments and spectroscopic measurements,this work studies the colour mechanism from amethyst from East African and ametrine from Bolivia and laboratory.Using micro observation and spectroscopy technology,many different types of inclusions such as hematites,violet bands and Brewster fringes were found to appear along the positive rhombohedron.That demonstrated Fe element has strict direction selectivity and trend to gather in some area.Brewster fringes which have darker violet colour are so important for the studies on colour mechanism.By Line and area scanning micro MIR spectrometer,Brewster fringes can be connected colour with structural defects.Following XAFS experiment suggested that Fe in the synthetic amethyst as the Fe³⁺in the substitutional and interstitial tetrahedron sites.Heat and irradiation treatment experiments can fade,intensify and reverse violet colour.In UV-Vis area,540 nm band decreased under heating while increased or reversed under irradiation.In MIR area,O-H band can be so different by these treatments.Heat treatment reduced the structural water in amethyst area in ametrine but strengthen the absorption of that in citrine area in the same sample.The phenomenon indicated the ionic immigration in the crystal.Irradiation treatment also can reduce some of the water peaks while create some new peaks.All the treatments demonstrated there is no direct correlation between MIR detectable water and violet colour.This paper confirmed the significant role played by Brewster fringes on helping study on colour mechanism in amethyst.And interstitial site have the profound effect on the coloration.Heated amethysts can change violet colour,while their previous spectral shapes wouldn't change a lot,and OH band detected in spectrometer has no connection with colour.The heat effected on the amethyst and ametrine has the regulation.Electron irradiation treatment do intense the violet with the dosage growing,the efficiency increases,and produce some new peaks in different samples.