Crystal-Chemical Features and Color Nature of Emeralds from the Khench Deposit(Afghanistan)

摘要

This paper describes the studies carried out on the crystal-chemical features and nature of the color of emeralds from the Khench deposit(Afghanistan).A brief geological characteristic of the emerald deposit is presented and the geochemical features of the host rocks,in which the veins are located,are shown.The color of emeralds is associated with Cr_(VI)~(3+)ions,which are isomorphic to Al~(3+)ions in the structure of the mineral.Using optical absorption spectra,electronic transitions in Cr~(3+)ions were calculated.Mostly electronic spin transitions that are allowed for ~4T1(~4F)and ~4T_(2g)(~4F)spins are responsible for coloring.The studies on the color of emeralds in polarized light and the features of absorption depending on the crystallographic direction in crystals were performed.The calculation of chromaticity coordinates was carried out using the International Commission on Illumination CIE-1931.