Optical Spectra and Color Nature of Lithium Amphiboles
Optical absorption spectra of two lithium amphiboles of the pedrisite group from rare-metal pegmatites of the Sangilen rare-metal province in the southeastern part of Tyva have been studied. One of them - a limit magnesian fluoro-sodium pedrisite of yellow-green color - was taken from the rocks hosting pegmatites, and another one - fluoro-sodium ferro-pedrisite of violet-blue color - was taken from pegmatites as such. It has been demonstrated that the color of the yellow-green mineral is associated with absorption bands of Cr<sup>3+ </sup>ions in the octahedral coordination. Absorption bands of Cr<sup>3+ </sup>ions in the spectrum of fluoro-sodium pedrisite are formed by a transmission window in the yellow-green region of the spectrum. Therefore, the color of this sample is yellow-green. The color of violet-blue pedrisite is defined by intensive absorption bands of charge transfer Fe<sup>2+</sup> → Fe<sup>3+</sup> 550, 680 nm. Very strong absorption bands of 550 and 680 nm are formed by a transmission window in the violet-blue region of the spectrum. Thus, the color of ferro-pedrisite is violet-blue.