Optical and Luminescence Spectroscopy of Varicolored Gem Spinel from Mogok, Myanmar and Lục Yên, Vietnam

Abstract

We studied 12 crystal fragments of natural spinel from Mogok, Myanmar and Lục Yên, Vietnam. All samples were crystal fragments of various shapes and sizes and several of them had gemological quality. Studied samples are enriched in Cr, V, Fe2+, Fe3+, Zn, which are responsible for its resulting color. They could be divided into groups of V-Cr spinels with Cr 0.001–0.006 apfu, V 0.001–0.004 apfu, and Fe spinels containing increased Fe2+ (0.001–0.017 apfu) and Fe3+ (0.004–0.012 apfu). Some samples show luminescence bands at 677, 685, 697, 710, and 718 nm assigned to Cr3+. The optical absorption spectra of spinels were divided into two groups of V-Cr and Fe spinels based on the dominant element acting on optical spectra. The optical spectrum of V-Cr spinels can be divided into two zones (1) 420–550 nm (V3+ and Cr3+ absorption); (2) 640–1000 nm (Fe2+-Fe3+ charge transfer). The optical absorption spectra of Fe spinels can also be divided into two zones (1) 410–650 nm (Fe2+-Fe3+ charge transfer) and (2) 770–1000 nm (Fe2+). This variation in chromophores results in the differences in color: V-Cr spinels are pink to red, Fe spinels are in shades of blue as well as yellow and pink.

Gemological and Spectral Characteristics of Gem-Quality Blue Gahnite from Nigeria

“…Five photoluminescence peaks at 675 nm, 686 nm, 698 nm, 708 nm, and 717 nm were present in all the samples in the 650∼750 nm band, although there were intensity differences in the fuorescence emission lines of diferent samples. Grinberg et al have revealed that lasers of wavelengths less than 600 nm can excite Cr 3+ ions in the crystal feld of spinel-phase strong octahedral crystals [18], which results in fve absorptions visible in the 650∼750 nm band [19]: (I) 650∼680 nm, (II) 680∼690 nm, (III) 690∼702 nm, (IV) 702∼712 nm, and (V) 712∼725 nm, which is consistent with the test results in this paper. Electron microprobe analyses showed that the gahnite samples all contained unequal amounts of Cr 2 O 3 (Table 2), and thus, the gahnite showed a photoluminescence spectrum that was highly similar to that of the other chromium-doped spinel species [19][20][21].…”

“…Malícková et al grouped the fuorescence spectra of Cr 3+ in natural spinel into three categories [19]: (a) purely electronic R-lines, known as the most intense luminescence centers, (b) a series of N-lines associated with eight pairs of Cr 3+ -Cr 3+ interactions related to the disruption of shortrange ordering of crystals [22], and (c) a series of phonon side bands on the low-energy side of the R-lines (R-PSB), related to the vibrational modes of spinel crystals.…”

A Comparison of Order-Disorder in Several Families of Cubic Oxides

“…The term “spinel” originates from the name of the naturally occurring mineral MgAl 2 O 4 , which is also well-known as a gemstone because of its variable color characteristics owing to the TM impurities ( Malíčková et al, 2021 ). The spinel structural family consists of numerous materials with the basic formula AB 2 X 4 where cation A can be either an alkali, alkaline earth, or a TM and cation B can be almost any TM or aluminum, gallium, and indium.…”

Gemological and Luminescence Characteristics of Taaffeites from Mogok, Myanmar

“…The Mogok region is located in the middle of the Mogok metamorphic band, which is a significant origin area for gems [14][15][16]. Systematic research on the taaffeite from Mogok, Myanmar, is still lacking.…”