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

Iveta Malíčková,Radek Hanus,Ján Štubňa,Daniel Furka,Samuel Furka,Jana Fridrichová,Ľudmila Illášová,Peter Bačík,Radek Škoda

doi:10.3390/min11020169

Iveta Malíčková, Radek Hanus + Show 7 more

Open Access

https://doi.org/10.3390/min11020169

Copy DOI

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.

Highlights

Due to their wide range of intense colors, high mechanical resistance, and high thermal and chemical stability, minerals belonging to the spinel group are actively sought as gemstones, while synthetic spinel powders are widely used as ceramic pigment
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
We studied 12 crystal fragments of natural spinel from Mogok SP–M–1 to 9 and Lục Yên, Vietnam SPV–1 to 3 (Table 1)

Summary

Introduction

Due to their wide range of intense colors, high mechanical resistance, and high thermal and chemical stability, minerals belonging to the spinel group are actively sought as gemstones, while synthetic spinel powders are widely used as ceramic pigment. The variability in color displayed by spinels is due to their ability to accommodate a wide range of transition metal cations of different valence states at their structural sites (Fe, Cr, V, Co, etc.) [1]. The spinel-supergroup minerals have the general formula AB2X4 [3], where A represents divalent and B trivalent cations at the tetrahedrally and octahedrally coordinated sites in a structure with a cubic nearly close-packed arrangement. Most natural and synthetic spinels are disordered, Al occupies tetrahedral site, and both Al and Mg occupies octahedral sites [4]. This disorder causes defects, electron traps, and vacancies in the crystal structure that complicate the interpretation of the spectra. The composition as well as cation distribution of spinel minerals and materials have a strong influence on their physical properties [7], as far as light absorption in concerned [1]

Objectives

Methods

Discussion

Conclusion