New Insights into Coloration Mechanism in Violet-Red Pyrope-Almandine

Puyue Yang,Ying Guo

doi:10.3390/cryst12030379

Puyue Yang, Ying Guo

Open Access

https://doi.org/10.3390/cryst12030379

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Abstract

The most common type of garnet is pyrope-almandine, whose color varies from red to violet. In this study, 36 faceted gem-quality samples are used for electron microprobe, infrared spectrum, and UV-Visible spectrum test to find the coloration mechanism and spectroscopic characters in red-violet pyrope-almandine. The gradually increasing content of Mg2+ at the X position in the lattice is connected to the variation in the infrared spectrum. The wavenumber increases with the decrease of cationic radius, which makes the distance between C and D bands further. The color mechanism is mainly affected by Fe2+, and Mn2+. We discuss the change in colors with the assumption of a certain MnO content. When it is above or below the standard of 1 wt%, the absorption intensity of the UV-Visible spectrum is completely different in the purple zone, which determines the color to be red or violet. Therefore, the effect of Mn2+ and Fe2+ should be combined instead of being considered respectively. The results show that the MnO content can be quickly inferred by the light purple/fancy purple color. To avoid harming the value, this new insight makes it possible to quickly classify the gem quality in mining as well as in the market.

Highlights

Garnet is a widespread mineral, existing on the crust and upper mantle with the chemical formula X3 Y2 (SiO4 )3 [1]
The isolated [SiO4 ] tetrahedra in garnet forms the crystalline base of the crystal, combining with [YO6 ] octahedron and
Most of the ions can be substituted by similar particles, which is due to the fact that garnet is a complete isomorphous series [4]

Summary

Introduction

Garnet is a widespread mineral, existing on the crust and upper mantle with the chemical formula X3 Y2 (SiO4 )3 [1]. The isolated [SiO4 ] tetrahedra in garnet forms the crystalline base of the crystal, combining with [YO6 ] octahedron and [XO8 ] dodecahedron repeatedly to make up the structure (Figure 1) [2]. The shared corners of both [YO6 ] octahedrons and isolated silicon-oxygen tetrahedrons make it workable to form a dodecahedron vacuum between them. This vacuum is centered on X2+ , shaped as a distorted cubic [3]. As a quick and non-destructive testing method, IR spectroscopy is indispensable for researchers to figure out which functional groups exist in the minerals and how they affect every aspect [5]. The infrared (IR) spectrum can be used as proof to infer the proportion of each garnet end-member through the different functional groups’ activities [6]

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