Composition Characteristics And Color Quality Evaluation Of Pyrope-almandine

In this paper,we investigated the relationship between the composition and spectroscopic characteristics of prope-almandine by EPMA,UV-Vis spectroscopy,infrared spectrum,and laser Raman spectroscopy test,revealed the effect of composition variation on color,explored the feasibility of determining the Mn O content based on the color of prope-almandine,and proposed the color grading evaluation schemes for two color types of prope-almandine.Fifty-nine gem-quality faceted garnets were used in this study,and their color belongs to the red to purple series.Basic gemmological tests combined with electron microprobe data proved that the samples tested were in the solid solution series of prope and almandine,and a few of garnets could be attributed to the almandine subspecies.The peak positions of the infrared and Raman spectra of the samples varies with the composition of prope-almandine garnet.It was found that the(Si O₄)^4-group vibration in the IR spectrum shifted toward the short-wave direction with the increase of Mg²⁺content at the X-position of the sample,while the C and D peak spacing increased.In addition,its shift in Raman spectra decreases in wave number with increasing approximate theoretical cell parameters,while the peak intensities at 211 cm^-1and 154cm^-1increase with increasing Mg²⁺end-member fraction of prope.The UV-Vis spectra shows that the color of prope-almandine is mainly caused by Fe²⁺,Mn²⁺and a very small amount of Cr³⁺.The color of the samples shifted toward a violet/purple hue with increasing Fe O/Mn O ratio,and the prope-almandine showed a lighter pink/pink-orange color when the ratio decreased.When the ratio is too low(Fe O/Mn O<1.454),the absorption peaks at 460 nm,610 nm,and 687 nm are suppressed and not visible.The increase of Mn²⁺content makes the absorption in the violet region stronger and the orange tone increases.When Mn O>1wt%,Mn²⁺produced strong absorption in the violet region,on the contrary,it's weak absorption.Therefore,the joint effect of Mn²⁺and Fe²⁺should be considered when exploring the effect of Fe²⁺on color.When Mn O<1wt%,the increase of Fe²⁺drives the hue violet,while when Mn O>1wt%,the increase of Fe²⁺leads to a reddish hue.Therefore,light pink-orange/intense violet prope-almandine can be quickly discriminated from Mn O content by color,but it is still difficult for red prope-almandine.In addition,the electron probe detected very low levels of Cr,but identified strong absorption peaks of Cr³⁺in the UV-Vis spectrum,indicate that very small amounts of Cr³⁺can still have a large effect on the color,resulting in a brighter red color for the prope-almandine.In this study,the color quality of two different color types of prope-almandine has been evaluated separately,and two grading standards with practical significance were proposed by combining their brightness and color intensity.According to the color grade from low to high,the purple prope-almandine can be divided into three grades:Light Pink,Fancy Purple/Reddish-purple,Fancy intense Purple/Reddish-purple,and the red prope-almandine is divided into five grades:Fancy Light Brownish red,Fancy Brownish red,Fancy dark Brownish red,Fancy intense red,and Fancy vivid red.