Mineralogical Characteristics and Coloration Mechanism of Yellow-Green Alunite Jade

BACKGROUNDAs a new kind of jade, yellow-green alunite jade is difficult to positively identify and evaluate due to the lack of research. Yellow-green alunite jade is a new type of jade that has appeared on the market in recent years. At present, there is relatively insufficient research on its gemological mineralogical characteristics, which makes it difficult for its scientific identification and quality evaluation.

OBJECTIVESTo study the mineralogical characteristics and coloration mechanism of yellow-green alunite jade.

METHODSInfrared spectroscopy, X-ray diffraction, Raman spectroscopy, scanning electron microscope, laser ablation-inductively coupled plasma-mass spectrometry and ultraviolet-visible spectroscopy were applied to analyze the mineral component, microtexture, and chemical composition.

RESULTSThe main mineral component of yellow-green alunite jade was alunite with a cryptocrystalline texture and grain size of less than 5μm. The very fine texture identified it as a delicate jade, but due to lack of interwoven fibric or scaly texture, its toughness was relatively poor. The alunite jade was composed of O, Al, S and K. The contents of transition metals such as V, Fe, Cr were relatively high, with average contents of 23591.52μg/g, 4717.99μg/g and 2077.67μg/g, respectively. Combined with the absorption spectrum in the ultraviolet region of 500nm and 826nm in the ultraviolet-visible spectroscopy, it was concluded that the isomorphous substitution of Al³⁺ by V³⁺, Cr³⁺ and Fe³⁺ was the main cause of the yellow-green color. The texture of weathering crust of alunite jade was relatively coarse. The mineral components were alunite, quartz, goethite with minor anatase. Yellowish-brown color of weathering crust was due to the serious pigmentation by limonite, and especially goethite.

CONCLUSIONSThe study confirmed the mineralogical characteristics and coloration mechanism of yellow-green alunite jade, which provides a theoretical basis for the scientific identification and further quality evaluation.