Spectral Characteristics and Color Genesis of Green Enstatite from Kenya

Enstatite is a mineral that crystallizes in the orthorhombic system, characterized by the general chemical formula XY[T2O6], with the cations Fe²⁺ and Mg²⁺ substituted respectively at the X and Y sites. Enstatite exhibits a diverse range of colors, including colorless, reddish-brown, brown-green, brown, and yellowish-green. Due to the rarity of dark green enstatite in nature, we utilized a suite of advanced analytical techniques to comprehensively investigate the unique spectral characteristics and color genesis mechanism of gem-quality green enstatite from Kenya. This study provides the first systematic analysis of the spectral features of the isomorphous substitution for Fe and Cr ions, thereby addressing a critical gap in the existing literature. It establishes a solid foundation for differentiating Kenyan enstatite from those of other origins and deepens the scientific understanding of this rare gem variety.

Fourier-transform infrared spectroscopy (FTIR), Laser Raman spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, photoluminescence (PL) spectroscopy, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for trace-element analysis were jointly employed to elucidate the origin specificity and color mechanism.

The analyses revealed that the green enstatite samples contained significant amounts of iron (4.060-5.080 wt.%) and trace amounts of chromium (0.313-0.502 wt.%). UV-visible spectral analysis has shown the green enstatite samples to have absorption peaks at 504, 645, 655, 684 nm, and a weak absorption peak at 545nm. The PL has shown a shoulder peak at 688 nm, is split into 681 nm and 686 nm sharp peaks, and a series of luminescence bands near 750 nm.

The green hue of the enstatite from Kenya is primarily attributed to the synergistic effects of Fe²⁺ and Cr³⁺ ions. Specifically, Fe²⁺ plays a dominant role in determining the depth and intensity of the base color, whereas Cr³⁺ significantly enhances the vividness and brightness of the green tone. By elucidating the distinctive coloration mechanism of Kenyan green enstatite and providing its precise spectral and chemical characteristic “signatures”, this study establishes a foundation for differentiating Kenyan specimens from those originating elsewhere, simultaneously contributing to the refinement of the classification system for pyroxene minerals.

Systematic spectral analysis of Kenyan green enstatite elucidates the mechanisms underlying its color formation, addressing a gap in existing research and providing a basis for comparative studies of pyroxenes. The green enstatite samples examined are magnesium-rich variants within the enstatite-ferrosilite solid solution series, contributing to the refinement of pyroxene classification. The coloration arises from the synergistic effects of Fe²⁺, which governs the depth and intensity of the base tone, and Cr³⁺, which enhances the vividness and brightness of the green hue.