Determining Jadeite and Omphacite in Fei Cui by Na/(Na+Ca) Atomic Ratio with a Semi-Quantitative FTIR Screening Method

Jadeite and omphacite, both pyroxene-group minerals, are the two most dominant component minerals in fei cui (a rock-origin gemstone that may be dominated by jadeite, omphacite, kosmochlor, or their intermediate compositions), and classifying jadeitic vs. omphacitic domains is key to identification, grading, and evaluation. However, determining their proportion quantitatively often needs destructive analyses, which is not suitable for high-value fei cui samples. In this study, 29 fei cui samples were selected to obtain electron probe micro-analysis (EPMA) data and infrared reflectance spectra. We found that six characteristic peaks and the intensity ratio (IB/IC) near 1085–1066 cm−1 and 963–946 cm−1 showed strong linear correlations with the Na/(Na+Ca) atomic ratio in the jadeite–omphacite series of the studied fei cui, upon which an infrared spectroscopy semi-quantitative screening method was then developed. In this method, the IB/IC ratio from infrared spectra was derived to provisionally estimate the Na/(Na+Ca) atomic ratio according to statistical models; for jadeite (Na/(Na+Ca) atomic ratio > 0.8), its IB/IC ratio > 1.68, whereas for omphacite (Na/(Na+Ca) atomic ratio ≤ 0.8), its IB/IC ratio ≤ 1.68. This finding has implications for provisionally discriminating jadeitic from omphacitic domains non-destructively. Furthermore, by FTIR mapping using micro-FTIR, heatmaps based on the calculated IB/IC ratio at each scanned spot allow observation of the spatial distribution of jadeite-dominant and omphacite-dominant domains classified as jadeitic or omphacitic according to the Na/(Na+Ca) ratio, providing a semi-quantitative estimate of their mapped areal proportion on the polished surface. The method and its application were reinforced by micro-XRF mapping; Na and Ca elemental heatmaps show the same distribution in the same region, and would satisfy the increasing demand in fei cui testing.