1. Gemmological Institute，China University of Geosciences (Wuhan)，Wuhan 430074，China
2. School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China
Abstract：Apatite is a kind of common gem species in jewelry market. It is popular because of its rich color. Apatite with the Alexandrite effect is rare and expensive. It shows yellow green under the D65 light source and pink under the A light source. Characteristics in the visible spectrum and Alexandrite effect origin of this apatite have not been studied in detail. Based on this, the sample of this study is an apatite crystal with the Alexandrite effect. Two thin slices were cut parallel and perpendicular to the c-axis, respectively. The two samples were polished, and their visible spectra and trace elements data were collected. The results show there are many peaks the in visible spectrum of Alexandrite effect apatite. There are double absorption peaks at 583, 578 nm with strong intensity, double absorption peaks at 748, 738 nm with middle intensity, two absorption peaks at 688 and 526 nm with weak intensity and some very weak absorption peaks at 514, 483, 473 and 443 nm. The absorption peaks at 748, 738, 583, 578 nm create a transmission window in red orange region, and the absorption peaks at 583, 578, 526 nm create another transmission window in the yellow green region. Due to different relative spectral power distributions between D65 and A light sources, the transmittance through transmission windows is also different, which leads to different colors of apatite. D65 light source has the more yellow green light, passing through transmission window in yellow green region, so apatite shows yellow green color. A light source has more red light, passing through another transmission window in red orange region, so apatite shows pink. Therefore, the Alexandrite effect of apatite is caused by the absorption peaks at 748, 738, 583, 578 and 526 nm. Based on trace elements data and crystal field theory of rare earth elements, it was found that these absorption peaks were caused by neodymium (Nd). It was found that the Alexandrite effect was better in the orientation parallel to the c-axis based on visible spectra. It is suggested that the table of the gemstone should be cut parallel to the c-axis. This research combines trace elements and visible spectra to analyze the Alexandrite effect origin of apatite. It also provides guidance for cutting apatite crystal.
Key words：Apatite; Alexandrite effect; Visible spectrum; Trace elements
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