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688 nm), and N3 (689 nm). N lines are caused by different Mg, Al environments of Cr 3+ . With increasing inversion parameter (x), the R line decreases in intensity and the N lines become prominent leading to strongly broadened peaks with a maximum shifted toward higher wave lengths (687.5 nm at 1,100 °C). Criteria for the detection of heat treatment on gemstone spinel applicable to gemological routine examination are provided. Extrapolation of u, a, and bond lengths from heat-treated Burma spinel toward the natural crystal suggests a retrograde "closing temperature" of ca. 400 ± 100 °C at which Mg, Al disorder was frozen.

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