Alexandrite, a rare form of the mineral chrysoberyl named for Tsar Alexander Ⅱ

Discovered in the 1830s in the Ural Mountains in Russia, the mineral alexandrite was named to honor Czar Alexander Ⅱ [1-3]. Alexandrite is a rare form of  chrysoberyl, a transparent, pale yellow-green mineral. By composition, chrysoberyl is beryllium aluminate with the formula BeAl₂O₄. Alexandrite contains impurities of chromium ions (Cr³⁺), which replace less than 0.5% of the aluminum ions (Al³⁺) [3]. This variation in ion-species composition results in a change of light-absorption behavior, making alexandrite appear yellow-green to greenish-blue in daylight and purple-red under artificial light.

In a recent American Scientist infographic, Andy Brunning introduced alexandrite as follows [2]:

Alexandrite is a rare form of the mineral chrysoberyl, which has the chemical formula Al₂BeO₄. Not content with being one color, it can display a whole range of hues, depending on the light falling on it. Deposits of alexandrite were first discovered in the 1830s, in the Ural Mountains in Russia. Mineralogist Count Lev Alekseevich Perovskii presented it to the future Tsar Alexander Ⅱ on his 16th birthday and named it in his honor.

It is interesting to find alexandrite typically associated with the formula BeAl₂O₄—not reflecting its nature of being a chromium variety of chrysoberyl (see, for example, its mindat entry [4]). To annotate data entries and abstracted text as specifically bound to alexandrite, the CurlySMILES language [5] provides the needed syntax: {*BeAl2O4}{IMc=[Cr+3]}. In this notation, the Stoichiometric Formula Notation (SFN) is augmented by annotation code beginning with the miscellaneous interest annotation marker IM for impurity and continuing with the specification of the impurity ion. CurlySMILES further allows integration of text and code. The above notation can be enhanced by explicitly stating the mineral name (using annotation dictionary key min):  
{*BeAl2O4}{IMc=[Cr+3];min=alexandrite} 


Keywords: mineralogy, chemistry, cheminformatics, nomenclature, mineral notation, mineral formula encoding, compact annotation.

References and more to explore

[1] Carly Wickell: Get the Facts About Alexandrite. The spruceCrafts, December 13, 2018. Link: www.thesprucecrafts.com/alexandrite-facts-2042901.
[2] Andy Brunning: The color of alexandrite. American Scientist Jan.-Feb. 2019. 107 (1).
[3] The Mineral Chrysoberyl. Minerals.net. Link: www.minerals.net/mineral/chrysoberyl.aspx.
[4] Alexandrite. Mindat.org. Link: www.mindat.org/min-109.html.
[5] Axel Drefahl: CurlySMILES: a chemical language to customize and annotate encodings of molecular and nanodevice structures. Journal of Cheminformatics 2011, 3:1. DOI: 10.1186/1758-2946-3-1.