Neptunite Inclusion in Benitoite
The authors recently examined a round, slightly zoned blue and near-colorless faceted benitoite that contained an interesting inclusion. Microscopic observation revealed a well-formed euhedral semitransparent reddish brown crystal surrounded by fine short needles and fluid fingerprints (see above). Because of the color and structure of the inclusion, both neptunite and joaquinite were considered as the possible identity. In benitoite, these minerals are often found in association and may have a similar color to the inclusion observed in this particular example. One method of separating neptunite from joaquinite is pleochroism. A yellow-orange, orange, and deep red pleochroism is consistent with neptunite, while light yellow and colorless pleochroism is consistent with joaquinite (webmineral.com). This inclusion showed a reddish orange to red pleochroism that was more consistent with neptunite. Raman analysis confirmed the host crystal was benitoite and the orangy red inclusion was neptunite, which has been previously documented (E.J. Gübelin and J.I. Koivula, Photoatlas of Inclusions in Gemstones, Vol. 1, 5th ed., 2008, Opinio-Verlag Publishers, Basel, Switzerland, p. 416).
Benitoite is a barium titanium silicate (BaTiSi3O9). This rare gem is found primarily in San Benito County, California, though it also occurs in other locations worldwide. Neptunite’s chemical formula is KNa2Li(Fe2+,Mn)2Ti2Si8O24, and it also occurs in San Benito County. Neptunite is found in association with other minerals in New Mexico, Greenland, and Canada, to name just a few (W.L. Roberts et al., Encyclopedia of Minerals, 2nd ed., 1990, Van Nostrand Reinhold, New York, pp. 603–604). The gem-quality benitoite material comes from the Benitoite Gem mine and the Junnila Claim (B. Laurs et al., “Benitoite from the New Idria District, San Benito County, California,” Fall 1997 G&G, pp. 166–187). This faceted benitoite contains one of the best examples of neptunite in benitoite seen by the authors.
