Origin of Asterism and Chatoyancy in Rare Six-Rayed Star Iolite

Bui, Thanh Nhan; Delaunay, Aurélien; Hennebois, Ugo; Steinbach, Martin P.; Gauthier, Jean-Pierre

doi:10.15506/jog.2021.37.7.670

Cited by 1 publication

(2 citation statements)

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“…The γ-axis, along which visible light is most strongly absorbed, is oriented parallel to the a-axis of cordierite (see figure 4 of Fritsch and Rossman, 1988). The Blue Bear chatoyancy-causing inclusions occur in the same orientation as similar colorless acicular inclusions previously reported in chatoyant iolite from South India (Kammerling and Koivula, 1991), as well as colored acicular hematite inclusions causing chatoyancy in iolite cabochons from an unknown country of origin (Bui et al, 2022). The nature and composition of the colorless acicular inclusions reported by Kammerling and Koivula (1991) were not identified.…”

Section: Cause Of Chatoyancysupporting

confidence: 60%

“…Inclusions reported in gem-quality cordierite include a variety of silicates, oxides, sulfides, and phosphates (table 2), with the most common being hematite and biotite. Hematite occurs in the "bloodshot" iolite variety (India and Sri Lanka; Kammerling and Koivula, 1991;Muyal and Mattlin, 2019) as oriented crystals causing asterism (Bui et al, 2022) or as small isolated inclusions (Thor-Odin dome; present study). Iron-rich phlogopite (biotite) inclusions are reported from Wyoming (Hausel, 2003), India (Das and Mohanty, 2017), and Thor-Odin (present study).…”

Section: Inclusionsmentioning

confidence: 64%

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Iolite from the Thor-Odin Dome, British Columbia, Canada: Geology, Chemical Composition, Inclusions, and Cause of Chatoyancy

Belley

2023

G&G

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Two gem-quality cordierite (iolite) localities, the Blue Arrow and Blue Bear occurrences, were recently discovered (in 2016 and 2021, respectively) in the mountainous Thor-Odin dome, southwest of Revelstoke in British Columbia, Canada (figure 1). They have so far produced small quantities of faceted stones (generally <2 ct, though the author has seen faceted windowed stones up to 4.76 ct from Blue Bear) and attractive cat's-eye cabochons (up to 12.02 ct, Blue Bear only). The present study seeks to gain new insights into: (1) the geological origin of the occurrences in a regional context and how they compare to the geology of other iolite deposits, (2) the nature of iolite mineralization at the Blue Arrow and Blue Bear localities, (3) the chemical composition and inclusions of Thor-Odin dome iolite, and (4) the nature of the inclusions responsible for chatoyancy. GEOLOGY OF IOLITE DEPOSITSDeposits of iolite (gem-quality cordierite) occur in magnesium-and aluminum-rich metamorphic rocks formed at amphibolite to granulite facies conditions. Many iolite occurrences are found around the world, but relatively few have been comprehensively characterized from a geological (i.e., petrological, geochemical, and geochronological) perspective.Significant deposits occur in the Bandarguha and Orabahala areas of Kalahandi District, India, where gem cordierite (with X Mg = [Mg]/[Mg + Fe] = 0.82-0.88) is hosted in granulite facies cordierite-biotite schists and gneisses, which can contain the following accessory minerals: quartz, potassium feldspar, plagioclase, sillimanite, corundum, and orthopyroxene (Das and Mohanty, 2017).

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