Violet-to-blue 'nuummite' from Simiuttat, Greenland: origin of colour appearance and conditions of formation

Freanz, Leander and Thye Sun, Tay and Wirth, Richard and De Capitani, Christian and Hui Ying, Loke. (2016) Violet-to-blue 'nuummite' from Simiuttat, Greenland: origin of colour appearance and conditions of formation. The Journal of Gemmology, 35 (4). pp. 330-339.

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Official URL: https://edoc.unibas.ch/81569/

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Iridescent orthoamphibole rock ('Nuummite') from the Nuuk District in south-west Greenland is an unusual gem material known in the trade since 1983. Unlike most Nuummite that shows a 'golden' brown iridescence, some specimens from Simiuttat in the Buksefjorden Archipelago display a violet-to-blue colour similar to that seen in some labradorite. The rock is a biotite-cordierite-anthophyllite granofels that formed in the Late Archean and subsequently experienced a polymetamorphic overprint, with peak metamorphism at amphibolite-facies conditions. While electron microprobe analyses and Raman spectroscopy classify the orthoamphibole as an Al-rich anthophyllite without any inhomogeneities on a micrometre scale, transmission electron microscopy revealed sub-microscopic exsolution lamellae of anthophyllite and gedrite parallel to (010). The periodic lamellae have an average spacing of 124-133 nm, which generates the violet-to-blue diffraction coloration. This study shows that the colour appearance of Nuummite is not due to chemical variations from grain to grain, but to the spacing of exsolution lamellae in orthoamphibole.
Faculties and Departments:05 Faculty of Science > Departement Umweltwissenschaften > Ehemalige Einheiten Umweltwissenschaften > Theoretische Petrologie (De Capitani)
05 Faculty of Science > Departement Umweltwissenschaften > Geowissenschaften > Quartärgeologie (Fleitmann)
UniBasel Contributors:Franz, Leander
Item Type:Article, refereed
Article Subtype:Research Article
Note:Publication type according to Uni Basel Research Database: Journal article
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Last Modified:25 Jan 2022 08:06
Deposited On:25 Jan 2022 08:06

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