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

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.