Gem-like iridescence is rare in amphibole minerals or amphibole-bearing rocks. The first known and most famous occurrence of amphibole iridescence is in high-grade metamorphic rocks called “nuummites” from the Precambrian of southwestern Greenland.
“Nuummite” is a trade name (= non-geologic name) for anthophyllite-gedrite amphibolite rocks having light- to dark-golden brown iridescence. They were discovered in the early 1900s. The amphibolite is reported to occur as pods in quartz-cordierite gneisses of the Akulleq Terrane. Amphibolite is a foliated or crystalline-textured metamorphic rock dominated by one or more amphibole minerals. Most amphibolites have basalt or gabbro as their original protolith rock.
The iridescence is caused by light interference along the boundaries between alternating, closely intergrown, exsolution lamellae of anthophyllite and gedrite - both are varieties of amphibole. Anthophyllite is Mg7Si8O22(OH)2 - magnesium hydroxy-silicate. Gedrite is Mg5(Si6Al2)O22(OH)2 - magnesium hydroxy-aluminosilicate. The same light interference phenomenon causes iridescent blue coloration in larvikites.
Age: Neoarchean, 2.68 to 2.77 Ga (dates are from included zircon and monazite). Anthophyllite-gedrite exsolution lamellae formed at that time or during a ~2.55 Ga reheating event or during a Proterozoic metamorphism event.
Locality: unrecorded island south of town of Nuuk, Nuuk District, southwestern Greenland.
“Nuummite” (= anthophyllite-gedrite amphibolite) (above & below) (8.2 cm across at its widest) from the Late Archean of southwestern Greenland. The 3 photos shown above and below are the same piece of “nuummite” illuminated from different angles. Notice that different crystals in the rock show their dark golden brown iridescence only at certain light angles.
“Nuummite” (= anthophyllite-gedrite amphibolite) (above & below) (8.2 cm across at its widest) from the Late Archean of southwestern Greenland.
“Nuummite” (= anthophyllite-gedrite amphibolite) (8.2 cm across at its widest) from the Late Archean of southwestern Greenland.
Mostly synthesized from:
Rodgers, K.A., P.D. Kinny, V.R. McGregor, G.R. Clark & G.S. Henderson. 1996. Iridescent anthophyllite-gedrite from Simiuttat, Nuuk District, southern West Greenland: composition, exsolution, age. Mineralogical Magazine 60: 937-947.
Champness, P.E. & K.A. Rodgers. 2000. The origin of iridescence in anthophyllite-gedrite from Simiuttat, Nuuk District, southern West Greenland. Mineralogical Magazine 64: 885-889.