“NUUMMITE”
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.