Ivigtut
World’s largest cryolite deposit
Ivigtut (or Ivittuut), on Arsuk Fjord on the far southwestern coast of Greenland, is famous for its strange minerals. At least 91 species are known, for which Ivigtut is the type locality of 17. Of those 17, the only one that is even remotely well-known is cryolite, sodium aluminum fluoride, Na3AlF6.
Cryolite, discovered at Ivigtut in 1798, was used as an ore of aluminum in the 19th century, when aluminum was more costly than silver because of the difficulty in extracting it from rocks despite its abundance in the earth’s crust. When electrolytic processes led to the cheaper production of aluminum metal, cryolite was used as a flux for bauxite, a rock which is now the primary ore of aluminum. A flux (from Latin for flow), in this case the fluoride, makes the molten ore in smelting more fluid and lowers the melting temperature while increasing the electrical conductivity of the melt. Although cryolite is unusual worldwide, it was exceptionally abundant at Ivigtut where more than 3 million tons was mined from 1854 until the deposit was essentially depleted in 1987.
The other weird Ivigtut minerals are mostly complex sodium aluminum fluorides, some including strontium, barium, magnesium, and lithium, but there are also rare silver-lead-bismuth and copper-zinc-tin sulfides present along with many more common minerals. The deposit is unique in the world, and consequently has been studied extensively.
The cryolite and other fluorides are related to the intrusion of the Ivigtut Granite about 1,250 million years ago (Blaxland, 1976, Rb-Sr isotopic evidence for the age and origin of the Ivigtut Granite and associated cryolite body, South Greenland: Economic Geology, 71 (5): 864–869). The alkali (rich in sodium) granite is part of the Gardar Igneous Province that includes carbonatites (formerly molten rocks that are dominantly calcium carbonate, like limestone but inorganic in origin), some of which are anomalously enriched in strontium’s only radiogenic isotope, 87Sr. The isotope 87 strontium forms by the radioactive decay of rubidium, which has a half-life of 48 billion years, more than three times longer than the age of the universe. So there must have been a lot of 87 rubidium to decay to produce elevated amounts of 87 strontium. Rubidium is an unfamiliar element in part because it rarely reacts to form specific minerals, but it is the 22nd most abundant element in the earth’s crust, more abundant than copper, zinc, lead, or gold.
The southern margin of the Archaean (more than 2,500 million years old) Greenland Craton (also known as part of the Nain or North Atlantic Province) has been repeatedly subject to tectonic activity.
The age of the igneous rocks hosting the deposit (1,250 million years) approximates the last stages of the break-up of the supercontinent of Columbia, which had been assembled about 2,100 to 1,750 million years ago. The assembly collision is called the Ketilidian Orogeny, whose earlier phases included island-arc collisions that helped generate copper, gold, and rare-earth mineralization in the Ivigtut region (Steenfelt and others, 2016, Metallogeny of South Greenland: A review of geological evolution, mineral occurrences and geochemical exploration data: Ore Geology Reviews 77, p. 194–245).
It was into the Ketilidian Orogen that the igneous rocks of the Gardar belt, including those at Ivigtut, were intruded 500 million years later, probably the result of pervasive rifting that is thought to have reached the lithospheric portion of the mantle (Upton and others, 2003, Magmatism of the mid-Proterozoic Gardar Province, South Greenland: chronology, petrogenesis and geological setting: Lithos, vol. 68, p. 43-65), or possibly even deeper, into the asthenosphere (80 to 200 km deep). That rifting probably led to the intrusion of the Ivigtut Granite mentioned above, as part of the breakup of the supercontinent Columbia, and a suggestion of how this might have happened is shown in the cartoon above.
The lithosphere includes the lower part of the earth’s crust as well as the solid, uppermost part of the mantle, about 60 to 80 kilometers below the surface, and presumably the deep faulting related to the rifting tapped unusual, differentiated magmas that were contaminated by their passage through younger rocks so that they became enriched in sodium, aluminum, and fluorine among other unusual elements. When they ascended into the shallower rocks as intrusive magmas, they solidified in the deposit at Ivigtut. Because the location is so unusual, it’s challenging to come up with simple explanations for it.
Cryolite has a refractive index (a measure of the way it transmits light) virtually identical to that of water, so colorless samples effectively become invisible in water. Cryolite is not really very photogenic – it just looks like milky quartz – so the photos here show some of the other obscure microminerals in my collection from Ivigtut. Pachnolite is hydrous sodium-calcium aluminum fluoride; prosopite is hydrous calcium-aluminum fluoride; hydrokenoralstonite is hydrous sodium-magnesium-aluminum fluoride; and thomsenolite is identical in chemistry to pachnolite but it has a different crystal structure, so it is a distinct mineral.
“Ivigtut” means “grassy place” in the West Greenlandic or Kalaallisut language. “Cryolite” is from Greek κρύος (kryos), cold, and λίθος (lithos), stone. Pachnolite is from Greek πάχνη, (pakhni), “frost.”