KAKORTOKITE
Kakortokite is a rare intrusive igneous rock. The name is traditionally
applied to eudialytic nepheline syenites found in southern Greenland. The
attractive sample shown below principally contains three minerals: whitish nepheline
((Na,K)AlSiO4), blackish arfvedsonite amphibole (NaNa2((Fe+2)4Fe+3)Si8O22(OH)2),
and reddish eudialyte (Na4(Ca,Ce)2(Fe,Mn,Y)ZrSi8O22(OH,Cl)2).
Notice that the amphibole and the eudialyte show
layering. Magmatic layering is expected to occur in the lower portions of
old magma chambers, and this kakortokite sample is indeed from the floor rocks
of an old igneous intrusion.
Geologic unit: Ilímaussaq Intrusion (Ilímaussaq Complex;
Ilímaussaq Alkaline Complex), an 8 x 17 km intrusion in the 1.12-1.35
b.y. Gardar Igneous Province, developed in a failed rift zone.
Age:
late Mesoproterozoic, 1.16 billion years.
Locality:
cliff outcrops on southern side of eastern end of Kangerdluarssaq Fjord (also
spelled Kangerdluarsuk Fjord), far-southern Greenland.
Kakortokite (cut surfaces; 9.5 cm tall) - magmatically layered kakortokite from
Greenland’s Precambrian-aged Ilímaussaq Complex.
White
= nepheline
Black
= arfvedsonite amphibole
Red =
eudialyte