"Rock of the Month #138, posted for December 2012" ---

Eudialyte is a striking mineral, with rather exotic chemistry, found in a number of alkaline igneous complexes worldwide. The pieces shown here are from the Kipawa complex in Temiscamingue county, near the confluence of the Kipawa and Ottawa rivers. See a related article on agrellite and eudialyte for more information on the unusual geology of the Kipawa complex.

While eudialyte is the best-known name in its family of trigonal, sodic zirconium silicates, a family of related minerals are known, with variable contents of Ca, Fe, Mn, the REE (rare-earth elements), Sr, Nb, K, Y, Ti, H and W (Johnsen et al., 2003). The name eucolite was coined in 1847 for material from the famed Langesundsfjord district of southern Norway. This name persisted for more than a century, probably due to the convenient characteristic of being optically negative while eudialyte is optically positive (e.g., Deer et al., 1966, pp.59-60). Declared superfluous by 1978, the term persists in the literature at least to 2001.

The MINLIB bibliographic database has over 120 records, dating back to 1910, on eudialyte. Approximately one-third of these concern Greenland. This is due to the famed concentration of the mineral, along with many other species, in the peralkaline igneous rock of the Proterozoic-age Gardar province of southwest and south Greenland.

The Ilimaussaq peralkaline layered intrusion is surely the "star attraction" of the Gardar province. It was rated significant enough that a large number of papers have been published in various journals as "CTTMOI" (Contributions to the Mineralogy of Ilimaussaq). The 17x8 km intrusive complex has been dated at 1168±21 Ma (Bailey et al., 1981). The combination of rare agpaitic (alkaline) chemistry and the efficient action of magma chamber processes allowing the sinking or flotation of both common and rare mineral species led to the development of some of the most mineralogically and chemically extreme rocks on Earth, such as sodalite foyaite, naujaite, kakortokite and lujavrite, within which roughly 190 mineral species were soon recognized (ibid., p.10).

At the Kipawa Zr-Y-REE deposit, one-third of the REE (rare earth elements) are the rarer, heavy elements (HREE), and coarse, friable eudialyte is the major host phase. Metallurgical test work gave total average REE yields of almost 90 percent using a leach process (Saywell, 2012).

The typical environment for eudialyte family minerals is peralkaline (agpaitic) nepheline syenites, including the late, pegmatitic facies of these unusual rocks. Late-magmatic and hydrothermal examples are known. The mineral-chemistry is very variable (Steenfelt and Bohse, 1975; Johnsen and Gault, 1997).

Other locations worldwide include alkaline intrusions in east Greenland (Johnsen et al., 1998), Morocco (Schilling et al., 2009), northeast India (Chakrabarty et al., 2012) and the Northwest Territories and Quebec, Canada (Sheard et al., 2012); Johnsen et al., 2003).

References

Bailey,JC, Larsen,LM and Sorensen,H (1981) Introduction to the Ilimaussaq intrusion with a summary of the reported investigations. In `The Ilimaussaq intrusion, South Greenland', GGU Rep. 103, 130pp., 5-17.

Chakrabarty,A, Pruseth,KL and Sen,AK (2012) Compositions and petrogenetic significance of the eudialyte group minerals from Sushina Purulia, West Bengal. J.Geol.Soc.India 79, 449-459.

Deer,WA, Howie,RA and Zussman,J (1966) An Introduction to the Rock-Forming Minerals. Longmans, 528pp.

Johnsen,O and Gault,RA (1997) Chemical variation in eudialyte. NJMA 171, 215-237.

Johnsen,O, Grice,JD and Gault,RA (1998) Kentbrooksite from the Kangerdlugssuaq intrusion, east Greenland, a new Mn-REE-Nb-F end-member in a series within the eudialyte group: description and crystal structure. Eur.J.Mineral. 10, 207-219.

Johnsen,O, Ferraris,G, Gault,RA, Grice,JD, Kampf,AR and Pekov,IV (2003) The nomenclature of eudialyte-group minerals. Can.Mineral. 41, 785-794.

Saywell,T (2010) Matamec: metallurgy fine at Kipawa. Northern Miner 96 no.41, 11,13,15, 29 November.

Schilling,J, Marks,MAW, Wenzel,T and Markl,G (2009) Reconstruction of magmatic to subsolidus processes in an agpaitic system using eudialyte textures and composition: a case study from Tamazeght, Morocco. Can.Mineral. 47, 351-365.

Natural History Museum, London, England, 13 November 2012.

The systematic mineralogy exhibit in this wonderful institution has been a favourite of mine since my first decade of mineral appreciation! The eudialyte family is represented by seven pieces, all rather dark and brownish in hue. These included: a "eucolite" with molybdenite and biotite mica from Brevik in Norway; another with arfvedsonite from "Kangerluarssuk" in Greenland (presumably Kangerdluarssuk in the Ilimaussaq complex of the Gardar alkaline province, S. Greenland); a brown example, with sodalite and astrophyllite, from Desourdy's quarry, Mont St. Hilaire (Quebec, Canada); and massive reddish-brown material in nepheline syenite from the "Khibin Tundra" of Russia. At least in colour, these interesting samples are all quite dull compared to the vivid example in this Rock of the Month.