Leakeite

From Norra Kärr

Leakeite was described as a new mineral in 1992 but following a major technical revamping of the nomenclature in 2012, it is now a group name for a series of amphiboles. I understand the rationale behind the chemistry-based names of these minerals that have complex continuous series between multiple end members, but you end up with crazy mineral names like ferro-ferri-fluoro-leakeite – a legitimate mineral name. Even a jargon-lover like me rolls his eyes at such things, and there’s a reason we usually just call them all “amphiboles”: you can’t really tell by looking that sodium is dominant in the “A” crystallographic position where sodium+potassium+calcium together are “greater than 0.5 atoms per formula unit.”

My specimen, the bluish blades in the photo, is almost certainly just fluoro-leakeite, a mineral described in 2009 as fluoro-aluminoleakeite but renamed in that 2012 update. It is from Norra Kärr, Sweden, which is designated as the type locality for fluoro-leakeite. It’s a complex sodium-magnesium-aluminum-lithium silicate with fluorine, NaNa₂(Mg₂Al₂Li)(Si₈O₂₂)F₂.

Norra Kärr, whose name means “north marsh,” is in southern Sweden about halfway between Stockholm and Göteborg. The rock in which the fluoro-leakeite occurs is agpaitic, meaning it is enriched in sodium and/or potassium and deficient in aluminum and silica, and typically enriched in fluorine and rare-earth elements. These are quite rare rocks worldwide, a variation on nepheline syenite called grännaite (or grennaite) which has phenocrysts of catapleiite and/or eudialyte, two sodium-zirconium-bearing minerals. The matrix in my specimen is probably mostly colorless sodium plagioclase (albite, anorthite), but the orange color likely comes from bits of catapleiite (not simple iron staining). The rock has a rough planar fabric as a consequence of metamorphism, so you could at least call it a somewhat gneissic texture, if not a full-fledged gneiss.

The Norra Kärr deposit, discovered in 2009, is one of the world’s largest occurrences of heavy rare earth elements (dysprosium, terbium, neodymium, and praseodymium), estimated by the mining operator (Leading Edge Minerals) to be adequate to supply all of Europe’s rare earth needs for at least 20 years. It also holds large reserves of zirconium, hafnium, yttrium, and niobium, all elements critical for modern high-tech applications.

Map from Saxon and others, 2015.

Saxon and others (2015, Geology, mineralogy, and metallurgical processing of the Norra Kärr heavy REE deposit, Sweden: Symposium on Strategic and Critical Materials Proceedings: Victoria, British Columbia) report that the agpaitic rock at Norra Kärr was emplaced about 1.49 billion years ago into the Trans-Scandinavian Igneous belt, a 1400-km-long zone of granitic rocks about 1.78 billion years old called the Växjö granite. Those rocks are younger than the primary underpinning of Sweden and Finland, formed by the Svecofennian Orogeny about 2.0 to 1.8 billion years ago and which established an important part of the core of the Baltica or East European Craton. The granitic rocks of the Trans-Scandinavian belt probably formed in some kind of subduction complex along the western flank of the Svecofennian craton, and most of them tend to be somewhat enriched in alkali elements (sodium and potassium).

The Norra Kärr agpaite might be a late-stage intrusion of material differentiated from those 1.78-billion-year-old granites and enriched in the rare earth elements, although 300 million years seems like a very long time for differentiation to be occurring. Alternatively, the complex might be related to rifting of the newly-formed craton (Åberg, 1988, Middle Proterozoic anorogenic magmatism in Sweden and worldwide: Lithos 21, 279–289). However it formed, it is further complicated by a metamorphic overprint at about 1.1 billion years ago, when equivalents to the Grenville Orogeny in North America affected what is now Scandinavia.

More information in Atanasova and others, 2017, Distinguishing Magmatic and Metamorphic Processes in Peralkaline Rocks of the Norra Kärr Complex (Southern Sweden) Using Textural and Compositional Variations of Clinopyroxene and Eudialyte-group Minerals: Journal of Petrology, Volume 58, Issue 2, February 2017, Pages 361–384.

“Agpaite” was named for occurrences of these kinds of rocks at Agpa, Greenland, whose name in turn is Greenlandic for the bird known in English as the thick-billed murre. Leakeite group minerals honor Scottish geologist Bernard Elgey Leake (1932- ), University of Glasgow, the chairman of the International Mineralogical Association subcommittee that revised amphibole nomenclature.