Eudialyte and rare earths

[Photo credit: Ole Johnsen]

Much of my research focuses on a rare breed of magmatic rocks known as agpaitic nepheline syenites. These rocks are fascinating to geologists, mineralogists and geeks alike, for many reasons. To me, the most obvious reason is their incredible mineralogical diversity. Hundreds of minerals were first discovered from these rocks; sodalite, aegirine, arfvedsonite, eudialyte are only a few examples. Famous agpaitic complexes that are record holders in terms of mineral discoveries, are Mont St Hilaire in Canada, Lovozero and Khibina in Russia, and Ilimaussaq in Greenland. And when I say mineralogically diverse, I mean that if you pick up any rock from these locations, chances are it contains a mineral that has not yet been named or identified. Also, they’re very pretty and colourful (especially in UV light).

Another reason why we are interested in these strange rocks is that agpaitic rocks present one of very few examples where magmatic enrichment processes alone led to the formation of potentially economic ore deposits of elements like Zr, Nb, Ta, Hf, and Rare Earth Elements. For a magma to become so enriched these elements, the magma needs to fractionate extensively and become highly enriched sodium and halogens. This requires a rather unique conditions, because normally, sodium and halogens are lost as soon as a fluid phase exsolves from the magma. Only in certain geodynamic settings, such as in continental rifts, magmas may be so reduced that volatiles remain dissolved in the melt until the very last stages. This subsequently increases the solubility of incompatible elements and significantly lowers the crystallisation temperatures. This is partly why agpaitic rocks are relative rare in the rock record.

By far the most characteristic mineral of agpaitic rocks is eudialyte. These are complex sodium-rich zirconosilicates with calcium, iron, manganese, niobium, and, importantly, rare earth elements. Where most igneous rocks contain zircon or ilmenite as the main hosts for zirconium and titanium, agpaitic rocks contain complex sodium and halogen bearing minerals like eudialyte, rinkite (Na-REE-Ti-silicate) and aenigmatite (Na-Fe-Ti-silicate). Eudialyte most commonly occurs as red to pink crystals, such as on the pictures below, but orange and yellow varieties are also known.

Eudialyte group minerals were first discovered in 1819, in the layered Ilimaussaq complex where they occur in large quantities. The mineral has now been reported from over 100 localities world-wide. Because they occur in such large quantities, eudialyte group minerals are considered as potential alternative resources for a range of technology metals, notably the heavy rare earths and niobium. These metals are indispensable in modern-day technologies, ranging from the permanent magnets used in tiny smart phone speakers, car engines or big wind-turbines [made of NdFeB], to the rare-earth phosphors giving colour to the screens of our electronic devices [using Ce, Eu, Gd, Tb and Y]. Niobium has important anticorrosive properties and significantly enhances the strength, flexibility and general lifetime of steel.

The problem with rare earth elements as well as niobium is that their supply chain is controlled by single producing countries; most of the world’s heavy rare earths come from China, and most niobium comes from Brazil. Needless to say, monopoly market positions are risky because the restricted supply of these metals could strangle global hi-tech industries. For this reason, alarmed governments around the world are working closely with geologists and exploration companies to seek and develop alternative resources and to safe-guard the supply chains.

Eudialyte-hosted REE deposits, such as in the Ilímaussaq complex, form an attractive alternative and current work focusses on developing cheap and environmentally friendly ways to extract the metals from these minerals.

My research specifically aims to understand how zoned crystals of eudialyte can be used to trace the evolution of the highly evolved magmas from which they crystallized, how they are replaced by secondary minerals when late-magmatic fluids percolate through the rocks after they crystallized, and how this affects the ore potential.

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Eudialyte crystals replaced by catapleiite – cross polarized light

In more recent work, I’ve worked out where inside the complex crystal structure of eudialyte the REE’s are hiding, using synchrotron-based techniques such as XANES and EXAFS. These techniques allow me to probe the type, number and distance of atoms surrounding the REE atoms inside the crystal lattice.

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