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 pretty awesome and colourful (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 melt needs to comprise high contents of sodium and halogens is essential(to be agpaitic, sodium contents must significantly exceed that of aluminium). This in turn requires a rather unique set of conditions, because normally, sodium and halogens are lost as soon as a fluid phase is exsolved from the magma. Only in certain geodynamic settings, such as continental rifts, magmas may be able to retain these fluids until their very last moments of crystallization. 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 occur as red to pink crystals, such as on the pictures below, but rare occurrences of orange and yellow varieties are also known.

Eudialyte group minerals were first discovered in 1819, from the layered Ilimaussaq complex where they occur in large quantities. They are now 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, on the other hand, has the unique property to lengthen the

The difficulty with these metals is that the supply chain is monopolized by single producing countries; most of the world’s heavy rare earths come from China, and most niobium comes from Brazil. Monopoly market positions introduce a significant risk factor, because the restricted supply of these metals could strangle global hi-tech industries. For this reason, governments around the world are working closely with geologists and exploration companies to seek and develop alternative resources.

Eudialyte-hosted REE deposits, such as the agpaitic rocks of the Ilímaussaq complex, form an attractive alternative and current work focusses on developing suitable extraction routes for the REE 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 crystallize, how they are replaced by secondary minerals when late-magmatic fluids percolate through the rocks after they crystallize, and how this would affect the ore potential.

109212_pseud_100_xpl1

Eudialyte crystals replaced by catapleiite – cross polarized light

In more recent work, I’m trying to work out where the REE sit inside the crystal structure of eudialyte, 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.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s