Rare earth revelry three: Bastnäsite

Bastnäsite is the principal ore mineral for the rare earth elements. Most of the rare earth elements in the Earth are dissolved into silicate minerals such as garnet in very low concentrations. In the continental crust, however, they can occasionally be enriched in accessory mineral phases and phosphates. Bastnäsite is none of these. Rather, it is a rare earth fluorocarbonate, (REE)CO3F*. The economic deposits chiefly occur in carbonatites, which are igneous rocks where the melt is mostly molten carbonate instead of silicate material. While the source and petrogenesis for carbonatites is (or at least was when I lived in the mantle) hotly debated, then basics are that they represent very small percentage melts of deep mantle rocks that have been metasomatized (enriched in trace elements in processes that are not necessarily well understood).


Figure 1. REE compositional spaces for various mantle melt types. Source unfortunately blocked.

As I mentioned last week**, the smaller the degree of melting, the more the LREE are enriched. And for deep mantle melting, HREE are retained in the source, so long as garnet is not completely melted out. So carbonatites, like all trace mantle melts, have strong enrichment of the light rare earths over the heavy ones. Those which end up hosting ore deposits generally retain this pattern.

I suspect that the reason that bastnäsite deposits are economic is that they tend to have high REE concentrations, and that extraction of the REE into oxides by decarbonation is easier than from phosphates.

So, while it is true that nucleosythetic processes that created all the elements made more of the light rare earths than the heavy ones, (as mentioned in the chondrite abundance puzzle and followup) this is only a secondary reason as to why the heavy rare earth elements are much less abundant. The main reason is that the geologic processes that create REE deposits preferentially enrich the light rare earth elements by several orders of magnitude.

*Most REE deposits also contain co-existing REE phosphates (generally monazite), and in these days of tight REE markets that mineral is also processed.

** er, month?

Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3
Week 4