Carbonatite
An igneous rock made of carbonate minerals rather than silicates — geologically improbable, and the source of most of the world's rare earths and much of its niobium.
A carbonatite is an igneous rock composed principally of carbonate minerals. That sentence was rejected for most of the twentieth century, because a rock of calcite that looks like marble and occurs in intrusive bodies was far more easily explained as limestone that had been caught up and recrystallised. The argument was settled by an eruption: Ol Doinyo Lengai in Tanzania erupts carbonatite lava, and has been observed doing so.
They are rare — a few hundred bodies are known worldwide — and economically far out of proportion to that rarity. Carbonatite is where the light rare earths are, and where most niobium is.
How it forms
Carbonatite magma forms by very small degrees of partial melting of carbonate-bearing mantle, or by separating out of an alkaline silicate magma as an immiscible liquid — the two do not mix, in the way oil and water do not. Either route produces a melt with an unusual property: it is a very effective solvent for elements that silicate magmas reject.
The rare earths, niobium, phosphorus, strontium and barium are all incompatible in ordinary rock-forming minerals and concentrate in whatever phase will accept them. A carbonate melt accepts them readily, so a carbonatite can arrive at the surface already enriched in rare earths by two or three orders of magnitude over average crust before any weathering or later fluid has done anything to help.
Many carbonatite bodies are then weathered into a residual cap, and because the carbonate dissolves while the rare earth minerals do not, that cap can be several times richer again than the rock beneath it.
Economic significance
Bayan Obo in Inner Mongolia and Mountain Pass in California are both carbonatite-hosted, and between them they have supplied most of the world's light rare earths for decades. Niobium is more concentrated still: a single carbonatite mine at Araxá in Brazil accounts for the majority of world supply, which is about as narrow a supply base as any industrial metal has.
The recurring difficulty is not the rare earths but what accompanies them. Carbonatite rare earth minerals carry thorium, and separating a saleable product from a radioactive residue is the step that has closed more rare earth operations than any question of grade.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Calcite — mineral · 40–90% · the dominant mineral of a calcite carbonatite, and the reason the rock was mistaken for marble for so long
- Bastnäsite — mineral · the rare earth fluorocarbonate, and the mineral most of the world's light rare earths are actually extracted from
- Monazite — mineral · the rare earth phosphate, carrying thorium with it — which is the difficulty rather than the resource
- Apatite — mineral · abundant enough in some bodies to be mined for phosphate in its own right
- Magnetite — mineral · a common accessory, and at Bayan Obo the reason the deposit was originally worked as an iron mine
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing