Carbonatite from the Auvergne

Carbonatite from the Auvergne
Carbonatite from the Auvergne

To call this sample my favourite is not quite correct, but it is certainly fairly unusual. It is a piece of carbonatite from the Auvergne.

Geologic info

Carbonatite is pretty rare, it is an igneous intrusive or extrusive rock which is found only at places where continental rifting is taking place, or had taken place, but where the rifting was interrupted.
This is case in the Auvergne, which together with the Rhine Valley and the Eger Valley, forms what is known as the European Cenozoic Rift System. The only region where carbonatite is being erupted today is in the East African Rift Valley.
It may at first sight look like a lump of chalk. This is due to its chemical  composition, which is defined as consisting of greater than 50% carbonate minerals, such as natrolite, sodalite and apatite. Most carbonatite deposits found today were originally intrusive. This is because lava flows react rapidly when they come into contact with moisture in the atmosphere.
The original, dark coloured lava starts to turn white within hours, leaving a soft endproduct, which tends to erode rapidly. Carbonatite is often confused with marble.

Where I found it
I saw my sample at the side of the path we were using in the Auvergne to reach the cone of a small volcano. I considered it slightly too large for my collection and ignored it initially.
On the way back, I noticed it again and picked it up. This was when I discovered the pyrite phenocryst, at the top corner of the sample. I was initially highly irritated by the fact that the face of the phenocryst is a pentagon, whereas pyrite usually occurs in a cubic form. However, it sometimes forms a so called ‘dodecahedron’.
A ‘dodecahedron’ is a 12-sided polyhedron, where the individual sides are pentagons. I think the phenocryst makes the sample even more unusual, otherwise it would not have survived to grace my rock collection, as I already had a simple sample, found during the very first excursion make by OUGS ME. That trip was to the ‘Kaiserstuhl’ in the Rhine Valley, following the inauguration of OUGS ME in Basel in 2000.

Mike Molloy

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