Eucrite (Bytownite Gabbro), Ardnamurchan.



Eucrite is an old name for gabbro containing feldspar with more than the usual proportion of calcium in its chemistry. The name was originally used for a type of meteroite of similar composition, and was later used for terrestrial rocks of the same type. The use of the name is now regarded as outdated in geology, but is so well known for its use to describe the world famous volcanic topography in Ardnamurchan, “The Great Eucrite”, that it will remain in use whatever the purists say. It describes the ring of crags that form a bowl containing the eroded magma chamber of the last volcano. Originally described in the 1930s as a ringdyke, recent work has suggested that it is the outer rim of a lopolith. This is an intrusion shaped like a golf tee.

The rock itself always contains feldspar, (technical name bytownite), which shows as black/grey/white in xpl view above, and glassy grey in hand sample, and pyroxene, bright colours in xpl, dark green to black in hand sample. The rock often, but not always contains olivine, also bright colours but with curved dark cracks and rounded shapes. Rusty spots on the weathered surface of hand samples are a good indication of olivine, the mineral is iron rich and the iron weathers out and ‘rusts’.

Achnaha in winter, and about as snowy as it ever gets here. Creag an Airgrid, one of the ring of crags in the Great Eucrite in the distance behind, this view gives a good idea of the vertical scale. This is also the source of the specimen that was cut to produce the slide.

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