SÖVITE (Carbonatite) from Loch Urigill, Elphin, Sutherland District (NC 2466 1048)

A (ppl) The orange-brown shape in the centre consists of a cluster of minute crystals of phlogopite, a silicate mineral belonging to the mica group and characteristic of this type of carbonatite known as SÖVITE.

C (xpl) Under crossed polars, the high order interference colours (birefringence), perfect rhombhedral cleavage can be seen as well as twinning in some crystals. 

B Hand specimen
The Loch Borralan intrusion, a Lower Palaeozoic plutonic complex in the SW corner of the Assynt region is of international importance for petrological reasons. Among several important exposures, the site includes the only British example of carbonatite, an igneous rock consisting almost entirely of carbonate as can be seen in thin section photo's A (ppl) and C (xpl).

The hand specimen was donated by Brian Young and the map indicates the locality where it was collected. 376 refers to the web editor's catalogue number.

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Carbonatites are rare, peculiar igneous rocks formed by unusual processes and from unusual source rocks. Three models of their formation exist:
1.direct generation by very low degree partial melts in the mantle and melt differentiation
2.liquid immiscibility between a carbonate melt and a silicate melt
3.peculiar, extreme crystal fractionation
Evidence for each process exists, but the key is that these are unusual phenomena. Historically, carbonatites were thought to form by melting of limestone or marble by intrusion of magma but geochemical and mineralogical data discount this.