Syenite Pegmatite with abundant vishnevite: Allt a' Mhuillin Quarry NC289 096

Hand specimen

Plane polarised light

Cross polarised light
This specimen of syenite pegmatite comes from a suite of rocks known as ‘borolanites' (sic), named after Loch Borralan and the only plutonic complex composed largely of silica-undersaturated (i.e. feldspathoid-bearing) igneous rocks in the British Isles.They are cut by a set of undeformed pegmatite veins, consisting of syenite pegmatite, containing an assemblage of minerals unique in Britain, among them being alkali feldspar (note the large crystals as long as 5cm), the feldspathoids nepheline, and a blue, sulphatic cancrinite (vishnevite). The blue tinges at the base of the hand specimen are characteristic of the latter mineral. The "night sky" effect consists mainly of feldspathoids.
The Loch Borralan complex has been dated at 430Ma, Silurian age.

 Map showing locality of Allt a' Mhuillin Quarry (the yellow cross; 395 refers to the web editor's catalogue number).

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