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