Above left: thin section (plane polarised light) showing a mafic clot consisting of titanaugite, possible olivine, apatite, iron ore associated with an “outgrowth” of lepidomelane (a variety of biotite), all surrounded by feldspars.

Above right: the same photo as above left (cross polarised light). Green interference colour is titanaugite, pink colour,– possible olivine, at least two blebs of apatite near centre, showing grey interference colours, opaque iron ore also near centre has outgrowths of lepidomelane, a variety of biotite.

The above two pictures are scanned (ppl & xpl) thin section photos of  Larvikite. It is classed as a syenite, i.e. consisting of not more than 20% quartz and dominant alkali feldspars (at least 65%) It is a Norwegian rock widely used for ornamental purposes. It is coarse grained and has distinctive feldspars said to consist of an intergrowth of orthoclase and oligoclase. On polished surfaces the feldspars display a blue schillerisation, no doubt due to the cryptoperthitic intergrowth of the two feldspars. The mafic (dark) minerals which form clots consist of titanaugite, olivine, apatite (often euhedral) and iron ore from which radiate crystals of lepidomelane. Cf. thin section photo’s above which are of the brightly coloured minerals seen near centre of the scanned photo.
Incidentally, the rock specimen was picked up from the Larvikite armoured sea wall leading to Skinningrove Jetty (GR: NZ712202). Skinningrove is an erstwhile fishing and ironstone mining village on the North Yorkshire Coast.
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