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Larvikite |
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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. |
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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. |
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