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Kyanite Gneiss. The rock specimen was taken from a hillside
about 400m. ENE of Craigoshina, Glen Esk, Grampian Region, NO575766
by Brian Young. It consists mainly of kyanite and micas,
predominently muscovite.
The high relief and strong "railway line" cleavage of
kyanite is readily seen centre and towards the bottom right of the
specimen in plane polarised light. The green mineral with a radiating
structure, seen towards the right could be a type of mica or possibly
chlorite. For scale, the dark "triangle" seen towards the
right is 2mm. in length.
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The above photograph shows the bright interference colours of
muscovite, especially towards the left. The dark patche at the top
right is a hole in the thin section.
Kyanite is an aluminium silicate, which, like andalusite and
sillimanite, develops in aluminium rich pelites (metamorposed
mud/siltstones) under different conditions of temperature and pressure.
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