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The specimen was taken from Sinen Gill at about NY302 282, 2.5 km NW
of Threlkeld, Lake District, UK. The location is near the contact
with the underlying Skiddaw Granite.
The thin sections show porphyroblasts of andalusite. A
& B are shown in PPL and XPL respectively in cross section
(at right angles to the length of the prism). Because it has a
distinctive darker rectangular centre and a hint of darker lines
going to each corner in a cruciform pattern this particular variety
is known as chiastolite. The crystal, an aluminium silicate (Al2SiO5)
has grown by replacing muscovite mica, biotite and quartz. In A
and B, cleavage planes are just recognisable cutting at right
angles to each other. Incipient cordierite in A & B
is seen to be forming at the top left and bottom right corners. |
C & D, PPL & XPL respectively, illustrate a
chiastolite cut parallel to the prism edge. The pale yellow is an
interference colour. The cleavage traces and dark inclusions running
parallel to each other are just discernible. Early forming cordierite
crystals are seen at bottom left and top right corners. Finegrained
quartz is visible, especially at the top right hand corner in photo D
The original mineral assemblage of the Skiddaw slates at Sinen Gill
has been altered through contact metamorphism due to the underlying
Skiddaw Granite. More details with regard to the contact metamorphism
of the Skiddaw Granite can be found in "Petrology of the
Metamorphic Rocks" by Roger Mason (obtainable via the internet).
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