Chiastolite (Andalusite)

                 A well exposed rock outcrop known as Chiastolite Slate, part of the Ordovician Skiddaw Group, can be seen in the Glenderaterra Valley which is located in the Northern area of the Lake District. The exposure, grid ref. NY299269, is by an unnamed stream next to the path leading from the Blencathra Centre, NY305255, accessible by road from Threlkeld. The outcrop displays the rather striking mineral chiastolite, a form of andalusite which contains cross-shaped inclusions of carbon.
                The Skiddaw Group consists of a succession of marine sediments varying in grain size from fine grained sandstones to mudstones as well as greywackes, all of which were laid down during Ordovician times about 480 - 470 Ma in the narrowing Iapetus Ocean which then separated "Scotland" from "England". As the ocean narrowed, the sediments underwent folding during end Silurian times. This led to the sediments undergoing low grade metamorphism.
                 Towards the end of the Caledonian Orogeny, about 394 Ma during Devonian times, the Skiddaw Granite was intruded into the slates. The resulting thermal as well as low grade metamorphism led to the formation of different minerals depending on the composition of the host rocks as well as their distance from the intrusion. Chiastolite (or andalusite) formed in the more argillaceous sediments. As seen in the photographs, the random orientation of the crystals indicates that their formation was due mainly to heat and not to pressure.

                The mineral andalusite, an aluminium silicate (Al2SiO5) gets its name from where it was first found, viz. Andalusia in Spain. The name chiastolite is derived from the Greek "chiastos" meaning "cross".

Red variety of Chiastolite Slate showing randomly orientated crystals. The cross sections show the typical characteristic black spot as part of the cruciform structure, e.g. on the extreme centre right.The cross sections are about 1.5mm square.

Grey variety of Chiastolite Slate again showing randomly orientated crystals. Paler grey roundish spots are early stage cordierite crystals which are more fully developed nearer the contact with the granite (inner hornfels zone).

Cross section of chiastolite photographed under a polarising microscope. The carbon inclusions can just be seen radiating towards the corners of the crystal forming the typical cruciform pattern.

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