This thin section in cross polarised light shows a crystal of andalusite, this particular variety being known as chiastolite. It is shown in cross section at right angles to the length of the prism and has a distinctive darker rectangular centre and a hint of darker lines going to each corner. The crystal, an aluminium silicate (Al2SiO5) has grown by replacing muscovite mica, biotite and quartz. The specimen was taken from Sinen Gill (cf. the garnet mica schist thin section) further away from the contact, i.e. in the andalusite slate zone. Incipient cordierite is seen to be forming at the top right and bottom left corners.