DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Chiastolite section. Iridescent colours of a chiastolite section viewed in polarised light. The section has passed across one of the elongated crystal columns formed by this mineral. Chiastolite is a form of the aluminium silicate andalusite. Symmetrical arrangement of impurities such as carbon or clay produce the dark cross seen across the exposed surface. Chiasma is the Greek for cross. The crystalline structure causes the iridescence seen here. The crystal column is 4.5 centimetres wide.
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