Same … but different. The structure of α-cristobalite

What does it look like?

(Left) photo from R. Weller/Cochise College. α-cristobalite located in a large vesicle in obsidian. 3.5 miles from Mono Lake, California. (Right) Crystallographic representation generated with Diamond.

What is it?

Do you remember the article on quartz and the crystal skull of Indiana Jones? Here is a polymorph, i.e. a material with the same chemical formula SiO₂ but a different structure. In this case this is the α-cristobalite crystal structure. It is commonly found in volcanic rock, where the crystals are microscopic and are found in the cavities of the host rocks.

The basic structural element of the α-cristobalite is the SiO₄ tetrahedron, the same as those observed in quartz. α-cristobalite consists of interconnected SiO₄ tetrahedra that build up a rigid three-dimensional network. There are many possible ways of connecting SiO₄ tetrahedra different from that found in quartz and α-cristobalite … we’ll meet another of these tomorrow!

Where did the structure come from?

The structure comes from a paper of J.J. Pluth, J.V. Smith and J. Faber Jr. published in the Journal of Applied Physics in 1985.

Tags: polymorph   silica   mineral   quartz