Titanite (Sphene) Structure

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The structure of titanite or sphene is not too complex but most mineralogy texts provide illustrations that are unclear and overly complicated.

The fundamental structure consists of octahedra with titanium atoms at the center and oxygens at each vertex. They share oxygens at opposite corners. Gaps between neighboring octahedra are bridged by silica tetrahedra so the octahedral chains zigzag.

For each octahedron there is a tetrahedron so the number of silicon and titanium atoms is equal.

The calcium atoms fill the gaps between the chains. The interstices between the chains are irregular and surrounded by seven oxygen atoms (gray). Five of them make a roughly coplanar cage and the remaining two are above and below the plane of the diagram. It's a cage with wide openings, but still effective.

The neighboring chains do not quite line up, so the mineral actually has monoclinic symmetry.

The calcium atoms are in purple with the shade indicating two different levels.

This diagram shows the titanium octahedra, silica tetrahedra, and calcium atoms.
If we represent the calcium surroundings in polyhedral form, the coordination polyhedron is an irregular pentagonal dipyramid. One calcium atom is shown along with the hidden edges of the polyhedron.
Here are two layers. The atoms above and below the pentagonal cage are shared with pairs of octahedra in the adjoining level.

 


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Created 22 Sept 1997, Last Update 22 Sept 1997

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