Putnisite Structure
Steven Dutch, Professor Emeritus, Natural and Applied Sciences,
No, not the love child of Vladimir Putin and Katniss Everdeen. It's a recently discovered mineral, so far rare, that is unusual for not being affiliated with any previously known chemical family of minerals. Its formula is SrCa4Cr3+8(CO3)8SO4(OH)16*25H2O, or pretty much what you'd get by dumping the contents of a chemistry set into a blender. The mineral was actually named for German mineralogists Andrew and Christine Putnis. I'd be thrilled to have something this unique and downright weird named for me.
This mineral is not here because it's abundant, quite the contrary, but because it's seemingly unrelated to anything else. Plus it has a very nifty structure. Viewed down the a-axis with the b-axis to the right.
Where to begin? Considering its complex formula, the structure is pleasingly regular and easy to picture. Putnisite's structure includes:
- Strontium ions (pink) in ten-fold coordination with oxygen. The coordination polyhedra are square anti-prisms capped with squere pyramids. In polyhedral terms, a gyroelongated square bipyramid (Johnson Solid J17).
- The strontium atoms are surrounded by a ring of eight chromium octahedra (green). We could nickname the Sr-Cr groups "saturn" groups.
- Each pair of chromium octahedra is also joined by a carbonate (CO3) group (gray).
- Calcium ions (purple) fit into the voids between the strontium-chromium "saturn" units and are linked to the carbonate groups and four chromium atoms. Pairs of Ca polyhedra share an apical atom. The coordination polyhedron is roughly an augmented square antiprism, with a square pyramid built outward on top of a square face. It's a pretty distorted version of that ideal polyhedron.
- Sulfate tetrahedra (yellow) that join layers
The symmetry is orthorhombic and pseudo-tetragonal.
Above: edge-on view. Atoms farther from the plane of the diagram are lighter. The light blue Ca units and the darker purple ones in the foreground are not linked. Viewed down c-axis, with b-axis to the right.
Above: edge-on view. Atoms farther from the plane of the diagram are lighter. Viewed down b-axis, with c-axis to the right. The slanting lines are carbonate groups viewed edge-on.
Showing Hydroxyl and Water Molecules
Above: viewed down a-axis, with b-axis to the right. Oxygen atoms are blue, hydrogen atoms are red.
Above: viewed down c-axis, with b-axis to the right.
Viewed down b-axis, with c-axis to the right. The slanting lines are carbonate groups viewed edge-on.
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Created 08 March, 2016, Last Update