Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
These are rare minerals. Their principal interest lies in showing the structure of minerals with three-membered rings of silica tetrahedra.
Benitoite, BaTiSi3O9, is a rare mineral. The name comes from San Benito County California, where the only important occurrence was discovered. As the formula suggests, the mineral contains three-membered rings of silica tetrahedra. Part of the unit cell is shown below.
|The rings are actually bisected by mirror planes but the tetrahedra are slightly skewed for visibility.
The next layer of rings is skewed in the opposite direction relative to the unit cell and is related to this layer by a vertical glide plane.
Gray represents a titanium atom, green is barium.
Below is a diagram of the lattice with two layers of rings. The titanium is octahedrally coordinated to the oxygen atoms, the barium also has six-fold coordination but the ring is much more open.
A ball model is shown below. Again, superposed oxygens have been skewed for visibility. Blue represents oxygen, red silicon, gray titanium and green barium.
The distortion necessary to maintain visibility cases the structure to be a bit more open that it is in reality. The barium ion is slightly smaller than an oxygen atom and is actually held in by six adjacent oxygens in a zigzag ring.
Benitoite is the only known mineral with symmetry 6*m2, reflecting the symmetry of its rings. Since vertically adjoining rings are actually mirror images of each other, the actual point group symmetry is P6*c2 (vertical glide planes).
Barium and titanium are not geochemically very much alike, and the barium ion is not held in place very tightly. Also the three-membered rings cause the anions to be jammed together pretty closely. The unusual chemistry makes benitoite rare to begin with, and the structure probably makes it not very stable.
Wadeite, K2ZrSi3O9, like benitoite, is a rare mineral. Like benitoite, it also includes two cations not exactly famous for being close drinking buddies. It is probably no accident that both contain large cations together with small, high charge cations. It probably takes a combination of exotic chemistry and wild mismatches of cation radius and charge to force silica into three membered rings.
Since K only has a +1 charge instead of the +2 of Ba, requiring twice as many large radius cations for charge balance, this mineral has a different structure than benitoite. It has hexagonal symmetry, space group P63m. In the diagrams below, purple represents potassium and dark blue zirconium.
Created 22 February, 2001, Last Update 31 May 2020