Tetrahedral coordination edge-sharing

The block iron silicate Si2 xFex Fe3ijFe 05(0H)4, cronstedtite, is representative of several mixed-valent minerals. Its structure consists of alternate layers of comer-shared tetrahedra and edge-shared octahedra. The tetrahedra Si " and Fe ions are coordinated by four oxide ions, the octahedral Fe " and Fe " ions share three common oxide ions with tetrahedral-site layers on one side and on the other are coordinated by three hydroxyl ions that hydrogen bond to the next tetrahedral-site layer. Mossbauer data at 35 K are able to resolve distinguishable Fe " and Fe " ions on the octahedral sites but in the range 300 < T < 500 K a rapid (t), < 10 s) charge transfer was found ... 

Coordination tetrahedra sharing faces would be present if all tetrahedral interstices in a hexagonal closest-packing were occupied. This would be an unfavorable arrangement for electrostatic reasons. On the other hand, the occupation of all tetrahedral interstices in cubic closest-packing results in an electrostatically favorable structure type the CaF2 type (F" ions in the interstices), which also is the structure of Ll O (Li" " in the Interstices). The tetrahedra are linked by common edges and common vertices. 

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