Zinnwaldite: Octahedral Ordering in Lithium-Iron Micas

Ordering of the octahedral sheet of some lithium-iron micas is proposed on the basis of chemical compositions and crystallographic properties. The model of the ordered sheet has large and small sites in the ratio 2 : 1 and explains all observed properties—the small layer thickness, the "dioctahedral reflections," and the n(120 deg) motif in polytypism.