The pyrochlore overgroup is a group of minerals whose composition corresponds to the formula A 2 B 2 X 7 or A 2 B 2 O 6 X, as well as O 6 X, as well as A 4 X 7 . The structure represents a three-dimensional framework of BX 6 octahedra (BO 6 ), atoms A occupy the voids of the framework.
Content
Nomenclature
| Pyrochlore group | Stibiconite group | Group of circelitis |
|---|---|---|
| Pyrochlor (Ca, Na, U, TR) 2-m (Nb, Ta, Ti) 2 O 6 (O, F) 1-n * nH2O | Stibiconite (Sb 3+ , Ca) 2-x Sb 2 5+ (O, OH) 6-7 * n H 2 O | Cirkelite CaZrTi 2 O 7 |
| Microlite (Ca, Na, U) 2-m (Nb, Ta, Ti) 2 O 6 (O, F) 1-n | Bindheimit (Pb, Ca) 2-x Sb 2 5+ 6-7 * n H 2 O | Polymignite (Ca, TR) (ZrTiNb) 2 O 6 |
| Betaphyt (Ca, U, TR) 2-m (Nb, Ta, Ti) 2 O 6 (O, F) 1-n * nH2O | Romaite (Ca, Na, Fe, Mn) 2 Sb 2 5+ O 6 (O, OH, F) | Kobeite Y (Ti, Zr, Fe, Nb) 3 O 7 |
| Westgrenite (Bi, Ca) (Ta, Nb) 2 O 6 (OH) | Monimolite (Pb Ca, Fe) 3 Sb 2 5+ O 8 | - |
| - | Hydromorheitis Ca 2-x Sb 2 5+ (O, OH) 6-7 * n H 2 O | - |
Crystal structure
The structure of pyrochlore is derived from the structure of fluorite : in a double-edge cell of fluorite composed of 32 cubes, half of the cubes have two opposite vertices; thus, half of the cubes are transformed into flattened octahedra in which atoms B are enclosed. In the ideal pyrochlore structure, each vertex of any polyhedron simultaneously belongs to three more, is common to two octahedra and two cubes in which atoms A are located. Atoms A are surrounded by six atoms oxygen and two X. The distance A — X is smaller (on average 0.224 nm ) than the distance A – O (on average 0.253 nm ). The presence of BO 6 octahedra connected by vertices brings the pyrochlore structure closer to the perovskite structure.
The structure of pyrochlore is characterized by the possibility of incomplete colonization of the skeleton BO 6 cavities by the A and X atoms, in connection with which there is often a shortage of A and X atoms (mainly A atoms), and the composition of minerals corresponds to the formula A <2 B 2 O 6 X <1 or A 2-m B 2 X 7-n .
Among the minerals, the structure of the pyrochlore type has titanium-tantalum-niobates, referred to the pyrochlore group, and some antimonates, combined into the group of stibiconite .
The structure derived from the fluorite- type structure also has a pseudo- cubic circlet and calcirtit .
Tolerance factors for compounds with pyrochlore structure (A 2 B 2 O 7 and A 1+ A 2+ B 2 O 6 X) are as follows:
where t is the possibility of placement between two atoms X atoms A or A 1+ and A 2+ ; t 1 and t 2 - the possibility of placing atoms A 1+ and, respectively, A 2+ between six oxygen atoms. In compounds with the pyrochlore structure, t is in the range of 0.94-1.16. [one]
Notes
- ↑ Chukhrov, 1967 , p. 126-128.
Literature
Chukhrov, FV, Bonstedt-Kupletskaya. E. M. Minerals. Directory. Issue 3. Complex oxides, titanates, niobates, tantalates, antimonates, hydroxides .. - Moscow: Nauka, 1967. - 676 p. |}