PEROVSKITE

THE PEROVSKITE GROUP OF MINERALS

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Its most common mineral, perovskite, lends its name to this important group of oxides. The importance of this group is two fold. First, every member of this group has rare earth metals as trace elements in their structures and second, the structure of this group is unique and many ceramic, synthetic and useful substances can be created using the structure. The Perovskite Group is a group of oxides with a general formula of AXO3. The A can be either cerium, calcium, sodium, strontium, lead and/or various rare earth metals. The X site can be occupied by titanium, niobium and/or iron. All members of this group have the same basic structure.

The XO3 atoms form a framework of interconnected octahedrons. Each octahedron has at its center an X ion and at their corners are six oxygens which are in themselves a corner of yet another octahedron. In the mineral perovskite's case, each oxygen (-2 charge) is connected to two titanium ions which have a charge of (+4) and are in themselves connected to six oxygens for a ratio of 1/3. The result is an imbalance of a -2 charge (as in (+4) + 3(-2) = -2) which must be counter balanced with a calcium ion (+2) situated between eight TiO3 octahedrons. If the X ion is niobium (+5) or iron (+2 or +3) then the A ions must have a charge appropriate for balancing the formula.

This simplified ideal structure is isometric in symmetry. Members of this group would all be isometric if it were not for the fact that the octahedrons of most of the natural members of the group are twisted or rotated so as to kink or bend the structure. The twisting or bending is to accommodate the large ions between the octahedrons. The result is a variety of symmetries from isometric to tetragonal to orthorhombic to monoclinic depending on the degree of distortion to the basic ideal structure. Most perovskite minerals show some pseudocubic tendencies due to the close to, but not quite, isometric structure.

These are the members of the Perovskite Group with their simplified ideal formula and type locality:

Latrappite	Ca(Fe, Nb)O3	Oka, Quebec, Canada
Loparite	(Na, Ce)TiO3	Khibina, Kola Peninsula, Russia
Lueshite	NaNbO3	Lueshe, Democratic Republic of the Congo (former Zaire)
Macedonite	PbTiO3	Crni Kaman, Macedonia
Perovskite	CaTiO3	Slatoust district, Ural Mountains, Russia
Tausonite	SrTiO3	Murun complex, Russia

The mineral brownmillerite, which is not a perovskite group mineral, has a layered perovskite structure where a periodic break occurs in the framework structure. Natural samples of the above minerals often are different from the give formulae due to substitution. In fact several solid solution series exist between these species which mean they can substitute ions freely and are therefore somewhere in between their end-member formulas.

There are several synthetic crystals that have been made with the perovskite structure. These synthetics have a wide range of electrical ceramic uses from insulators right on through to superconducting material. Due to these uses and the increasing need for rare earth metals, the Perovskite Group will continue to be important and intensely studied.