Nature of the coordination polyhedra around M 2 cations in pigeonite

✍ Scribed by Necip Güven

• Publisher: Springer
• Year: 1969
• Tongue: English
• Weight: 395 KB
• Volume: 24
• Category: Article
• ISSN: 0010-7999
• DOI: 10.1007/bf00376052

✦ Synopsis

An understanding of the irregular coordination polyhedra exhibited by the M2 cations in clinopyroxenes is of prime importance in connection with the behaviour of solid solutions and polymorphism in pyroxenes.

As shown by "unfolding" the silica chains in pigeonite, cations with octahedral stereochemistry like Mg ++ and Fe ++ cause "kinking" of the silica chains, whereas larger cations like Ca ++ and Na+ have an opposite namely "straightening" effect. This is a probable factor causing immiscibility in the Mg++--Ca ++ solid solutions in pyroxenes. In pyroxene structures with "kinked" silica chains the arrangement of oxygen atoms approaches "close packing"; consequently different stacking sequences will be energetically possible leading to polymorphism in pyroxencs.

When Pauling's electrostatic valence rule is applied to clinopyroxenes, the charge balance will be exceedingly improved if "effective" coordination number of M2 cation is assigned and if the distortions of M2 coordination polyhedron is considered.