Question

The following is a chemical formula for a pyroxene:

(Ca,Mg, Fe^2₊, Al)₂(Si,Al)₂0₆

Which element(s) occupy the tetrahedral sites?

Which element(s) occupy the octahedral sites?

Answer 1

Si, Al - Tetrahedral sites

Ca, Mg, Fe2+, Al- Octahedral sites

Pyroxenes have a general structural formula of XYZ₂O₆ , where Z= Si⁴⁺ or Al³⁺ in tetrahedral coordination

X= Na^+, Ca²⁺ ,Mn²⁺, Mg²⁺ ,or Fe²⁺ filling octahedral sites called M2

Y= Mn²⁺, Mg²⁺, Al³⁺, Cr³⁺ or Ti⁴⁺ filling smaller octahedral sites called M1

Although Al is commonly present only in small amounts in the pyroxene structure, it can occur in both tetrahedral and octahedral coordination. High temperatures and relatively low pressures favour tetrahedral coordination for Al.

High pressures favour increased coordination number with negligible tetrahedral Al.