Crystal-chemistry of natural and heated aluminous orthopyroxenes

X-ray single crystal diffraction data of natural and heated Al-rich orthopyroxenes were used to study the cation ordering and the geometrical changes induced by Si+R ²⁺=Al^IV+R ³⁺ substitution. The calculated site populations and the observed bond distances in tetrahedral and octahedral sites suggest a total ordering of Al^IV in the TB tetrahedron and of R ³⁺ cations in the M1 octahedron, even in samples heated at 1000° C. The mismatch between the tetrahedral and octahedral layers along the c axis seems to play a crucial role in determining the limits of Si/Al^IV substitution.