Crystal-chemical Implications of the Mg-2+-Fe-2+ Distribution In Orthopyroxenes

Structure reflnement data of natural and heated orthopyroxenes with different Fe/ (Fe + Mg) ratios were used for a systematic analysis of structural variations as a function of chemical composition and ordering degree. The mean T-O bond distances do not vary significantlyw ith increasingir on content and thereforea ny changeo f (TB-O) distancesm ay provide a measure of Si/Alrv substitution, which occurs only at the TB site. The mean Ml-O distancei ncreasesli nearly with iron content as a consequencoef a homogeneouse xpansiono f Ml site.T he mean M2-O distancea lso increasesw ith iron content,b ut the linear plot is the sum of very different trends of the individual bond lengths; among these, the M2-O3 distancese xhibit a peculiarb ehaviorc onsistentw ith a 4 + 2 coordination of the M2 polyhedron. The R3+ cations, which enter the structure with the Si/Altr substitution, appear to order totally in the Ml site.