The johannsenite-hedenbergite complete solid solution: clinopyroxenes from the Campiglia Marittima skarn
Capitani, Gian Carlo; Mellini, Marcello
European Journal of Mineralogy Volume 12 Number 6 (2000), p. 1215 - 1227
published: Nov 17, 2000
manuscript accepted: Jun 6, 2000
manuscript received: Apr 4, 2000
ArtNo. ESP147051206011, Price: 29.00 €
Abstract Clinopyroxenes from the Campiglia Marittima skarn range from hedenbergite to johannsenite (Jh7 to Jh100). Twenty single-crystals were selected for structural refinements and electron microprobe analyses. (100) twins were present and treated using a halved data set. End-member johannsenite is best approximated by the crystal Jh7GJ2, with a formula Na0.01Ca0.96Mn0.99Mg0.02 Fe0.01Al0.01Si2.00O6 , which was refined to Robs = 0.024. The structural change along the johannsenite-hedenbergite join is interpreted using the concept of the I-beam module. For instance, the variation of lattice parameters with composition is markedly anisotropic, being minimal along c (the I-beam direction), intermediate along a (perpendicular to the bases of adjacent I-beams) and maximal along b (parallel to the base of the I-beam). The replacement of manganese for iron expands and deforms the M1 polyhedron, and determines further coupling effects in the M2 and T polyhedra. These effects mostly consist of a more deformed M2 geometry, with longer M2-O bonds, and larger differences between the bridging and non-bridging T-O bond distances. Both the compositional and structural data indicate a complete solid solution between johannsenite and hedenbergite. All the different crystal structure parameters (lattice constants, bond distances, distortion parameters) vary continuously. Their trends suggest small values for the thermodynamic excess functions, implying limited deviations from the ideal solid solution along the johannsenite-hedenbergite join.