Structural systematics of Ge substitution in primitive pyroxenes

database, nuove specie, discrediti,
ridefinizioni, classificazioni, ecc.
Avatar utente
Marco E. Ciriotti
Messaggi: 24283
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy

Structural systematics of Ge substitution in primitive pyroxenes

Messaggio da Marco E. Ciriotti » sab 02 gen, 2016 19:06

Prossima pubblicazione.

▪ Welch, M.D. & Pawley, A.R. (2016): Structural systematics of Ge substitution in primitive pyroxenes. Physics and Chemistry of Minerals, 43, (in press).

Pyroxenes of general stoichiometry Mg(Ge x Si1−x )O3 were encountered in attempts to synthesise Ge-substituted talcs at 0.2 GPa, 650–700 °C. Orthopyroxenes (Pbca) of compositions x = 0.21, 0.30, and 0.34 were identified, and also a P21/c clinopyroxene of composition x = 0.63, and C2/c clinopyroxenes of compositions x = 0.91 and 1. End-member clinoenstatite MgSiO3-P21/c synthesised at 16 GPa, 1300 °C and transformed from C2/c was also included in the study. Crystal structure refinements using single-crystal XRD data showed that unit-cell parameters vary linearly with Si–Ge for the Pbca and P21/c pyroxenes, both of which have two symmetrically non-equivalent tetrahedral chains. Refinement of Si–Ge occupancies at tetrahedral sites showed that the two chains of all primitive pyroxenes have very different compositions, with XGe(TB) ≫ XGe(TA). This difference arises from the greater flexibility of the B-chain to rotate in response to tetrahedral expansion due to increasing Ge content. The TA-M2 shared polyhedral edge imposes significant constraints on the flexibility of the A-chain, which can accommodate much less Ge than the B-chain. Linear trends of cell parameters, site occupancies, and structural parameters for the primitive pyroxenes, when extrapolated to published data for MgGeO3–Pbca, extend across the entire Si–Ge join.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»


Chi c’è in linea

Visitano il forum: Nessuno e 4 ospiti