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Leverage analysis of X-ray single crystal diffraction data from orthopyroxene and pigeonite

¹ Dipartimento di Scienze della Terra, Università degli Studi di Pavia, via Ferrata 1, I–27100 Pavia, Italy
² CNR-Centro di Studio per la Cristallochimica e la Cristallografia, via Ferrata 1, I–27100, Pavia, Italy

^* E-mail: merli{at}crystal.unipv.it

Leverage analysis, a statistical procedure for calculating the influence of each reflection on the estimate of each refined variable, was applied to the structure refinement of a Pbca orthopyroxene (Wo₁En₄₈Fs₅₁) and that of a P2₁/c pigeonite (Wo₁₀En₄₇Fs₄₃). For both minerals this procedure was carried out on single-crystal X-ray diffraction data collected up to 55° and, for pigeonite, also on data collected in the range 2–28° during in situ annealing at 750 and 850°C. Leverage analysis revealed the role of certain groups of reflections in constraining some crucial variables of the structure refinement, such as atomic positions, atomic displacement parameters and site occupancies. The problem of which reflections to choose to achieve reliable results in terms of atomic coordinates and site population, and thus of Fe/Mg ordering degree, is discussed.

The treatment of data collected on pigeonite during in situ annealing to induce P2₁/c C2/c displacive phase transition is described in terms of leverage analysis. Some suggestions for correct data collection strategy and structure refinement are made.

Key-words: leverage analysis, pigeonite, orthopyroxene, order-disorder, phase transformation.

This article has been cited by other articles:

				M. C. Domeneghetti, M. Zema, and V. Tazzoli Kinetics of Fe2+-Mg order-disorder in P21/c pigeonite American Mineralogist, November 1, 2005; 90(11-12): 1816 - 1823. [Abstract] [Full Text] [PDF]

				M. MERLI and F. CAMARA Topological analysis of the electron density of the clinopyroxene structure by the maximum entropy method: an exploratory study European Journal of Mineralogy, October 1, 2003; 15(5): 903 - 911. [Abstract] [Full Text] [PDF]