SITE POPULATIONS IN MINERALS: TERMINOLOGY AND PRESENTATION OF RESULTS OF CRYSTAL-STRUCTURE REFINEMENTS

FRANK C. HAWTHORNE*, LUCIANO UNGARETTI and ROBERTA OBERTI
CNR - Centro di Studio per la Cristallochimica e la Cristallografia (CSCC), via Abbiategrasso 209, I-27100 Pavia, Italy
* Permanent address: Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2.

Abstract

Definitions are proposed for the terms site, structural formula, site occupancy, site-scattering value, and site population. The results of site-scattering refinement should be presented in a chemically independent manner in terms of effective epfu (electrons per formula unit) for X-ray structure- refinement and in terms of aggregate scattering-length for neutron structure-refinement. With the terminology proposed here, there is a direct correspondence between the refined site- scattering values, site populations, the structural formula and the formula unit of the mineral. Hopefully, this will make the results of structure refinements more clear to other branches of the Earth Sciences.

Keywords: structure refinement, site, site occupancy, site population, site-scattering value, structural formula, formula unit.