SHORT-RANGE ORDER IN AMPHIBOLES FROM THE BEAR LAKE DIGGINGS, ONTARIO

Abstract

The crystal structures of four amphiboles from the Bear Lake diggings, in the Bancroft area of Ontario, fluorotremolite, fluororichterite and two samples of potassian fluor-magnesiokatophorite, have been refined to R indices in the range 1.5–2.2% using ~1380 unique reflections measured with Mo K α X-radiation. The crystals used in the collection of the X-ray intensity data were subsequently analyzed by electron and ion microprobes, and formulae were calculated using the Fe3+ and Fe2+ contents established by crystal-structure refinement. Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. Difference-Fourier maps through the A (2/ m ) site show that the A ( m ) site is occupied and the A (2) site is vacant in all crystals. Occupancy of the A ( m ) site, and not the A (2) site, by Na imposes strong constraints on the possible patterns of short-range order in these amphiboles. The site populations of the A ( m ), M (4) and O(3) sites are in accord with the local arrangement A ( m )(Na + K)–O(3)F– M (4)Na, with lesser amounts of A ( m )(Na + K)–O(3)OH– M (4)Na and major to minor amounts of A □–O(3)OH– M (4)Ca. The coupling of elements to specific short-range-ordered arrangements in these crystals suggests that the overall chemical variability of amphiboles is strongly constrained by local bond-valence restraints in addition to overall electroneutrality.