Sodium at the A site in clinoamphiboles: The effects of composition on patterns of order

The crystal structues of 12 amphiboles with Na at the A site have been refined to R indices of -1.57o using single-crystal MoKo X-ray data. Site populationsw ere assignedf rom the refined site-scatleringv alues and results of electron-microprobe analysis. Of pdmar/ interest is the variation in ordering of Na at the A site as a function of composition and site populations in the rest of the structure. Higb-resolution data were collected with Ag radiation for one crystal, but the results were not materially different from refinements with normal-resolution data collected with Mo radiation. High conelations among parametersa nd convergencea t false minima preventedd erivationo f reliable partitioning of the A-site cationsb efweent he A(rra) andA (2) sites;t hus, tlle charactero f the electrond ensityi n the A cavity was examinedw ith difference-Fourierm apsc alculated with the A cations omitted from the structural model. The variation in electron-density distnbution at the A site can be rationalize<li n terms of a number of prefened short-range-orderedc onfigurations involving Na or Ca at the M(4) site, OH or F at the O(3) site, and Na at the A(m) or A(2) sites. The relevant anangements and their relative strbility are M(a)11*o(3)yi-A(a)N>a ir(a)Ca_o(3)OH_ia(2)>N aM (a la_o(3)OH-a(rNa >> M(qCe-.o(3)F-A(n)N=a l/(4)6u-o(:)Fj(z)Na. this sequencec an be understoodin terms of local bond-valencer equirementsw ithin the amphiboles tructure.