SODIUM AT THE A SITE IN CLINOAMPHIBOLES - THE EFFECTS OF COMPOSITION ON PATTERNS OF ORDER

The crystal structures of 12 amphiboles with Na at the A site have bee n refined to R indices of similar to 1.5% using single-crystal MoK alp ha X-ray data. Site populations were assigned from the refined site-sc attering values and results of electron-microprobe analysis. Of primar y interest is the variation in ordering of Na at the A site as a funct ion of composition and site populations in the rest of the structure. High-resolution data were collected with Ag radiation for one crystal but the results were not materially different from refinements with no rmal-resolution data collected with Mo radiation. High correlations am ong parameters and convergence at false minima prevented derivation of reliable partitioning of the A-site cations between the A(m) and A(2) sites; thus, the character of the electron density in the A cavity wa s examined with difference-Fourier maps calculated with the A cations omitted from the structural model. The variation in electron-density d istribution at the A site can be rationalized in terms of a number of preferred short-range-ordered configurations involving Na or Ca at the M(4) site, OH or F at the O(3) site, and Na at the A(m) of A(2) sites . The relevant arrangements and their relative stability are (M(4))Na- (O(3))F-(A(m))Na > (M(4))Ca-(O(3))OH-(A(2))Na > (M(4))Na-(O(3))OH-(A(m ))Na >> (M(4))Ca-(O(3))F-(A(m))Na = (M(4))Ca-(O(3))F-(A(2))Na; this se quence can be understood in terms of local bond-valence requirements w ithin the amphibole structure.