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7464855 
Journal Article 
Sodium at the a site in clinoamphiboles: The effects of composition on patterns of order 
Hawthorne, FC; Oberti, R; Sardone, N 
1996 
Yes 
Canadian Mineralogist
ISSN: 0008-4476
EISSN: 1499-1276 
34 
3 
577-593 
English 
WOS:A1996UY79100007 
The crystal structures of 12 amphiboles with Na at the A site have been refined to R indices of ∼1.5% using single-crystal MoKα X-ray data. Site populations were assigned from the refined site-scattering values and results of electron-microprobe analysis. Of primary 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. High-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 correlations among 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 was examined with difference-Fourier maps calculated with the A cations omitted from the structural model. The variation in electron-density distribution 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) or 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 sequence can be understood in terms of local bond-valence requirements within the amphibole structure. 
structure refinement; electron density; amphibole; short-range order; Na ordering