The Canadian Mineralogist
Volume 34, pages 577-593 (1996)
SODIUM AT THE A SITE IN CLINOAMPHIBOLES: THE EFFECTS OF COMPOSITION
ON PATTERNS OF ORDER
FRANK C. HAWTHORNE*, ROBERTA OBERTI and NICOLA SARDONE
CNR - Centro di Studio per la Cristallochimica e la Cristallografia, via Abbiategrasso 209, I-27100 Pavia, Italy
* Permanent address: Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2
Abstract
The crystal structures of 12 amphiboles with Na at the A site have been refined to R indices of 1.5% using single-crystal MoK 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.