Kôzulite, an Mn-rich alkali amphibole

The crystal structure of kôzulite, an Mn-rich alkali amphibole with the ideal formula NaNa2[Mn4 2+(Fe3+,Al)]Si8O22(OH)2, trisodium tetramanganese iron/aluminium octasilicate dihydroxide, was refined from a natural specimen with composition (K0.20Na0.80)(Na1.60Ca0.18Mn2+ 0.22)(Mn2+ 2.14Mn3+ 0.25Mg2.20Fe3+ 0.27Al0.14)(Si7.92Al0.06Ti0.02)O22[(OH)1.86F0.14]. The site occupancies determined from the refinements are M1 = 0.453 (1) Mn + 0.547 (1) Mg, M2 = 0.766 (1) Mn + 0.234 (1) Mg, and M3 = 0.257 (1) Mn + 0.743 (1) Mg, where Mn and Mg represent (Mn+Fe) and (Mg+Al), respectively. The average M—O bond lengths are 2.064 (1), 2.139 (1), and 2.060 (1) Å for the M1, M2, and M3 sites, respectively, indicating the preference of large Mn2+ for the M2 site. Four partially occupied amphibole A sites were revealed from the refinement, with A(m) = 0.101 (4) K, A(m)′ = 0.187 (14) Na, A(2) = 0.073 (6) Na, and A(1) = 0.056 (18) Na, in accord with the result derived from microprobe analysis (0.20 K + 0.80 Na), considering experimental uncertainties.


Crystal data
The site occupancies determined from the refinements are M1 = 0.453 (1)   Four partially occupied amphibole A sites [A(m), A(m)', A(2), and A(1)] were revealed from the structure refinements.
The refinement shows that K prefers the A(m) site, whereas Na is distributed among the other three sites. The refined A site occupancies are 0.208 K + 0.764Na [A(m) = 0.208 (4) K, A(m)' = 0.374 (14) Na, A(2) = 0.146 (6)Na, and A(1) = 0.224 (18) Na], consistent with the result derived from microprobe analysis (0.20 K + 0.80Na), considering experimental uncertainties. The presence of two distinct A sites on the mirror plane, A(m) and A(m)' has also been observed in many other alkali amphiboles (e.g., Hawthorne et al. 1996;Hawthorne and Harlow 2008).

Experimental
The kôzulite specimen used in this study is from the type locality Tanohata Mine, Iwate Prefecture, Tohoku Region, Honshu Island, Japan and is in the collection of the RRUFF project (deposition No. R070122; http://rruff.info). The crystal chemistry was determined with a CAMECA SX50 electron microprobe (http://rruff.info) on the same single-crystal used for the collection of X-ray intensity data. The av-

Refinement
The chemical analysis and crystal-chemical considerations show that the C-group cations consist of Mn 2+ , Mn 3+ , Fe 3+ , Mg, and Al 3+ . Because of similar X-ray scattering powers, Fe and Mn were grouped together (represented by the scattering factor for Mn) and Mg and Al together (represented by Mg) throughout the structure refinements. No refinement was made for the cations in the M4 site (= Na 0.80 Ca 0.09 Mn 2+ 0.11 ); they were assigned based on crystal-chemical considerations and previous studies on amphiboles (Hawthorne 1983). The total Mn and Mg in the M1 + M2 + M3 sites were fixed to those from the chemical analysis. To dampen the extreme correlations that would otherwise occur among the refined A-site variables, the isotropic displacement factors of these A sites were constrained to be equal (Hawthorne & Harlow 2008).

Figures
Fig . 1 Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (