Three new alluaudite-like protonated arsenates: NaMg3(AsO4)(AsO3OH)2, NaZn3(AsO4)(AsO3OH)2 and Na(Na0.6Zn0.4)Zn2(H0.6AsO4)(AsO3OH)2
Ðorðević, Tamara; Wittwer, Astrid; Krivovichev, Sergey V.
European Journal of Mineralogy Volume 27 Number 4 (2015), p. 559 - 573
published: Aug 1, 2015
Three new arsenates, NaMg3(AsO4)(AsO3OH)2 (1), NaZn3(AsO4)(AsO3OH)2 (2) and Na(Na0.6Zn0.4)Zn2(H0.6AsO4) (AsO3OH)2 (3), were synthesised under low-temperature hydrothermal conditions. 1 is the first example of a synthetic hydrous Na-Mg arsenate, and 2 and 3 are the first synthetic hydrous Na-Zn arsenates with octahedrally coordinated Zn atoms. They are all isostructural with the protonated members of the alluaudite-supergroup compounds; 2 is synthetic end-member of o'danielite. Their crystal structures were determined from the single-crystal X-ray diffraction data collected at 293 K [space group C2/c, a = 11.972(2)/12.023(2)/12.458(3), b = 12.367(3)/12.385(3)/12.484(3), c = 6.743(2)/6.750(2)/6.839(2) Å, β = 112.67(3)/112.79(3)/113.83(3)°, V = 921.3(4)/926.7(4)/973.0(4) Å3, Z = 4, for 1, 2, and 3 respectively]. The crystal structures consist of chains of edge-sharing M1O4(OH)2 and M2O6 octahedra (M1, M2 = Mg for 1, M1, M2 = Zn for 2, and M1 = Na, Zn, M2 = Zn for 3) stacked parallel to (101) and connected by the AsO4 tetrahedra. These chains form two types of channels parallel to the c-axis. While hydrogen atoms of the OH groups are situated in the channels of the first type, Na atoms are located in the second-type channels. Infrared and single-crystal Raman spectra were measured and evaluated in order to obtain further information on the anion groups and especially on the short hydrogen bonds. The OH stretching frequency is in good agreement with the observed O · · · O distances. Furthermore, the crystal chemistry of synthetic alluaudite-like arsenates is discussed in terms of structure distortion and measure of similarity among them.