Refinement of the crystal structure of arseniopleite: confirmation of its status as a valid species

Arseniopleite, ideally (Ca,Na) (Na,Pb2+) Mn2+ (Mn2+,Mg,Fe2+)2 As3 O12, is monoclinic, a 6.8113(6), b 13.0358(11), c 11.3245(10) A, β 100.25(3)o, V 989.5(1) A3, I2/a, Z = 2, has been refined to an R index of 2.3% for 1280 observed (|Fo| ≥ 4σF) reflections measured with a Bruker P4 diffractometer equipped with a CCD detector and MoKα X-radiation. The crystal used for the collection of the X-ray intensity data was subsequently analyzed with an electron microprobe. In arseniopleite, the M(1) site is octahedrally coordinated and is occupied completely by Mn2+ with an observed distance of 2.218 A. The M(2) site is also octahedrally coordinated and is occupied by Mn2+, Mg and Fe3+ with a distance of 2.129 A. The X(1) site is disordered off its ideal special position, locally decreasing its coordination number from [8] to [7]; X(1) is occupied by dominant Ca and less Na, with a distance of 2.584 A. The X(2) site is [8]-coordinated with a distance of 2.732 A, and is occupied predominantly by Na, with minor Pb2+ and Ba. The resulting unit formula is (Ca0.68 Na0.32) (Na0.93 Pb2+0.0+6 Ba0.01) Mn2+ (Mn2+1.21 Mg0.52 Fe3+0.27) (As2.98 P0.02) O12. Hence arseniopleite is confirmed as a valid mineral species distinct from caryinite. Arseniopleite has Mn2+ dominant at M(1), and caryinite has Ca dominant at M(1). The end-member compositions for these species are as follows: arseniopleite: Ca Na Mn2+ Mn2+2 As3 O12; caryinite: Ca Na Ca Mn2+2 As3 O12. (author)