SIGISMUNDITE, ,K,PB)NA-3(CA,SR)(FE,MG,MN)(14)AL(OH)(2)(PO4)(12), A NEW BA-RICH MEMBER OF THE ARROJADITE GROUP FROM SPLUGA VALLEY, ITALY

The new mineral species sigismundite ,K,Pb)Na-3(Ca,Sr)(Fe,Mg,Mn)(14)Al (OH)(2)(PO4)(12), a Ba-rich member of the arrojadite family, occurs as greyish green lumps in phengitic quartzite at Madesimo, Spluga Valley , Italy, associated with quartz, apatite, and lazulite. Sigismundite i s monoclinic, space group C2/c, with a 16.406(5), b 9.945(3), c 24.470 (5) Angstrom, beta 105.73(2)degrees, D-x 3.544 g/cm(3), n((average)) 1 .65(1). The structure has been refined using 2846 observed reflections [1 > 3 sigma(I)] down to R = 0.066. This mineral differs from arrojad ite by the replacement of K by Ba, the presence of considerable conten t of Mg in place of transition metals, and vacancies in the Na site. T he strongest lines in the powder-diffraction pattern [d in Angstrom(l) (hkl)] are: 3.010(100)(42 42(4) over bar), 3.178(51)(206), 2.678(42)(2 26), 2.523(27)(60(6) over bar), 2.805(25)(31(8) over bar), 4.519(23)(1 14), 2.775(21)(134), 2.741(21)(13(5) over bar), and 2.732(21)(60(2) ov er bar).