Strontiomelane, SrMn4+6Mn3+2O16, a new mineral species of the cryptomelane group from St. Marcel Praborna, Aosta Valley, Italy

Strontiomelane is a new mineral species from the Praborna mine near Saint-Marcel, Aosta Valley, Italian Alps. It occurs as small anhedral inclusions (about 100-200 mu m) and veinlets associated with braunite, cryptomelane, hollandite, quartz, fluorapatite, titanite, jadeite and strontiopiemontite. The new species is opaque. black with a submetallic luster and a strong grey-white pleochroism: in plane-polarized light in oil immersion, it presents a medium anisotropy with grey-blue ro white tints. Strontiomelane, whose ideal formula is SrMn64+Mn23+O16, crystallizes in space group P2(1)/n (14) with a 10.00(1), b 5.758(7), c 9.88(1) Angstrom, beta 90.64(6)degrees and Z = 2. The strongest lines in the X-ray powder pattern [d in Angstrom(I)(hkl)] are: 3.15(100)(103), 3.13(80)(103), 2.409(80)(122), 2.170(60)(320) and 1.556(50)(225). The calculated density is 4.66(1)g.cm(-3). Strontiomelane is the Sr end-member of the cryptomelane group. The proposed name is derived from the chemical element strontium and the color black, melas,- anos in Greek.