The new mineral challacolloite, KPb_2Cl_5, the natural occurrence of a technically known laser material

摘要

The new mineral challacolloite has the ideal chemical formula KPb_2Cl_5. The type locality of challacolloite is the Chal-lacollo silver mine SE of Iquique, Atacama desert, northern Chile. In the year of discovery challacolloite was also found on leu-cotephrite lava from the Vesuvius eruption of May 1855. It occurs closely associated with cotunnite as a hydrothermal phase (Chile) or as the product of fumarole activities (Italy). Challacolloite is colourless to white with an adamantine luster. Mohs's hardness is about 2-3. The calculated density is 4.77g/cm~3. Challacolloite is biaxial (+), 2V_(calc) = 67 deg with n alpha = 2.004(2), n beta = 2.010 (2) and n gamma = 2.024 (3). Chemical analyses of the type material give (wt. percent) K 5.45, Pb 66.30, Cl 28.69. Challacolloite is monoclinic with space group P2_1/c. Cell parameters refined from powder diffraction data of the type material are a = 8.864(8), b = 7.932 (8), c = 12.491 (11) (A), beta = 90.153 (5) deg, V = 878.2 (1) A~3 with Z = 4. Challacolloite is isotypic with NH_4Pb_2Cl_5 and PbU_2Se_5. Synthetic REE-doped equivalents of challacolloite are known as technical laser materials. The strongest reflections in the X-ray powder diffraction data of the type material are as follows [d_(meas).(A)(I, hkl)]: 3.686(100, 211), 3.609(49, 20-2), 2.669 (42, 22-2), 8.855 (39, 100) and 3.961 (31, 020). The mineral was named after its type locality.