BRAITHWAITEITE, NaCu5(TiSb)O2(AsO4)4[AsO3(OH)]2(H2O)8, A NEW MINERAL SPECIES FROM LAURANI, BOLIVIA

Abstract

Braithwaiteite, NaCu 5 (Ti 4+ Sb 5+ )O 2 (As 5+ O 4 ) 4 [As 5+ O 3 (OH)] 2 (H 2 O) 8 , is a new and very rare secondary mineral of copper, discovered in the supergene zone of an epithermal Cu–Au–Ag deposit at Laurani, near Sica Sica, Bolivia. This deposit is the type locality for lammerite, described as a new species in 1981. Braithwaiteite is closely associated with lammerite, lavendulan– lemanskiite and quartz. It formed by oxidation of Sb-bearing enargite in an arid environment. Traces of pyrite, covellite, anatase, albite–oligoclase, kaolinite and a mineral of the chlorite group are present as well. Electron-microprobe analysis gives Na 2 O 1.7, CuO 25.8, FeO 0.2, TiO 2 4.5, Sb 2 O 5 11.7, As 2 O 5 42.3, H 2 O (calc.) 10.17, for a total of 96.37 wt.%. The empirical formula based on the results of crystal-structure solution and refinement [34 anions and 2(OH) + 8H 2 O] is Na 0.87 Cu 2+ 5.17 (Ti 4+ 0.90 Sb 5+ 1.15 ) ∑2.05 O 2 (As 5+ 0.98 O 4 ) 4 [As 5+ 0.98 O 3 (OH)] 2 (H 2 O) 8 . The simplified formula is NaCu 5 (TiSb)O 2 (AsO 4 ) 4 [AsO 3 (OH)] 2 (H 2 O) 8 , which requires Na 2 O 2.04, CuO 26.13, TiO 2 5.25, Sb 2 O 5 10.63, As 2 O 5 45.30, H 2 O 10.65, total 100.00 wt.%. Infrared spectroscopy indicates the presence of H 2 O and OH, and As–O and As–OH stretching vibrations. Single-crystal X-ray studies show braithwaiteite to be triclinic, space group P 1, a 7.0308(4), b 9.8823(5), c 10.6754(6) A, α 106.973(1), β 104.274(1), γ 93.839(1)°, V 679.76(11) A 3 , Z = 1, and a:b:c = 0.7115:1:1.0803. The strongest seven X-ray powder-diffraction lines [ d in A(I)( hkl )] are: 9.825(100)(001), 5.887(50)(011), 4.635(30)(102), 3.354(30)(122), 3.232(30)(211), 2.947(60)(022), 2.736(30)(222). Two crystals of braithwaiteite were found, one on matrix and the other loose, each of them less than 1 mm in size, plus a few very minute fragments. The forms that could be determined with certainty include the following pinacoids: {001} dominant, {010} less prominent, {100}, {103} and {203}, all subordinate. The crystals are transparent and have a sky-blue color, a vitreous luster and a very pale blue streak. Braithwaiteite is brittle and has a perfect cleavage parallel to {001}. Optically, braithwaiteite is biaxial negative, 2 V meas 59(2), 2 V calc 65°. Indices of refraction are α 1.698(2), β 1.757(5), γ 1.783(5) at 589.3 nm. The mineral is weakly pleochroic with pale blue to greenish blue hues. The hardness (Mohs) is ~2. Using the empirical formula and the unit-cell parameters derived from the single-crystal study, the calculated density is 3.753 g cm −3 . Braithwaiteite is named after Dr. Richard S.W. Braithwaite (born 1930), prominent chemist and mineralogist at the University of Manchester, U.K. The mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA (2006–050).