Lead-tellurium oxysalts from Otto Mountain near Baker, California: X. Bairdite, Pb_2Cu_4^(2+)Te_2^(6+)O_(10)(OH)_2(SO_4)(H_2O), a new mineral with thick HCP layers

Bairdite, Pb_2Cu_4^(2+)Te_2^(6+)O_(10)(OH)_2(SO_4)(H_2O), is a new tellurate-sulfate from Otto Mountain near Baker, California, U.S.A. It occurs in vugs in quartz associated with khinite, cerussite, goethite, and hematite. It is interpreted as having formed from the partial oxidation of primary sulfides and tellurides during or following brecciation of quartz veins. Bairdite is monoclinic, space group P2_1/c, with unit-cell dimensions a = 14.3126(10), b = 5.2267(3), c = 9.4878(5) Å, β = 106.815(7)°, V = 679.41(7) Å^3, and Z = 2. Bairdite occurs as diamond-shaped tabular crystals up to about 250 μm long and 5 μm thick, in subparallel and fan-shaped aggregates. The color is lime green, the streak is pale lime green, and the luster is adamantine. The Mohs hardness is estimated at between 2 and 3. Bairdite is brittle with an irregular fracture and one perfect cleavage on {100}. The calculated density based on the empirical formula is 6.062 g/cm^3. Bairdite is biaxial (+), with calculated indices of refraction of α = 1.953, β = 1.966, and γ = 2.039. The measured 2V is 47(2)°, dispersion is r 4σF) contains edge-sharing chains of Te^(6+)O_6 and Cu^(2+)O_6 octahedra parallel to b that are joined by corner-sharing in the a direction, forming thick stair-step-like hexagonal close packed layers parallel to {100}. The polyhedral sheet has similarities to those in the structures of timroseite and paratimroseite. The thick interlayer region contains PbO_(10) polyhedra and half-occupied SO_4 groups. Raman and infrared spectral data are presented