Lead-tellurium oxysalts from Otto Mountain near Baker, California; X, Bairdite, Pb (sub 2) Cu (sub 4) (super 2+) Te (sub 2) (super 6+) O (sub 10) (OH) (sub 2) (SO (sub 4) )(H (sub 2) O); a new mineral with thick HCP layers

Bairdite, Pb (sub 2) Cu (sub 4) (super 2+) Te (sub 2) (super 6+) O (sub 10) (OH) (sub 2) (SO (sub 4) )(H (sub 2) O), 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 (sub 1) /c, with unit-cell dimensions a = 14.3126(10), b = 5.2267(3), c = 9.4878(5) Aa, beta = 106.815(7) degrees , V = 679.41(7) Aa (super 3) , and Z = 2. Bairdite occurs as diamond-shaped tabular crystals up to about 250 mu m long and 5 mu m 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 (super 3) . Bairdite is biaxial (+), with calculated indices of refraction of alpha = 1.953, beta = 1.966, and gamma = 2.039. The measured 2V is 47(2) degrees , dispersion is r < v, strong and the optical orientation is Y = b; Z ^ a = 34 degrees in obtuse angle beta . The pleochroism is strong: Z (pale green) <<< X (green) < Y (green). Electron microprobe analyses (average of 4) provided: PbO 34.22, CaO 0.06, CuO 23.80, TeO (sub 3) 26.34, SO (sub 3) 5.74, H (sub 2) O 2.81 (structure), total 92.97 wt%. The empirical formula (based on 17 O atoms pfu) is: Pb (sub 2.05) Ca (sub 0.01) Cu (super 2+) (sub 3.99) Te (super 6+) (sub 2.00) S (sub 0.96) O (sub 17.00) H (sub 4.16) . The eight strongest powder X-ray diffraction lines are [d (sub obs) in Aa (hkl) I]: 4.77 (110,102) 50, 4.522 (002,011,111) 66, 3.48 (multiple) 62, 2.999 (311,411) 97, 2.701 (502,113,213) 79, 2.614 (013,020) 100, 1.727 (multiple) 65, and 1.509 (911,033,324) 83. The crystal structure of bairdite (R (sub 1) = 0.072 for 1406 reflections with F (sub o) > 4sigma F) contains edge-sharing chains of Te (super 6+) O (sub 6) and Cu (super 2+) O (sub 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 (sub 10) polyhedra and half-occupied SO (sub 4) groups. Raman and infrared spectral data are presented.