Lead–tellurium oxysalts from Otto Mountain near Baker, California: 

X. Bairdite, Pb2Cu2+4Te6+2O10(OH)2(SO4)ĚH2O, a new mineral with thick HCP layers


Anthony R. Kampf
Mineral Sciences Department, Natural History Museum of Los Angeles County
900 Exposition Blvd., Los Angeles, CA 90007, U.S.A.
Stuart J. Mills
Geosciences, Museum Victoria, GPO Box 666, Melbourne 3001, Victoria, Australia
Robert M. Housley, George R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA 91125, U.S.A
Joseph Marty
5199 E. Silver Oak Road, Salt Lake City, UT 84108, U.S.A.
Brent Thorne
3898 S. Newport Circle, Bountiful, UT 84010, U.S.A.


Abstract

Bairdite, Pb2Cu2+4Te6+2O10(OH)2(SO4)ĚH2O, is a new tellurate 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 P21/c, with unit cell dimensions a = 14.3126(10), b = 5.2267(3), c = 9.4878(5) ┼, β = 106.815(7)░, V = 679.41(7) ┼3, and Z = 2. Bairdite occurs as diamond-shaped tabular crystals up to about 250 Ám long and 5 Ám 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/cm3. 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 < v, strong and the optical orientation is Y = b; Z ^ a = 34░ in obtuse angle β. 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, TeO3 26.34, SO3 5.74, H2O 2.81 (structure), total 92.97 wt%. The empirical formula (based on 17 O atoms pfu) is: Pb2.05Ca0.01Cu2+3.99Te6+2.00S0.96O17.00H4.16. The eight strongest powder X-ray diffraction lines are [dobs in ┼ (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 (R1 = 0.072 for 1406 reflections with Fo > 4sF) contains edge-sharing chains Te6+O6 and Cu2+O6 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 10-coordinated Pb and SO4. Raman and infrared spectral data are presented.


Bairdite crystal
Bairdite crystal (100) surface