A small andychristyite crystal
Photo credit: Tony Kampf.
Anthony R. Kampf
Mark A. Cooper
Department of Geological Sciences, University of Manitoba,
Winnipeg, Manitoba, R3T 2N2, Canada
Stuart J. Mills
Geosciences, Museum Victoria, GPO Box 666, Melbourne 3001, Victoria,
Australia
Robert M. Housley, and George R.
Rossman
Division
of Geological and Planetary Sciences, California Institute of
Technology
Pasadena,
California 91125, USA
ABSTRACT
Andychristyite, PbCu2+Te6+O5(H2O), is a new tellurate mineral from Otto Mountain near Baker, California, U.S.A. It occurs in vugs in quartz in association with timroseite. It is interpreted as having formed from the partial oxidation of primary sulfides and tellurides during or following brecciation of quartz veins. Andychristyite is triclinic, space group P-1, with unit cell dimensions a = 5.322(3), b = 7.098(4), c = 7.511(4) Å, α = 83.486(7), β = 76.279(5), γ = 70.742(5)°, V = 260.0(2) Å3, and Z = 2. It forms as small tabular crystals up to about 50 μm across, in sub-parallel aggregates. The color is bluish green and the streak is very pale bluish green. Crystals are transparent with adamantine luster. The Mohs hardness is estimated at between 2 and 3. Andychristyite is brittle with an irregular fracture and one perfect cleavage on {001}. The calculated density based on the empirical formula is 6.304 g/cm3. The mineral is optically biaxial, with large 2V, strong dispersion, and moderate very pale blue green to medium blue green pleochroism. The electron microprobe analyses (average of 5) provided: PbO 43.21, CuO 15.38, TeO3 35.29, H2O 3.49 (structure), total 97.37 wt%. The empirical formula (based on 6 O atoms pfu) is: Pb0.98Cu2+0.98Te6+1.02O6H1.96. The Raman spectrum exhibits prominent features consistent with the mineral being a tellurate, as well as an OH stretching feature confirming a hydrous component. The eight strongest powder X-ray diffraction lines are [dobs in Å(I)(hkl)]: 6.71(16)(010), 4.76(17)(110), 3.274(100)(120,102,012), 2.641(27)(-102,211,-1-12), 2.434(23)(multiple), 1.6736(17)(multiple), 1.5882(21)(multiple) and 1.5133(15)(multiple). The crystal structure of andychristyite (R1 = 0.0165 for 1511 reflections with Fo > 4σF) consists of stair-step-like octahedral layers of Te6+O6 and Cu2+O6 octahedra parallel to {001}, which are linked in the [001] direction by bonds to interlayer Pb atoms. The structures of eckhardite, bairdite, timroseite, and paratimroseite also contain stair-step-like HCP polyhedral layers.
last revised 10-Sep-2015