Abstract

The new mineral reynoldsite, Pb2Mn4+2O5(CrO4), occurs at the Blue Bell claims, near Baker, San Bernardino County, California, U.S.A., and at the Red Lead mine, Dundas, Tasmania, Australia. At the Blue Bell claims, reynoldsite occurs in subparallel growths and divergent sprays of thin prisms with a square cross section. At the Red Lead mine, it occurs as thin rectangular blades. At both occurrences, crystals are small (≤0.2 mm), and ubiquitously and multiply twinned. At both deposits, reynoldsite formed as a secondary mineral derived from the weathering of primary minerals including oxides and sulfides in the presence of acidic groundwater. Reynoldsite is dark orange-brown to black in color and has a dark orange-brown streak. Its luster is subadamantine and its Mohs hardness is about 4½. The mineral is brittle with irregular to splintery fracture and a poorly developed {001} cleavage. The calculated density is 6.574 g/cm3 (Red Lead mine). The very high indices of refraction and dark color permitted only partial determination of the transmitted light optical properties. Electron microprobe analyses of Blue Bell and Red Lead reynoldsite provided the empirical formulas (based on nine O atoms): Pb1.97Mn2.01O5(Cr1.01O4) and (Pb2.07Sr0.04)∑2.11Mn2.15O5(Cr0.87O4), respectively. The strongest powder X-ray diffraction lines for Red Lead reynoldsite are [d(hkl)I]: 3.427(02̄1,110)52, 3.254(021,11̄2,12̄1)85, 3.052(1̄1̄2,111,02̄2,1̄03)100, 2.923(013,1̄22)40, 2.5015(004,2̄11,1̄30)47, 1.9818(01̄5,1̄05,202,23̄1)42, 1.7694(11̄5,13̄4,203,1̄42,1̄3̄3)36, and 1.6368(2̄2̄3,04̄3,221,124,22̄4)36. Reynoldsite is triclinic with space group P1̄ and unit-cell parameters: a = 5.0278(7), b = 7.5865(11), c = 10.2808(15) Å, α = 91.968(12), β = 99.405(12), γ = 109.159(10)°, V = 363.81(9) Å3, and Z = 2 (for a Red Lead mine crystal). The crystal structure of reynoldsite (R1 = 10.2% for 902 reflections with Fo > 4σF for a Red Lead crystal) contains close-packed layers of edge-sharing Mn4+O6 octahedra parallel to {001}. These layers are composed of edge-sharing double chains of octahedra extending along [100], which in turn are linked to one another by sharing edges in the [010] direction. The thick interlayer region contains Pb2+ cations and CrO4 tetrahedra. The 6s2 lone-electron pair of the Pb2+ is stereochemically active, resulting in a one-sided Pb-O coordination arrangement. The structure bears strong similarities to those of the phyllomanganates, such as chalcophanite and birnessite.

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