Langhofite, Pb2(OH)[WO4(OH)], a new mineral from Långban, Sweden

Langhofite, ideally Pb2(OH)[WO4(OH)], is a new mineral from the Långban mine, Värmland, Sweden (IMA-CNMNC 2019-005). It occurs in a small vug in hematite-pyroxene skarn associated with calcite, baryte, fluorapatite, mimetite and minor sulfide minerals. Langhofite is triclinic, space group P–1, and unit cell-parameters a = 6.6154(1) Å, b = 7.0766(1) Å, c = 7.3296(1) Å, α = 118.175(2)º, β = 94.451(1)º, γ = 101.146(1)º and V = 291.06(1) Å3 for Z = 2. The 7 strongest Bragg peaks from powder X-ray diffractometry are [dobs Å (I)(hkl)]: 6.04(24)(010), 3.26(22)(11-2), 3.181(19)(200), 3.079(24)(1-12), 3.016(100)(020), 2.054(20)(3-11) and 2.050(18)(13-2). Langhofite occurs as euhedral crystals up to 4 mm, elongated along the a-axis, with lengthwise striation. Mohs hardness is ca. 2½, based on VHN25 data obtained in the range 130˗192. The mineral is brittle, with perfect {010} and {100} cleavages. The calculated density based on the ideal formula is 7.95(1) g·cm-3. Langhofite is colourless to white (non-pleochroic) and transparent, with a white streak and adamantine luster. Reflectance curves show normal dispersion, with maximum values 15.7˗13.4% within 400-700 nm. Electron microprobe analyses yield only the metals Pb and W above the detection level. The presence of OH-groups is demonstrated with vibration spectroscopy, from band maxima present at about 3,470 and 3,330 cm-1. A distinct Raman peak at ca. 862 cm-1 is related to symmetric W-oxygen stretching vibrations. The crystal structure is novel and was refined to R = 1.6 %. It contains [W2O8(OH)2]6- edge-sharing dimers (with highly distorted WO6-octahedra) forming chains along [101] with [(OH)2Pb4]6+ dimers formed by (OH)Pb3 triangles. Chains configure (010) layers linked along [010] by long and weak Pb-O bonds, thus explaining the observed perfect cleavage on {010}. The mineral is named for curator Jörgen Langhof (b. 1965), who collected the discovery sample.