Langhofite, Pb2(OH)[WO4(OH)], a new mineral from Långban, Sweden. (24th June 2020) - Holtstam, Dan
Abstract: Langhofite, ideally Pb2 (OH)[WO4 (OH)], is a new mineral from the Långban mine, Värmland, Sweden. The mineral and its name were approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA2019-005). 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 $\bar{1}$, and unit-cell parameters a = 6.6154(1) Å, b = 7.0766(1) Å, c = 7.3296(1) Å, α = 118.175(2)°, β = 94.451(1)°, γ = 101.146(1)° and V = 291.06(1) Å 3 for Z = 2. The seven strongest Bragg peaks from powder X-ray diffractometry are [ d obs, Å ( I )( hkl )]: 6.04(24)(010), 3.26(22)(11 $\bar{2}$ ), 3.181(19)(200), 3.079(24)(1 $\bar{1}$ 2), 3.016(100)(020), 2.054(20)(3 $\bar{1}$ 1) and 2.050(18)(13 $\bar{2}$ ). 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 lustre. 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 ~3470 and 3330 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 [W2 O8 (OH)2 ] 6– edge-sharing dimers (with highly distorted WO6 -octahedra) forming chains along [101] with [(OH)2 Pb4 ] 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.