Abstract
AbstractLanghofite, 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) Å3 for Z = 2. The seven strongest Bragg peaks from powder X-ray diffractometry are [dobs, 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 [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.
Highlights
Moore (1970) characterised the Långban deposit in Värmland, Sweden, as “the most remarkable mineral locality on Earth”. The validity of this statement is a matter of taste, Långban is still the most prolific ore deposit based on objective criteria, with 73 valid type species, and presently only challenged by Tsumeb (72)
Only one Pb–W mineral has been identified with certainty from here; stolzite crystals occur as a rarity, confined to vugs in hematite–garnet skarn (Boström, 1965)
The Fe ores in Långban-type deposits are generally enriched in W whereas Pb is, in the present case, most likely to have been mobilised from Mn-rich lithologies, where a number of Pb silicate and arsenate minerals occur in the skarns (Nysten et al, 1999)
Summary
Moore (1970) characterised the Långban deposit in Värmland, Sweden, as “the most remarkable mineral locality on Earth”. Only one Pb–W mineral has been identified with certainty from here; stolzite crystals occur as a rarity, confined to vugs in hematite–garnet skarn (Boström, 1965) We present another one, langhofite, approved by the International Mineralogical Association (IMA) Commission on New Minerals, Nomenclature and Classification (IMA2019-005, Holtstam et al, 2019). The Fe ores in Långban-type deposits are generally enriched in W (sequestered by scheelite; Holtstam, 2001) whereas Pb is, in the present case, most likely to have been mobilised from Mn-rich lithologies, where a number of Pb silicate and arsenate minerals occur in the skarns (Nysten et al, 1999).
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