Friisite, Pb ₈ Al ₃ Si ₈ O ₂₇ Cl ₃ , a new mineral with a polysomatic relation to jagoite, from Långban, Sweden

Friisite, ideally Pb8Al3Si8O27Cl3, is a new mineral discovered in a museum sample from the Långban mine in Värmland, Sweden. It occurs as subhedral, flaky grains up to 150 µm in size, forming aggregates within a medium-grained skarn matrix and contiguous to jagoite. Both are associated with melanotekite, aegirine–augite, albite, baryte, fluorapophyllite-K, margarosanite, alamosite, native lead, a serpentine group mineral, and a wickenburgite-like mineral. Friisite is white to colorless with a white streak and sub-adamantine luster. The mineral is transparent and does not fluoresce under UV light. It is brittle, with an uneven fracture and perfect cleavage on {001}. Mohs hardness is 4–5 (by analogy with jagoite). The calculated density is 5.54(1) g cm−3. Optically, friisite is non-pleochroic and uniaxial (–). Point analyses by means of an electron microprobe using wavelength-dispersive spectroscopy resulted in an empirical formula (based on 30 O+Cl): (Pb7.89Na0.11Ca0.08)∑=8.08(Al2.19Si0.31Fe0.203+Zn0.13Mn0.112+)∑=2.94Si8O27.02Cl2.98. Friisite is hexagonal, P6‾2c (#190), with unit-cell parameters a=8.5955(1) Å, c=23.4092(2) Å, and V=1497.82(4) Å3 for Z=2. The eight strongest powder X-ray diffraction lines are [d, Å (Irel) (hkl)]: 5.848 (31) (004), 5.375 (20) (103), 4.040 (96) (11‾2, 112), 3.680 (40) (201), 3.463 (100) (114), 2.886 (21) (116), 2.795 (20) (21‾1), and 2.4828 (35) (300). Friiste is a phyllosilicate and forms a polysomatic series with jagoite characterized by a layer sequence of SiO4 tetrahedra (T) and metal octahedra (O) between double layers (*) corresponding to *TOT*, whereas jagoite is described as *TOTOT*. Friisite forms from transformation of melanotekite or barysilite in the presence of albite and a Cl-enriched fluid at relatively high aSiO2. The mineral (IMA2024-047) is named in honor of Danish mineralogist Henrik Friis (b. 1977), professor at the Natural History Museum, University of Oslo, Norway.