Zinkgruvanite, Ba₄Mn²⁺₄Fe³⁺₂(Si₂O₇)₂(SO₄)₂O₂(OH)₂, a new ericssonite-group mineral from the Zinkgruvan Zn-Pb-Ag-Cu deposit, Askersund, Örebro County, Sweden

Abstract

Zinkgruvanite, ideally Ba4Mn42+Fe23+(Si2O7)2(SO4)2O2(OH)2, is a new member of the ericssonite group, found in Ba-rich drill core samples from a sphalerite- and galena- and diopside-rich metatuffite succession from the Zinkgruvan mine, Örebro County, Sweden. Zinkgruvanite is associated with massive baryte, barytocalcite, diopside and minor witherite, cerchiaraite-Al, and sulfide minerals. It occurs as subhedral to euhedral flattened and elongated crystals up to 4 mm. It is almost black and semi-opaque with a dark-brown streak. The lustre is vitreous to sub-adamantine on crystal faces and resinous on fractures. The mineral is brittle with an uneven fracture. VHN100=539, and HMohs ≈ 4.5. In thin fragments, it is reddish-black, translucent and optically biaxial (+), 2Vz > 70∘. Pleochroism is strong and deep brown-red (E ⊥ {001} cleavage) to olive-pale-brown. Chemical point analyses by WDS-EPMA (wavelength-dispersive X-ray spectroscopy electron probe microanalyser) together with iron valencies determined from Mössbauer spectroscopy yielded the empirical formula (based on 26 O+OH+F+Cl anions): (Ba4.02Na0.03)Σ4.05(Mn1.79Fe1.562+Fe0.423+Mg0.14Ca0.10Ni0.01Zn0.01)Σ4.03(Fe1.743+Ti0.20Al0.06)Σ2.00Si4(S1.61Si0.32P0.07)Σ1.99O24(OH1.63Cl0.29F0.08)Σ2.00. The mineral is triclinic, in space group P1¯, with unit-cell parameters a=5.3982(1) Å, b=7.0237(1) Å, c=14.8108(4) Å, α= 98.256(2)∘, β= 93.379(2)∘, γ= 89.985(2)∘ and V= 554.75(2) Å3 for Z=1. The eight strongest X-ray powder diffraction lines are the following (d Å (I %; hkl)): 3.508 (70; 103), 2.980(70; 114‾), 2.814 (68; 12‾2), 2.777 (70; 121), 2.699 (714; 200), 2.680 (68; 201‾), 2.125 (100; 124, 204) and 2.107 (96; 2‾21). The crystal structure (R1=0.0379 for 3204 reflections) is an array of TS (titanium silicate) blocks alternating with intermediate blocks. The TS blocks consist of HOH sheets (H for heteropolyhedral and O for octahedral) parallel to (001). In the O sheet, the Mn2+-dominant MO(1,2,3) sites give ideally Mn42+ pfu (per formula unit). In the H sheet, the Fe3+-dominant MH sites and AP(1) sites give ideally Fe23+Ba2 pfu. In the intermediate block, SO4 oxyanions and 11 coordinated Ba atoms give ideally 2× SO4Ba pfu. Zinkgruvanite is related to ericssonite and ferroericssonite in having the same topology and type of linkage of layers in the TS block. Zinkgruvanite is also closely compositionally related to yoshimuraite, Ba4Mn4Ti2(Si2O7)2(PO4)2O2(OH)2, via the coupled heterovalent substitution 2 Ti4++ 2 (PO4)3-→2 Fe3++ 2 (SO4)2− but presents a different type of linkage. The new mineral probably formed during a late stage of regional metamorphism of a Ba-enriched, syngenetic protolith, involving locally generated oxidized fluids of high salinity.