Abstract
AbstractGarpenbergite is a new mineral (IMA2020-099) from the Garpenberg Norra mine, Hedemora, Dalarna, Sweden. It occurs with carlfrancisite and minor stibarsen, paradocrasite and filipstadite in a fractured skarn matrix of granular jacobsite, alleghanyite, kutnohorite and dolomite. Crystals are short-prismatic, up to 1.5 mm in length. They have a blackish to greyish brown colour, and are lustrous semi-opaque, with brown streak. Garpenbergite is brittle, with an uneven to subconchoidal fracture. Cleavage is distinct on {010}. Hardness ≈ 5 (Mohs) and VHN100 = 650(40). Dcalc = 4.47(1) g⋅cm−3, overall ncalc = 1.85. Maximum specular reflectance values (%) obtained are 9.2 (470 nm), 9.1 (546 nm), 9.0 (589 nm) and 8.9 (650 nm). The empirical chemical formula of garpenbergite, based on electron microprobe data, is (Mn2+3.97Mg1.48Mn3+0.26Zn0.29)Σ6.00(As0.89Fe3+0.04Mn3+0.06Si0.01)Σ1.00(Sb0.98Fe0.02)Σ1O10[(OH)1.99Cl0.01]Σ2.00. The five strongest Bragg peaks in the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are 3.05 (30) (002), 2.665 (100) (161), 2.616 (40) (301), 2.586 (25) (251) and 1.545 (45) (462). The orthorhombic unit-cell dimensions (in Å) are a = 8.6790(9), b = 18.9057(19) and c = 6.1066(6), with V = 1001.99(18) Å3 for Z = 4. The crystal structure was refined from single-crystal X-ray diffraction data in the space-group Ibmm to R1 = 3.7% for 957 reflections. Garpenbergite, ideally Mn6As5+Sb5+O10(OH)2, is isostructural with manganostibite, Mn7AsSbO12, but possesses a cation vacancy (□) at an octahedrally coordinated structural site; the two minerals are thus related by the exchange Mn2+ + 2O2– → □ + 2(OH)–. The presence of hydroxyl groups is supported by vibration bands at 3647 and 3622 cm−1 in the Raman spectrum of garpenbergite and by bond-valence considerations.
Highlights
Metal mining at Garpenberg, Hedemora, Dalarna (Dalecarlia), Sweden, has been described in annals since the fourteenth century, but recent studies of Holocene lake-sediment records in the area indicate that small-scale extraction of copper occurred already in the pre-Roman iron age, from ca. 375 B.C. (Bindler et al, 2017)
The sample containing the new mineral presented here was collected in situ at the 900 m level of the Garpenberg Norra mine, where various exotic skarn mineral assemblages have been noted to exist (Eriksson and Kalinowski, 2001; Holtstam et al, 2001; Holtstam, 2002; Kolitsch and Holtstam, 2002; Nysten, 2003)
A single-crystal X-ray diffraction (SCXRD) study was performed on a 55 × 45 × 40 μm crystal fragment using a Bruker D8 Venture diffractometer equipped with Photon III CCD detector, with graphite-monochromatized MoKα radiation (λ = 0.71073 Å), with 20 s exposure time per frame; the detector-to-sample distance was 60 mm
Summary
Metal mining at Garpenberg, Hedemora, Dalarna (Dalecarlia), Sweden, has been described in annals since the fourteenth century, but recent studies of Holocene lake-sediment records in the area indicate that small-scale extraction of copper occurred already in the pre-Roman iron age, from ca. 375 B.C. (Bindler et al, 2017). The sample containing the new mineral presented here was collected in situ at the 900 m level of the Garpenberg Norra mine The Garpenberg polymetallic Zn Pb Ag (Cu Au) sulfide deposit stretches about seven km in a SW NE direction, and comprises several ore bodies, mainly confined to a marble horizon. Most or all of the Fe oxide and complex sulfide ores in the Garpenberg area formed within the time span 1895–1890 Ma during a synvolcanic event (Jansson and Allen, 2011). The mineral-forming episode is likely coeval with the transition from ductile to brittle deformation that occurred in the area at about 1.8 1.7 Ga (Stephens and Jansson, 2020)
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