IMA 2020-099 = garpenbergite

[Marco E. Ciriotti]

Prossima pubblicazione.

Referenza:
▪ Holtstam, D., Bindi, L., Förster, H.J., Karlsson, A., Gatedal, K. (2022): Garpenbergite, Mn6□As5+Sb5+O10(OH)2, a new mineral related to manganostibite, from the Garpenberg Zn−Pb−Ag deposit, Sweden. Mineralogical Magazine, 86, (in press).

Abstract:
Garpenbergite is a new mineral (IMA No. 2020-099) 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}. H ~ 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 EPMA data, is (Mn2+3.97Mg1.48Mn3+0.26Zn0.29)Σ6(As0.89Fe3+0.04Mn3+0.06Si0.01)Σ1(Sb0.98Fe0.02)Σ1O10[(OH)1.99Cl0.01]Σ2. The five strongest Bragg peaks in the X-ray powder 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.

[Marco E. Ciriotti]

Pubblicazione effettuata.

Referenza:
▪ Holtstam, D., Bindi, L., Förster, H.J., Karlsson, A., Gatedal, K. (2022): Garpenbergite, Mn6□As5+Sb5+O10(OH)2, a new mineral related to manganostibite, from the Garpenberg Zn−Pb−Ag deposit, Sweden. Mineralogical Magazine, 86, 1-8.