Mn3+-bearing pargasite from the Långban Fe-Mn oxide mineralisation, Bergslagen, Sweden

Abstract

Supplying new data on the nature of amphibole-group minerals, a recent find in the Palaeoproterozoic carbonate-hosted Långban Fe-Mn oxide deposit comprises a manganian plumbian pargasite. Here we report textural and mineralogical data on this clinoamphibole, based on optical microscopy, powder X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analyses, electron microprobe techniques and optical absorption spectroscopy. The mineral occurs as relatively dark cherry-red, mostly subhedral, often strongly poikiloblastic crystals dispersed in a hausmannite-braunite-bearing, silicate-impregnated calcitic carbonate rock. Electron microprobe analysis of the amphibole gives the composition (Na,K)0.96(Ca,Pb)1.97(Mg4.24Mn0.40Al0.25Fe0.08Ti0.01) (Si6.40Al1.59)O22(OH)2. Refinement of powder XRD data resulted in a unit cell with a = 9.897 (6), b = 17.97 (2), c = 5.260 (7) Å, and β = 105.22 (8)°. Combining compositional data with the results from optical absorption spectroscopy, it is evident that the intense colour of this amphibole is due to electronic transitions in Mn3+. We infer that Mn3+-centered polyhedra in pargasite are more distorted from octahedral symmetry, and that mean Mn3+–O distances of the Mn-centered polyhedra in pargasite are shorter than corresponding sites in previously described manganian riebeckite and winchite. Associated species at Långban include arsenate-substituted apatite, hedyphane, kentrolite and forsterite. The Mn3+-bearing pargasite formed during a metamorphic stage affecting the Mn ore-bearing carbonate rock, post-dating hausmannite formation.