Raman spectroscopy as a tool in mineral discoveries

Abstract

Preliminary investigations of unknown minerals are frequently based on physical and optical observations, chemical analysis and collection of powder X-ray diffraction data. At the Swedish Museum of Natural History, laser Raman micro-spectrometry has been adopted as a standard method, which led to discoveries that otherwise might have been unnoted. The new mineral garpenbergite (IMA2020-099), with formula Mn6□AsSbO10(OH)2, is found at the Garpenberg Zn-Pb-Ag deposit, Dalarna (Dalecarlia), Sweden. Preliminary data indicated a close relationship to manganostibite, ideally Mn7AsSbO12. The two minerals are not easily distinguished using routine procedures, because of almost identical powder patterns and similar chemical compositions. In the Raman spectra, a characteristic feature, with distinct bands at 3647 and 3622 cm-1 related to OH-stretching vibration modes, appears for garpenbergite only. Structural refinement from single-crystal X-ray diffraction data yielded an orthorhombic unit cell, with a = 8.6919(10), b = 18.927(3), c = 6.1110(6) Å for Z = 4. The crystal structure is distinct by the presence of a vacancy, corresponding to an octahedrally coordinated Mn2+ site in manganostibite (Moore, 1970), and by incorporation of protons via the exchange mechanism Mn2+ + 2O2- → □ + 2(OH)- that leaves the space-group symmetry, Ibmm, invariant. Parahibbingite (IMA2020-038a) was recently approved, with the Karee mine in the Bushveld complex, South Africa, as type locality. This mineral, with formula Fe2+2(OH)3Cl, has independently also been identified on samples of corrosion crust from weathered fragments of the Muonionalusta iron meteorite, collected in the Kitkiöjärvi area, Pajala, in northernmost Sweden. From initial energy-dispersive X-ray microanalysis, it was identified as hibbingite or possibly another polymorph of Fe2+2(OH)3Cl , and the identity with the rhombohedral β-form was then confirmed with Raman spectroscopy, when compared with data for the synthetic analogue. A subsequent refinement of the crystal structure, including the hydrogen positions, from single-crystal diffraction data gave a = 6.9362(4), c = 14.6730(11) Å with Z = 6 for the R-3m unit cell, in good agreement with previous Rietveld refinement of synthetic material and corrosion products from artefacts. Magnussonite, ca. Mn10As6O18OH2, is a rare arsenite mineral with inferred cubic crystal symmetry that has been the subject of several studies. Brattforsite, ideally Mn19(AsO3)12Cl2 (IMA2019-127), has a similar structural topology as magnussonite and is a monoclinic bona fide Cl-analogue . The close relationship between the two minerals is supported by their resemblance in the Raman spectra overall, but there is also a distinct shift (ca. 30 cm-1) in the bands originating from As-O stretching in the (AsO3)3- groups, related to differences in mean bond lengths between the corresponding atoms.