Chemical composition and Raman spectroscopy of aerugite, xanthiosite, and a natural analog of KNi3(AsO4)(As2O7) from Johanngeorgenstadt, Germany

• Published in: Journal of geosciences (Prague) 2022-01, Vol.67 (4), p.299-310
• Main Authors: Korybska-Sadło, I., Szuszkiewicz, A., Prell, M., Gunia, P.
• Format: Article
• Language: English
• Subjects: Analytical methods; Arsenates; Barite; Chemical composition; Concretions; Crusts; Crystal structure; Crystals; Deformation; Dolomite; Dolostone; Electron microprobe; Electron microscopy; Electron probe microanalysis; Electron probes; Empirical analysis; Interstices; Lattice vibration; Mineralogy; Minerals; Modes; Museums; Polyhedra; Prisms; Quartz; Raman spectra; Raman spectroscopy; Scanning electron microscopy; Spectroscopy; Stretching; Vibration; Vibration mode; Vibrations
• ISSN: 1802-6222; 1803-1943
• DOI: 10.3190/jgeosci.361

Aerugite Ni8.5(AsO4)2As5+O8 and xanthiosite Ni3(AsO4)2, two rare anhydrous arsenates, have been identified in a historic sample from Johanngeorgenstadt, Saxony, Germany. The minerals have been characterized through scanning electron microscopy, electron microprobe analysis and Raman spectroscopy for the first time. They are mostly dark-green (aerugite) to light-green (xanthiosite) fine-grained or microcrystalline crusts on a quartz matrix in association with barite, bunsenite, dolomite, and rooseveltite. Aerugite forms up to 200 gm large pseudo-hexagonal platy crystals, whereas xanthiosite forms short prisms to nearly equant forms, often with indistinct, poorly-developed and rounded faces. The chemical composition of the two minerals can be expressed by the empirical formulas: (Ni7.92Co0.52Cu0.06)Σ8.50(As1.00O4)2As1.00O8 with traces of Bi (aerugite, mean of 4 analyses, based on 32 oxygens) and (Ni2.85Co0.12Cu0.03)Σ3.00(As1.00O4)2 (xanthiosite, mean of 5 analyses, based on 32 oxygens). The Raman spectra of both minerals lack bands related to OH stretching vibrations and are dominated by antisymmetric v3 and symmetric v1 As-O vibrations in AsO4 polyhedra centered at 817, 846 and 886 cm-1 in the case of aerugite and at 786, 808, 826 and 843 cm-1 in xanthiosite. Bands from stretching vibrations As-O in AsO4 polyhedra are located at 728 and 735 cm-1 in aerugite and are slightly displaced to 726 and 747 cm-1 in xanthiosite. The Raman spectrum of aerugite also contains well-defined 692, 675 and 658 cm-1 bands due to the stretching mode of NiO6 octahedra, a broad feature at 576 cm-1 probably from a number of modes connected with AsO6 octahedra. On the other hand, the xanthiosite spectrum displays a number of low-intensity, well-defined bands related to antisymmetric v4 and v2 symmetric bending vibrations in AsO4 below 700 cm-1 as well as to lattice vibrational modes and Ni-O interactions below 250 cm-1. Locally, the interstices between xanthiosite grains are filled with cryptocrystalline mass with the mean chemical composition of (K0.90Ba0.01)Σ0.91(Ni2.86Co0.11Cu0.05)Σ3.02(As1.00O4)(As2.1O7) with traces of Na (mean of 7 analyses, based on 11 oxygens). The recorded Raman spectrum, with a strongly overlapping xanthiosite-related signal, lacks bands of water molecules or OH groups and contains bands related to the As-O-As vibration modes attributed to pyroarsenate As2O7 groups. Although it was impossible to obtain more detailed data on crystal structure, we suggest t