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  (Formerly Journal of the Czech Geological Society)

Original paper

Iwona Korybska-Sadło, Adam Szuszkiewicz, Marta Prell, Piotr Gunia

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

Journal of Geosciences, volume 67 (2022), issue 4, 299 - 310


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 μm 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)2 As1.00O8 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 ν3 and symmetric ν1 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 ν4 and ν2 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 this is the first reported natural occurrence of KNi3(AsO4)(As2O7) phase.

Journal of Geosciences, Published by © Czech Geological Society, with support from the Czech Geological Survey.
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ISSN: 1802-6222

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