Chemical composition and Raman spectroscopy of aerugite, xanthiosite, and a natural analog of KNi 3 (AsO 4 )(As 2 O 7 ) from Johanngeorgenstadt, Germany

[摘要] Aerugite Ni8.5(AsO4)2As5+O8 and xanthiosite Ni3(AsO4)2, two rare anhydrous arsenates, have been identified in a historicsample from Johanngeorgenstadt, Saxony, Germany. The minerals have been characterized through scanning electronmicroscopy, 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 xanthiositeforms short prisms to nearly equant forms, often with indistinct, poorly-developed and rounded faces. The chemicalcomposition of the two minerals can be expressed by the empirical formulas: (Ni7.92Co0.52Cu0.06)Σ8.50(As1.00O4)2As1.00O8with 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 stretchingvibrations and are dominated by antisymmetric ν3 and symmetric ν1 As–O vibrations in AsO4 polyhedra centered at817, 846 and 886 cm–1 in the case of aerugite and at 786, 808, 826 and 843 cm–1 in xanthiosite. Bands from stretchingvibrations As-O in AsO4 polyhedra are located at 728 and 735 cm–1 in aerugite and are slightly displaced to 726 and 747cm–1 in xanthiosite. The Raman spectrum of aerugite also contains well-defined 692, 675 and 658 cm–1 bands due to thestretching mode of NiO6 octahedra, a broad feature at 576 cm–1 probably from a number of modes connected with AsO6octahedra. On the other hand, the xanthiosite spectrum displays a number of low-intensity, well-defined bands relatedto antisymmetric ν4 and ν2 symmetric bending vibrations in AsO4 below 700 cm–1 as well as to lattice vibrational modesand Ni-O interactions below 250 cm–1. Locally, the interstices between xanthiosite grains are filled with cryptocrystallinemass 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 xanthiositerelated signal, lacks bands of water molecules or OH groups and contains bands related to the As–O–As vibration modesattributed 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.

[学科分类] 自动化工程

[关键词] aerugite;xanthiosite;KNi3 (AsO4 )(As2 O7 );arsenates;chemistry;Raman spectroscopy