Site occupancy in richterite-winchite from Libby, Montana, USA, by FTIR spectroscopy

Abstract

Three natural amphibole samples collected from the former vermiculite mine near Libby, Montana, USA, have been analysed by Rietveld X-ray powder diffraction (XRPD) refinement and Fourier transform infrared spectroscopy (FTIR) in the OH-stretching region. The same materials have been analysed previously by electron microprobe analysis (EMPA), Mössbauer spectroscopy and structure refinement (SREF) single crystal X-ray diffraction (SC-XRD), which revealed that these amphiboles have a crystal chemical formula very close to an intermediate composition between winchite and richterite, i.e. A A 0.5 B NaCa C Mg 4.5 M 3+ 0.5 T Si 8 O 22 (OH) 2 (A = Na and/or K; M 3+ = Fe 3+ and/or Al). The Rietveld analysis showed the powder samples used for the experiments here to be composed only of amphibole. This in turn allowed us to use FTIR OH-stretching data to derive cation ordering on these powder samples. The three FTIR spectra are quite similar and up to four components can be fitted to the patterns. The two lower-frequency components (labelled A and B) can be attributed to a local O(3)ÀH dipole surrounded by M(1)M(3) Mg 3 and M(1)M(3) Mg 2 Fe 2+ (respectively), an empty A site and T Si 8 environments; on the contrary, the higher-frequency C and D bands indicate the presence of an occupied A site. The FTIR OH-stretching data alone allow us to calculate the site occupancy of the A, M(1)ÀM(3) and T sites with confidence, as compared with previously published data. By contrast M(4)-and M(2)-site occupancies are more difficult to evaluate. This study takes advantage of the large database of well characterized synthetic amphiboles, built over the last two decades. The comparison of vibrational spectroscopy data with micro-chemical and crystallographic data reported in this study demonstrate that the FTIR OH-stretching method alone is a valuable and rapid method to derive or at least sensibly constrain site occupancy for natural amphiboles. A much more detailed cation site occupancy can be obtained by combining micro-chemical and FTIR OH-stretching data.

FTIR Spectroscopy of the Regulated Asbestos Amphiboles

“…Considering that the exact wavenumbers of the OH-stretching bands are also sensitive, albeit weakly, to the nextnearest neighbours, i.e. the M (4) and M (2) cations [8,46,52], the calcic amphiboles tremolite-actinolite can be easily distinguished from the ferromagnesian amphiboles of the anthophyllite-grunerite type. For simple binary M (1,3) Mg-M(1,3) Fe 2+ solid-solutions, as is typically the case for asbestos amphiboles, the relative amount of these cations in the OH-bonded octahedra can be calculated with a high degree of confidence [26], and, provided that the Al and Fetot amounts are known from EMPA, the complete cation distribution at all octahedral sites can be derived.…”

“…FTIR and Raman spectroscopy are frequently used for the characterisation of Earth and analogue materials [17][18][19]. Both methods have been used extensively for rapid identification of asbestiform minerals and the characterization of their site occupancies [8,[20][21][22][23]. Vibrational spectra can be collected either on powders or on single crystals, requiring only a very small amount of material (mg), such as tiny crystals with sizes ranging from some µm to few tens of µm.…”

“…From a legal perspective, six fibrous minerals are regulated as asbestos minerals: five species of the amphibole group, and chrysotile, which belongs to the serpentine group of the layer silicates [3][4][5]. From a mineralogical point of view, chrysotile has only limited variations from its ideal crystal-chemical composition described as Mg3(OH)4Si2O5 [6], whereas amphiboles exhibit extremely wide ranges in composition, even in samples from the same outcrop, in the same rock or within the same (zoned) crystal [7][8][9][10]. Such chemical variability results in significant challenges for identification and classification of amphiboles, implying difficulties for health and legal issues arising from exposure to the amphibole fibres [11][12].…”

Infra Red Spectroscopy of the Regulated Asbestos Amphiboles

“…FTIR and Raman spectroscopy are frequently used for the characterization of earth and analogue materials [17–19]. Both methods have been used extensively for rapid identification of asbestiform minerals and the characterization of their site occupancies [8,20–23]. Vibrational spectra can be collected either on powders or on single crystals, requiring only a very small amount of material (mg), such as tiny crystals with sizes ranging from some μm to few tens of μm.…”

OH in zoned amphiboles of eclogite from the western Tianshan, NW-China

“…On the basis of previous work, the E band centred at *3,672 cm -1 (Fig. 4d-a; Table 2) can be assigned to a M(1),(3) (MgMgMg)-O(3)-H-A h-T Si 8 configuration (e.g., Iezzi et al 2007), i.e., to the same cation arrangement responsible for the D component. This does not appear to agree with the micro-chemical data, however, (Table 1).…”