Nickel-bearing clay minerals: I. Optical spectroscopic study of nickel crystal chemistry

Abstract

A B S T R A C T:Nickel crystal chemistry was systematically studied in various phyllosilicates, mainly the natural phases selected from the 'garnierites' of the supergene ore deposits of New Caledonia. Minerals which do not usually occur in New Caledonian parageneses were synthe sised, as they could represent intermediate phases of genetic importance. In the kerolitepimelite series, a linear relationship occurred between the ratio I (13,20~/I(02,11 ) of the hk bands and Ni-content. Diffuse reflectance spectra were used to derive the crystal chemical parameters of Ni. These confirmed its divalent character and its occupation of octahedral sites; the resulting structural distortion was slight and could not be detected in some minerals. There was no optical evidence for Ni atoms in 4-fold coordination. The two main parameters which showed significant variations among the studied phases were site distortion and crystal field stabilization energy (CFSE). Site distortion was at a maximum in trioctahedral smectites and sepiolite. CFSE depended on the mineralogy, crystallinity and chemical composition (Al-content) of the phase. Finally, clay minerals are classified according to the increasing stability of Ni in the octahedral sheet, which has been tentatively related to the geochemical distribution of this element. Secondary minerals are usually enriched vs. primary ones and among them are nepouite and kerolite which exhibit a high CFSE in contrast to sepiolite.Although the green-coloured 'garnierites' have been studied for many years (Faust, 1966), their chemical and structural characteristics are not entirely clear. An accurate study of these hydrous Ni-Mg silicates requires investigations at various microscopic scales, from the study of the relations between the various layers to the determination of the precise nature of the sites occupied by Ni atoms.In this paper we present some results quantifying Ni crystal chemistry, which have been derived from the study of diffuse reflectance spectra of various phyllosilicates. Special emphasis is laid on parameters such as coordination number, site distortion, nickel-oxygen bond covalency and crystal field stabilization energy (CFSE). These results should promote a better understanding of Ni geochemistry during the formation of supergene ore deposits. A subsequent paper will be devoted to the intracrystalline distribution of Ni studied by X-ray absorption spectroscopy in the same phases. EXPERIMENTALPure phases were selected on the basis of X-ray diffraction (XRD) and wet chemical analyses. The ignition losses of minerals previously heated at 110~ (H20 +) were 9 1985 The Mineralogical Society

Spectroscopic Studies of Nickel(II) and Nickel(III) Species Generated upon Thermal Treatments of Nickel/Ceria-Supported Materials

“…20 The spectrum of the untreated Ni/CeO 2 sample can be analyzed as the superposition of the spectra of Ni(OH) 2 and The band observed at 1120 nm for Ni(OH) 2 is shifted to 1230 nm for Ni/CeO 2 , probably because of a crystallinity effect, Ni II ions in a poorly organized deposit being submitted to a weaker crystal field than in crystalline Ni(OH) 2 . 23 Furthermore, two very broad bands indicate a distribution of hexacoordinated Ni II species. According to Tanabe-Sugano diagrams, 26 their average positions (λ 2 , λ sf ) 820 nm and λ 1 ) 1400 nm), indicate that the Ni II ions involved are submitted to very weak crystal fields.…”

Domain Segregation in Ni-Fe-Mg-Smectites

“…The shoulders at 22,400 and 27,300 cm x are in the same energy region as in nontronite (Bonnin et al, 1985) and are due to octahedral Fe 3+ atoms. The two other absorption bands at lower energy are due to Ni in 6-fold coordination (Manceau et al, 1985). These latter authors showed that the energy of the crystal field band at 9000 cm -~ depends on the type of Ni-bearing phyl-J I I 12000 9,000 6,000 WAVENUMBER (cm-1) Figure 6.…”

Optical spectroscopic study of synthetic NaScSi2O6–CaNiSi2O6 pyroxenes at normal and high pressures

“…In diffuse reflectance spectra of Ni-bearing phyllosilicates, for instance, such splitting is attributed to the low symmetry of the coordination octahedron of Ni 2+ (e.g. Manceau et al 1985, Manceau andCalas 1987). However, in the type I spectra of our pyroxenes this band is seen as a singular band slightly overlapped by the much weaker spin-forbidden band c. This observation is supported by results of the curve resolution of the Ni 0.3 pyroxene spectrum showing that all three spin-allowed bands a, b and e, as well as the two spin-forbidden ones, c and d, can be satisfactory approximated by singular Gaussians (Fig.…”