Spectroscopic and structural properties of synthetic micas on the annite-siderophyllite binary: Synthesis, crystal structure refinement, Mössbauer, and infrared spectroscopy

The effect of the incorporation of Al-Tschermak’s molecule to the trioctahedral potassium mica annite {K}[Fe3]O10(OH)2 on local and average structure has been investigated by hydrothermal synthesis, structure refinement of X-ray powder diffraction data, Mössbauer and infrared spectroscopy. The various types of brackets indicate different structural sites. Samples with compositions {K}[Fe3–xAlx]O10(OH)2 were prepared by hydrothermal techniques. The maximum solubility of Al is limited to x = 0.92 at 500 °C and to x = 0.82 at 700 °C. The main factor controlling the substitution limits is the ditrigonal distortion of the tetrahedral rings. Lattice parameters decrease linearly with increasing Al content of the mica. A considerable decrease of M2-O and nearly no change of M1-O bond lengths with increasing Al contents is indicative of preferred occupation of the M2 site by [Al]. Changes in K-O distances are also very pronounced and reflect the ditrigonal distortion of the tetrahedral sheet. The bimodal ferrous quadrupole splitting distribution (QSD) in annite, extracted from Mössbauer spectra, becomes narrower and more centered around 2.60 mm/s with increasing Al contents, and its evolution suggests an increasing deviation from ideal octahedral coordination of Fe by O, illustrated by the increasing octahedral flattening angle ψ. The population of individual QSD components proves that it is impossible to resolve cis and trans M-sites in micas by Mössbauer spectroscopy. In the hydroxyl stretching region, up to 7 bands are observed in the infra-red spectra which correspond to OH groups adjacent to 3 Fe (N-bands), to OH groups coordinated by Fe, Al, and Fe (I-bands) and to configurations having one octahedral vacancy (V-bands). Nand I-type bands are shifted toward lower wavenumbers with increasing Al content because of increasing OH···Otet interactions. Al, Ga and Ge can enter the tetrahedral site < >. To understand the complex crystal chemistry and the effect of these various possible substitutions on stability, global and local structure and cationic environments, the “simplified” solid solutions series between the two main end-member compositions annite (ann) and siderophyllite (sid) was studied. We describe syntheses, stability, and structure refinements of X-ray powder patterns of the title compounds and compositions along the annsid binary to investigate possible cationic ordering of Fe and Al on the octahedral sites. Mössbauer and infrared spectroscopy were used to address changes in the local and electronic environments of the cations-mainly on the octahedral sites. The Mössbauer results demonstrate that the two different octahedral M sites in the mica structure are impossible to resolve by this method. The structure of trioctahedral micas is built up by a negatively charged 2:1 composite layer. Two tetrahedral sheets with the apices pointing together, sandwich one octahedral sheet. Charge balance is achieved by the large interlayer cations. The structures of annite and phlogopite were described in detail by *E-mail: [email protected] REDHAMMER ET AL.: STRUCTURE AND SPECTROSOPY OF AL-ANNITE 450 Hazen and Burnham (1973), but except for the numerous investigations on natural biotites (e.g., Brigatti and Davoli 1990; Brigatti et al. 1991), there is a lack of structural information for the very Fe rich and Fe-Al rich biotites, and no systematic study does exist so far regarding the changes of structural parameters as a function of progressive Al-Tschermak’s substitution in annite. Syntheses of annite were reported by Mercier et al. (1996), Redhammer et al. (1993), Dachs (1994) and references in these papers, the substitution of aluminum in annite by Rutherford (1973) and Hewitt and Wones (1975), who both found the substitution to be limited to about 0.8 apfu (atoms per formula unit) on the octahedral sites. In contrast, Levillain et al. (1981) report the synthesis of a siderophyllite with approximately ideal composition. Benisek et al. (1996) and Benisek et al. (1999) investigated the activity-composition relationship and the heat capacities of Tschermak’s substituted Fe-biotites. Mössbauer spectroscopy has used to define valence state and coordination of iron in natural and synthetic (e.g., Annersten 1974; Levillain et al. 1981; Dyar and Burns 1986; Dyar 1990 and references therein). Mössbauer spectroscopy also has high potential to characterize the local electronic and geometric environment around the iron atom and to record changes in polyhedral distortion (e.g., deviations from ideal octahedral coordination) as a function of chemical composition (e.g., Rancourt et al. 1994a, 1996; Redhammer 1998). The approach to refine Mössbauer spectra of mica with Lorentzian shaped doublets and applying different kinds of constraints to extract M2/M1 site occupation ratios (e.g., Annersten 1974; Dyar and Burns 1986; Dyar 1990; Redhammer et al. 1993, 1995) was critically discussed by Rancourt et al. (1994a, 1994b) and Rancourt (1994a, 1994b). To fit the spectral resonance absorption contributions of micas, a Voigt based quadrupole-splitting distribution (QSD) method (Rancourt and Ping 1991; Ping et al. 1991) was introduced. The application of the QSD method to characterize synthetic trioctahedral micas is outlined in detail by Rancourt et al. (1994a) for the annitephlogopite join, by Rancourt et al. (1996) for (OH)-F annites and by Redhammer (1998) for micas with various octahedral and tetrahedral substitutions. Natural and synthetic micas have been investigated by infrared spectroscopy with emphasis on the OH regions (Farmer 1974; Jenkins 1989; Boukili 1996; Benisek et al. 1999). Robert and Kodama (1988) generalized the correlation between hydroxyl-stretching wavenumbers and compositions of synthetic diand trioctahedral Mg-Al micas. Some discussions on the OH-stretching region of annite is also found in Redhammer et al. (1993) and in Rancourt et al. (1994b), however, substantial discrepancies do exist concerning band assignment, espe-