Fe-F and Al-F avoidance rule in ferrous-aluminous (OF, F) biotites.

The results of infrared and Raman spectroscopic investigations in the OH-slrelching and lattiee-mode regions in synthetic ferrous-aluminous (OH.F)-biotites are presented. In the OH-stretching region (3800-3200 cm '). all micas studied present a high intensity peak at high frequencies [3669 cm ' for (OH)-annite and 3641 cm-' for (OH)-Es] which can be decomposed into two bands and a low intensity peak at low frequencies [3535 er1 for (OH)-annite and 3589 cm ' lor (OH)-Es] which suggests rather a vacant octahedral site. Along the (OH.F)-annite join. the intense peak at 3669 cm ' shifts to lower frequencies as XF increases from 0 to 0.4. In contrast. this peak shifts to higher frequencies along the (OH.F)-Es join (Es K(Fe:25Al())(Si225Al,75)Ol()(OH.F)2). The low-intensity V-band re¬ mains roughly unchanged. The two bands that compose the 3669 cm ' peak are assigned to a N-band resulting from OH-Fe2+Fe2+Fe2+ (Tri-6p vibrations and a Ib-band due to OH-Fe2*Fe2+AP* (Tri-7-) vibrations. As the amount of fluorine increases in micas of the (OH.F)-annite join. the N-band frequency varies weakly but its intensity decreases significantly, while the Ibband becomes more intense. In contrast, these two bands show an opposite behaviour in (OH.F)-Es micas. TTie Nband intensity increases whereas that of the Ib-band decreases. The opposite evolution of the two main bands in the OH-stretching region shows that F is preferentially linked to Fe rather than to AI in (OH.F)-anmte, whereas F is preferentially linked to AI rather than to Fe in (OH.F)-Es. Conse¬ quently. the bond strengths Al-F or Fe-F are not controlled by the Fe-F or Al-F avoidance rule (which would predict lhat fluorine is preferentially associated to Fe in all micas). but by structural constraints. The Al-F or Fe-F avoidance rule may not play a determining role on the fluorine content of the micas as it is generally agrecd.The maximum fluorine contents in micas mainly depend on the ability of the dimensional adaptation of tetrahedral and octahedral layers.

[1]  A. Beran,et al.  Spectroscopic and structural properties of synthetic micas on the annite-siderophyllite binary: Synthesis, crystal structure refinement, Mössbauer, and infrared spectroscopy , 2000 .

[2]  F. Hawthorne,et al.  Near-infrared study of short-range disorder of OH and F in monoclinic amphiboles , 1999 .

[3]  D. Rancourt,et al.  Octahedral-site Fe2+ quadrupole splitting distributions from Mössbauer spectroscopy along the (OH, F)-annite join , 1996 .

[4]  M. Dyar,et al.  The influence of structural fluorine on biotite oxidation in copper-bearing, aqueous solutions at low temperatures and pressures , 1995 .

[5]  J. Robert,et al.  Fluorine in micas: crystal-chemical control of the OH-F distribution between trioctahedral and dioctahedral sites , 1993 .

[6]  R. Mason.  MODELS OF ORDER AND IRON-FLUORINE AVOIDANCE IN BIOTITE , 1992 .

[7]  D. Jenkins.  Empirical study of the infrared lattice vibrations (1100–350 cm−1) of phlogopite , 1989 .

[8]  M. Dyar,et al.  Mossbauer spectral study of ferruginous one-layer trioctahedral micas , 1986 .

[9]  J. Robert,et al.  Muscovite solid solutions in the system K2O-MgO-FeO-Al2O3-SiO2-H2O: an experimental study at 2 kbar P H2O and comparison with natural Li-Free white micas , 1986, Mineralogical Magazine.

[10]  J. Sanz,et al.  NMR applied to minerals. V. The localization of vacancies in the octahedral sheet of aluminous biotites , 1984 .

[11]  S. Ludington,et al.  Fluoride-hydroxyl exchange in biotite , 1974 .

[12]  R. Gilkes,et al.  The Oxidation of Octahedral Iron in Biotite , 1972 .

[13]  J. L. Ahlrichs,et al.  Evidence for loss of protons and octahedral iron from oxidized biotites and vermiculites , 1971, Mineralogical Magazine.

[14]  P. Rouxhet.  Hydroxyl Stretching Bands in Micas: A Quantitative Interpretation , 1970, Clay Minerals.

[15]  V. Farmer,et al.  Replacement of OH by OD in layer silicates, and identification of the vibrations of these groups in infra-red spectra , 1970, Mineralogical Magazine.

[16]  H. Eugster,et al.  Experimental control of fluorine reactions in hydrothermal systems , 1969 .

[17]  J. Munoz.  F-OH and Cl-OH exchange in micas with applications to hydrothermal ore deposits , 1984 .

[18]  C. Guidotti.  Micas in metamorphic rocks , 1984 .

[19]  V. Farmer.  The Layer Silicates , 1974 .

[20]  S. Ross.  The Infrared Spectra of Minerals , 1974 .

[21]  T. Shimanouchi,et al.  Far infra-red absorption spectra of layer silicates , 1967 .

[22]  E. E. SroNr.  NMR study of micas , II . Distribution of Fe ' * , F-, and oH-in the octahedral sheet of phlogopites , 2022 .