Prehnite has been postulated as a low-subgreenschist facies metamophic mineral hydrothermally formed in basic igneous rocks. It can also be formed as a product of contact metamorphism (Deer et al., 1962; Artioli et al., 1995). The mineralogy and petrology of prehnite has been sufficiently investigated but the crystal chemistry of this mineral has not been yet completely understood (Stebbins, 1992). The aim of the present study was to provide new data concerning the structural characteristics of prehnite based on spectroscopic and nuclear analytical techniques (Hawthorne, 1988; Vaughan and Patrick, 1995; Petit et al., 1990).
Well-formed prehnite crystals from Loanhead Quarry (Ayshire, Scotland), characterized by Powder-XRD, EPMA and TGA, have thoroughly been investigated using
1H-NMR, FT-IR, 29Si-MAS NMR, EPR and TPD. Especial emphasis was given to the correlation of the hydrogen content of the mineral determined by a 1H-NMR probe developed at the Institute of Inorganic Chemistry of the Russian Academy of Sciences (Novosibirsk) (Gabuda et al., 1989) with relevant data obtained using the 1H(15N,ag)12C nuclear reaction at the 7 MV van de Graaff accelerator of the Nuclear Physics Institute of the University of Frankfurt. The above mentioned nuclear reaction has a sharp resonance at projectile energy of 6.385 MeV and emits g-rays of energy 4.43 MeV allowing the determination of the hydrogen concentration and depth distribution in near-surface layers of materials (Lanford et al., 1976).
The hydrogen content of the mineral determined by
1H-NMR was found to be 0.479 (+ 0.005) wt%. This value is in good agreement with the corresponding value obtained by nuclear reaction analysis techniques. Complementary information concerning the nature of the H-atoms in the prehnite structure has been obtained from FT-IR spectra that revealed, in accordance to preceding data (Dominguez Bella et al., 1987), the presence of different types of O-H bonds into the lattice. The TPD study showed that these Brønsted acidic sites make the material slightly acidic (3.11 mg NH3 /g of prehnite) and dehydroxylation phenomena appear at ca. 750°C. The 29Si-MAS NMR data confirmed the proposed chemical environment around the Si-tetrahedra (Stebbins, 1992), whereas the EPR spectra provided the evidence for the existence of paramagnetic centres possibly due to ions such as Mn2+ and Fe3+. The indicated oxidation state of iron coincides with previous Mîssbauer (Artioli et al., 1995; Michailidis et al., 1995) and XANES (Artioli et al., 1995) measurements.
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