Gradual H/D substitution in synthetic germanium bearing talcs: a method for infrared band assignment F. MARTIN, S. PETIT*, O. GRAUBY { AND M. P. LAVIE Universite´ Paul Sabatier , Laboratoire de Mine´ralogie-Cristallographie, UMR 5563 CNRS, 39 Alle´es Jules Guesde, F-31000 Toulouse, *Universite´ de Poitiers, Laboratoire ‘Hydr. A.S.A.’, UMR 6532 du CNRS, 40 Avenue du Recteur Pineau, F-86022 Poitiers Cedex, and { CRMC2-CNRS, Campus Luminy, Case 913, F-13288 Marseille Cedex 9, France (Received 8 September 1997; revised 9 June 1998) ABSTRACT: The deuteration technique was used to assign unknown infrared (IR) bands of Ge- bearing talcs. Procedures to achieve partial or complete H/D replacement in synthetic Ge-bearing (Mg, Ni and Co)-talcs have been investigated. From the spectral shift of IR absorption bands in the 4000300 cm 1 region resulting from the H-D substitution, the IR bands of vibrations of Ge-bearing talcs could be identified and synthetic Ge-bearing talcs showed significantly different IR spectra from those for Si-talcs. After deuteration of synthetic Ge-bearing talcs, the unknown bands were attributed either to hydroxyl groups or to GeO vibrations of the clay skeleton, and the spectra were interpreted fully. The use of D 2 O during hydrothermal synthesis of clay minerals leads to the conversion of some or all of their OH groups to OD groups. The degree of exchange can be determined from the IR spectra. This technique of deuteration is commonly used to identify the IR absoption bands which arise from vibrations involving hydroxyls (Stubican & Roy, 1961; Russell et al., 1970; Farmer et al., 1968; Shirozu & Ishida, 1982; Ishida, 1990; Grauby et al., 1991). Infrared spectra of Ge-bearing talcs, synthesized under hydrothermal conditions, have two major differences compared to Si-bearing ones: (1) the high crystallinity of the Ge-bearing talcs implies very well defined absorption bands; and (2) a shift of all the absorption bands to lower frequencies due to the Ge-Si atomic weight difference. These effects produce complex spectra with unassigned absorp- tion bands (Martin et al., 1992, 1996; Petit et al., 1995). In this paper, experimental conditions to achieve gradual replacement of H by D were investigated in synthetic Ge-bearing talcs. These products with various H/D ratios were used to identify all the IR absorption bands in the 1200300 cm 1 region. EXPERIMENTAL Ge-talc synthesis Few authors have synthesized Ge-bearing talcs at high pressure (>1 kbar) and high temperature (>5008C), and the products obtained were mixed with other mineral phases like spinel (Roy & Roy, 1955; Stubican & Roy, 1961). In order to prevent mixtures, Martin et al. (1992, 1996) used the hydrothemal process at low temperature (502008C) described by Decarreau (1983) to synthesize Ge-bearing talcs. They synthesized Ge- bearing talcs with various octahedral cations (Ni, Co and Mg) and characterized three solid-solutions (Ni, Co and Mg) between the Ge and Si talc end- members with a random distribution of the Ge and Si cations in the tetrahedral sheet of the Ge-Si- bearing talcs (Petit et al., 1995; Martin et al., 1996). # 1999 The Mineralogical Society Clay Minerals (1999) 34, 365–374