The spectrum of Ni-substituted lizardite, (Mg,Ni)3Si2O5(OH)4, has been discussed by many authors, who have successively given contradictory interpretations concerning the symmetry of the Ni-sites. Lakshman & Reddy (1973) affirmed that in garnierites Ni is in a tetrahedral site. Faye (1974) rejected this conclusion and showed that Ni2+ occupies an octahedral site. However, the Ni … O distance of 2·01 Å which he deduced from the intensity of the crystal field is unacceptable. Recently, using a completely different approach to the interpretation of the spectrum, Cervelle & Maquet (1982) claimed that Ni2+ occupies a six-coordinated site of C3v symmetry, and concluded that the mean Ni … O distance is 2·06 Å.The aim of this note is to show that it is possible to interpret the spectrum of Ni-lizardite as arising from a predominantly octahedral field with a weak C2v component which provokes a small increase in width of the absorption bands.

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This paper is dedicated to Professor Carr~ille Sandorh, on the occasion of' his 65th birthday ANDRE JULG and ODERE JULG. Can. J. Chem. 63, 1955Chem. 63, (1985. With an accuracy of 0.1 eV, all the electron transitions in any 3d transition element and in the corresponding ions, isolated or inside a crystal, can be obtained with the Hartree-Fock model by using hydrogen d orbitals with suitable effective charges 5. When the ion is embedded in a crystal, the value of 5 is generally weaker than that in the isolated state. For a given ion, the value of the reduction factor depends on the nature of the crystal. This phenomenon is explained by the boundaries imposed on the ion by the presence of the first neighboring ions (box effect). The application to some crystals (oxides and carbonates) corroborates this interpretation. An empirical relationship between the size of the ion and that of the cavity in which it is confined is proposed. ANDRE JULG et ODERE JULG. Can. J. Chem. 63, 1955 (1985 The interpretation of the spectra of atoms and ions, in the isolated state and inside a complex or a crystal, is often considered one of the greatest successes of quantum mechanics. In fact, the quantitative interpretation of these spectra necessitates the replacing of the exactly calculated values of the various terms which appear in the calculation of the transition energies, by values fitted to experiment (Slater-Condon integrals, Racah parameters) (1). Moreover, the Coulomb parameters corresponding to an ion in a complex or a crystal are, in general, weaker than those corresponding to the isolated state (1, 2). In order to explain this reduction, a partial covalency is often invoked (2, 3). Certain authors have also envisaged a modification in the form of the atomic orbitals (4, 5). We shall see that another factor, essentially geometric in origin, can also be invoked. Whatever that may be, it would be very helpful to the chemist (non specialist in sophisticated quantum calculations) to know the reduction factor directly. Indeed, the a priori knowledge of the absorption spectrum not only permits the identification of impurities inside the minerals, but also helps to lead the experimentalist in preparing inorganic compounds which possess well-determined optical properties (artificial gems (6), colored filters, and lasers). Some results concerning atoms in the isotated stateand within a crystal field In order to simplify the discussion, we shall consider 3dn ions only, the core-electrons being assumed to be completely localized around the nucleus. Since the reduction we propose to study brings modifications in energy for various levels much more important (ca. 100 times) than the splittings between the components of a given multiplet arising from the spin-orbit coupling, the latter will be neglected. The actual levels corresponding to a given ' S t ' X term (leaving J out of account) will be replaced by their center of gravity.Using the Hartree-Fock model, the various states are described by linear combinations of Slater determina...