CHEMICAL AND STRUCTURAL VARIATIONS IN SERPENTINES FROM SOME METAULTRAMAFITES OF THE ROMANIAN CARPATHIANS (ROMANIA). Lucia Robu, Gabriela Stelea, I.N. Robu, Geological Institute of Romania, 1, Caransebes str., 78344-Bucuresti-32, Romania

Serpentine mineral samples come from some metaultramafic bodies included in the geological formations of the Romanian Carpathians.

For the most part of the samples mesh-texture are characteristic. Its one is determined due to the substituted processes of the olivene or/and pyroxene by chrysolile and/or lizardite in the marginal zones, and antigorite in the central ones. Sometimes, some small antigorite flasks are spread in the serpentine mass.

Crystallochemical data have been calculated according to M₆N₄O₁₀(OH)₈, general formula, whereM=Mg, Fe²⁺, Ni, Fe³⁺, sometimes Cr and N=Si, Al.

However, Si and Al contents show some evident substituted processes in the tetrahedral levels, between Si and Al. A lack of silica in these levels, in the calculated formula suggests a completion of these ones by Fe³⁺ cations.

Mg, Ni, Fe²⁺, Fe³⁺ cations were identified as filling of the octahedral chains. The exchanges between Mg and other above mentioned cations are insignificantly, so that their sum varies among 0.267-0.617. Sometimes Al cations are presented in these structural levels.

IR absorption spectra confirm these substitutions processes, registering absorption bands at the characteristic frequencies (cm^-1).

However the registered bands about 610 cm^-1 varies directly proportional to the Mg substitution by Fe,Ni, Cr, at the octahedral levels. Its intensity increases when Mg content is increasing. There is no possibility to evidence the proportion in which took place this substitution, but some possible combination would be suppose, MgMgMg, NiMgMg, NiFeMg, FeFeMg, FeMgMg, sometimes CrNiMg or CrFeMg.

Some of IR spectra present at the 663cm^-1 frequency a very low intensity peak, which could be assigned to NiNiNi combination.

These high inhomogeneous and discontinuous substitutional exchanges in the octahedral level have determined these variations of the IR absorption data.

At OH-stretching region, about the 3700 cm^-1 domain the allures of the curves are similar to these ones characteristics for low Ni content.