Calcination of magnesium hydroxide, which was prepared from natural bischofite MgCl(2)(.)6H(2)O, leading to dehydration 2(equivalent to MgOH) -> equivalent to Mg-O-Mg equivalent to + H2O, is accompanied by transition of phase not only to MgO but also to MgOx at x < I (assigned to Mg4O3) at moderate temperatures. At higher temperatures, MgOx, is completely transformed into MgO. Magnesium hydroxide and oxide heated at different temperatures were studied using the TEM, XRD, IR, PCs, TG-DTA, nitrogen and argon adsorption methods. The electronic structure of MgO and Mg4O3 was studied using the ab initio quantum chemical method with periodic conditions. According to TEM images, the morphology of particles changing from Mg(OH)(2) laminae to aggregates of interpenetrated MgO cubelets and foils depend strongly on the calcination temperature. Significant changes in surface area are observed mainly at 325-470 degrees C on desorption of a major portion of eliminated water corresponding to 28.4 wt.% at its total amount of 30.9 wt.%. Pore size distribution (PSD) is sensitive to treatment conditions and the main PSD peaks shift towards larger pore size with elevating temperature. The characteristics of the surface hydroxyls as well as of the bulk Mg-O bonds depend on heating conditions, as noticeable changes are observed in the XRD patterns and the IR spectra of the samples undergoing the mentioned transformation of phase Mg(OH)(2) -> MgOx - MgO. (c) 2005 Elsevier B.V. All rights reserved.