ON THE PRESENCE OF OH- GROUPS IN ZABARGAD OLIVINE GEMS

A single crystal X-ray diffraction study on an olivine gem from Zabarg ad island (Red Sea) reveals a structure fully consistent with an essen tially anhydrous phase. The eventual presence of hydrogen (BERAN & PUT NIS, 1983; MILLER et al., 1987) is at concentration limits far beyond the detection capability of the procedure. The internal disorder K(D) = (Fe(M1) . Mg(M2))/(Fe(M2) . Mg(M1)) indicates that intracrystalline exchanges ceased out at a temperature of about 600-degrees-C in agreem ent with the geochemistry of heterogeneous equilibria (SCIUTO, 1993) a nd with fluid inclusion estimates (CLOCCHIATTI et al., 1981). Defect e nergetics, based on the refined structure have been calculated followi ng the procedure of OTTONELLO et al. (19,90) to assess how the presenc e of OH- groups would affect the phase energetics. Based on calculatio n OH.O defects would preferentially form on 03 positions. However the estimated energy terms are too much elevated and additional stabilizat ion processes as the formation of neutral trimers {OH.O3 VIIMOH.O3}x a nd neutral associates {OH.O3FeISi}x are necessary to reconduct the req uested energies into a reasonable range of values. The presence of OH- groups suggests that in protonated systems the defect scheme of NAKAM URA & SCHMALZRIED (1983) would be balanced by a concomitant equilibriu m of the type: 5Fe(x)Si + 2Si(x)Si + 4H2O + 2O2gas <-> {OH.O3FeISi}x 3 {OH.O3 VIIMOH.O3]x.