Computer simulation of Al-Mg ordering in glaucophane and a comparison with infrared spectroscopy

TheorderingofMgandAlovertheoctahedralsitesinglaucophane, (A) (8) Na2 (6) (Mg3Al2) (4) Si8O22(OH)2,hasbeenstudied by Monte Carlo simulation using a model Hamiltonian parameterised using both empirical interatomic interactions and ab initio calculations. It is found that Al is fully ordered at the M(2) site, with disorder beginning to appear for temperatures above ~1000 K. Infrared spectra of threesynthetichigh-PT glaucophane-nybo ite amphiboles were also collected and theOH-stretching frequencies usedtoinferthestateofAl-Mgordering.Thespectraofallthreeamphibolescompriseonlytwopeaksat ~3662cm -1 and~3720cm -1 , correspondingtoMgMgMg- OH- (A) andMgMgMg- OH- (A) Na,respectively.TheseinfraredspectrashowunequivocallythatM(1) andM(3) sites are fully occupied by Mg, and, therefore, (6) Al isfully orderedat M(2),in agreement withthebehaviour predicted by the computational studies and bond-valence considerations. Such a highly (6) Al-ordered state for alkali amphiboles contrasts starkly with calcic amphiboles synthesized under similar pressure-temperatureconditions,whichhaveahighdegreeof (6) AldisorderoverM(2)andM(3)sites.Thisdifferencebetweenalkali and calcic amphibolesshows the major influence that the M(4) cation (monovalent versus divalent) has, via its bonding relations to O(4), in controlling the ordering of trivalent cations over the octahedral sites in amphiboles.