ALUMINOSILICATE AND BOROSILICATE SINGLE 4-RINGS - EFFECTS OF COUNTERIONS AND WATER ON STRUCTURE, STABILITY, AND SPECTRA

Rings containing four tetrahedrally coordinated atoms connected by bri dging O atoms in a single four ring (S4R) geometry have been used to m odel the properties of ring species occurring in aluminosilicate and b orosilicate crystals, glasses, and melts. We recently established a mo lecular basis for the Al avoidance, or ''Loewenstein's rule,'' using m olecular quantum mechanical methods applied to the non-Loewenstein pai red, i.e., Al... Al... Si... Si..., and the Loewenstein alternating, i .e., Al... Si... Al... Si..., geometric isomers of the Si2Al2O4H8-2 mo lecular anion. We here extend this model to the Si2Al2O4H8-2 and Si2B2 O4H8-2 molecules neutralized by various counterions such as H+, CH3+, Na+, and Ca2+. Our calculations show that the paired geometry isomer c an be strongly stabilized by coordination of counterions to the underb onded bridging O in the Al-O-Al or B-O-B linkages. For the Si2Al2... c ase, coordination of H+ or Ca2+ to the underbonded O actually makes th e paired geometry isomer more stable than the alternating geometry one . This result helps us to explain why the energy penalty for Al-O-Al b onds is smaller in Ca aluminosilicates than in Na aluminosilicates. Fo r the Si2B2O4H8(CH3)(2) and Si2B2O4H8Ca molecules the paired geometry is again the more stable. Underbonded O atoms in B-O-B linkages are fo und to be stabilized more strongly by counterions than are those in Al -O-Al linkages. This provides an explanation for the different T,T' or dering patterns in anorthite, CaAl2Si2O8, and danburite, CaB2Si2O8. Ou r results also indicate that it may be possible to systematically synt hesize gas phase and condensed phase species in which the Al avoidance rule is violated. We have also used the S4R species to model rings in Na aluminosilicate glasses and their reaction with H2O. Calculated NM R shieldings are reduced (deshielded) by about 5-6 ppm for both Si and Al when two Na+ ions are coordinated to Si2Al2O12H8-2. Reaction of H2 O with Si2Al2O12H8Na2 does not cause hydrolysis of the Si-O-Al bond or protonation of the bridging O, rather the H2O coordinates strongly to Na+ while H-bonding to the bridging O. This reaction is exoergic by a bout 35 kJ/mol and produces a slight shielding of the Si and a reducti on in the quadrupole coupling constant of the Al. No single species wa s found to be consistent with all the NMR spectral data for hydrous al uminosilicate glasses. Copyright (C) 1997 Elsevier Science Ltd.