Aluminosilicate and borosilicate single 4-rings: Effects of counterions and water on structure, stability, and spectra
- Univ. of Maryland, College Park, MD (United States)
Rings containing four tetrahedrally coordinated atoms connected by bridging O atoms in a single four ring (S4R) geometry have been used to model the properties of ring species occurring in aluminosilicate and borosilicate crystals, glasses, and melts. We recently established a molecular basis for the Al avoidance, or {degrees}Loewenstein`s rule,{close_quotes} using molecular quantum mechanical methods applied to the non-Loewenstein paired, i.e., Al...Al...Si...Si..., and the Loewenstein alternating, i.e., Al...Si...Al...Si..., geometric isomers of the Si{sub 2}Al{sub 2}O{sub 4}H{sub 8}{sup {minus}2} molecular anion. We here extend this model to the Si{sub 2}Al{sub 2}O{sub 4}H{sub 8}{sup {minus}2} and Si{sub 2}B{sub 2}O{sub 4}H{sub 8}{sup {minus}2} molecules neutralized by various counterions such as H{sup +}, CH{sub 3}{sup +}, Na{sup +}, and Ca{sup 2+}. Our calculations show that the paired geometry isomer can be strongly stabilized by coordination of counterions to the underbonded bridging O in the Al-O-Al or B-O-B linkages. For the Si{sub 2}Al{sub 2}...case, coordination of H{sup +} or Ca{sup 2+} to the underbonded O actually makes the paired geometry isomer more stable than the alternating geometry one. This result helps us to explain why the energy penalty for Al-O-Al bonds is smaller in Ca aluminosilicates than in Na aluminosilicates. For the Si{sub 2}B{sub 2}O{sub 4}H{sub 8}(CH{sub 3}){sub 2} and Si{sub 2}B{sub 2}O{sub 4}H{sub 8}Ca molecules the paired geometry is again the more stable. Underbonded O atoms in B-O-B linkages are found to be stabilized more strongly by counterions than are those in Al-O-Al linkages. This provides an explanation for the different T.T. {prime} ordering patterns in anorthite. CaAl{sub 2}Si{sub 2}O{sub 8}, and danburite, CaB{sub 2}Si{sub 2}O{sub 8}. Our results also indicate that it may be possible to systematically synthesize gas phase and condensed phase species in which the Al avoidance rule is violated. 44 refs., 4 figs., 1 tab.
- OSTI ID:
- 554910
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 61, Issue 6; Other Information: PBD: Mar 1997
- Country of Publication:
- United States
- Language:
- English
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