AB-INITIO CALCULATIONS ON (OH)4 DEFECTS IN ALPHA-QUARTZ

Ab initio total energy calculations based on a new optimized oxygen ps uedopotential have been used to study the structures and relative ener gies of alpha-quartz, a partly (OH)4 substituted version of the alpha- quartz structure, and interstitial water molecules in alpha-quartz. Hy drogen bonds formed from two hydroxyl groups of the (OH)4 defects in t he substituted alpha-quartz structure promote a stable structure for t he defect alpha-quartz at low temperature. Comparable ab initio calcul ation of the energy of the interstitial water molecule in the quartz s tructure indicates that, energetically, the (OH)4 defect is likely to be strongly favoured as a mode for the incorporation of water. Ab init io stress calculations confirm that the (OH)4 defect in quartz has a l arge associated stress field which is likely to lead to segregation of these defects on supersaturation in wet quartz. The calculations indi cate that segregation should occur in the plane (1010BAR) of the alpha -quartz structure.