Irradiation effect on physico-chemical properties of clay minerals

The purpose of the present work was to study the exposure of various doses of y-irradiation (10(5), 10(6), 10(7) 3 x 10(7) Gy) on types and concentration of radiation-induced defects, deuteration, changes in specific surface and solubility of minerals irradiated in D2O solution in closed capsules. Some clay minerals (perfect and imperfect kaolinites, palygorskite, montmorillonite), muscovite, synthetic gibbsite and brucite were investigated. A sharp increase of the A- (Si-O-) and B-centres (Al-VI-O--Al-V1 bridge) concentration in the dose interval 10(5) - 10(7) Gy in both investigated kaolinites has been established. These concentrations are higher both in the initial and irradiated samples of well-ordered kaolinite. In irradiated palygorskite, montmorillonite and muscovite, such concentration is considerably lower than in kaolinites. The concentration of the A-centres and their stabilization are determined by the composition of the tetrahedral structural fragment of a mineral: kaolinite ([Si4O10])>montmorillonite, palygorskite ([Al0.5Si3.5O10])>muscovite ([AlSi3O10]), and also by the presence of mobile water molecules on the surface and in the interlayer space of a mineral: perfect kaolinite (water molecules are practically absent) > imperfect kaolinite > montmorillonite (the maximum amount of water molecules). Concentration of the B- and B'-centres is even more sensitive in the presence of mobile water molecules in mineral like the A-centres concentration. In irradiated montmorillonite, the W-centre was not found. The B-centre model has been refined: an impaired electron in the B-centre in kaolinite (1:1 structure) is located on an oxygen ion of an outer OH-group, and in the B-centre in minerals with 2:1 structure-on an inner OH-group. The deuteration begins at lower irradiation doses in those minerals, in which the maximum amount of the radiation-induced centres appears. The lower limit of the beginning of deuteration at small irradiation doses rises in the following sequence: brucite < kaolinite < montmorillonite < palygorskite < gibbsite. The irradiation dose growth leads to the increase of specific surface and change of solubility of the irradiated samples of clay minerals: the going out degree of the Al3+ ions from kaolinite increases, but that of Si4+ ions-decreases, for montmorillonite and palygorskite the inverse dependence takes place. It can be explained by the increase in the amount of the different types of radiation-induced defects in these minerals. (C) 2002 Elsevier Science B.V. All rights reserved.