X-RAY-DIFFRACTION AND MOSSBAUER-SPECTROSCOPY STUDIES OF FE,SI-ORDERING IN FERRISILICATE FELDSPARS

Processes of Fe,Si-ordering are studied in synthetic ferrisilicate ana logues of potassium and rubidium feldspars. Under low-temperature hydr othermal conditions, the rates of Fe,Si-ordering are found to be lower in RbFeSi3O8 than in KFeSi3O8. The changes in the unit-cell parameter s in the course of ordering are attributed to the changes in the bridg ing bond angles formed by iron-silicon-oxygen tetrahedra, An increase in ordering leads to an enhancement of ionicity of Fe3+-O bonding and reduction in both electron charge density and electric field gradient at the Fe-57 nuclei. We believe that in ferrisilicate feldspars under study, the rule forbidding the Fe3+-O-Fe3+ bonding (an analogue of the empirical Lowenstein rule for aluminosilicates) does not hold.