“…As for Al and Si ions, it is dif fcult to determine the degree of ordering by the X-ray diffraction technique, since Al and Si ions have nearly equal X-ray scatter ing factor. The structure refinements of CaAI2SiO6,CaFeAlSiO6,CaScAlSiO6,and Cal.OOSc0.84Ti0 .27Al1.16Si0.73O6 pyroxenes show that Al and Si ions are randomly distributed throughout tetrahedral sites in average and the aluminum avoidance rule is violated in these structures (Okamura et al, 1974;Ghose, et al, 1975;Ohashi and Ii, 1978;Ohashi et al, 1979). However, this fact does not nec essarily indicate that short-range Si/Al ordering is ruled out.…”