Thle 13 .5A and 22.5 A blocks along a-axis of pyroxeaies from Qingzhen chondrite and auorthosite were observed under HRTEM. This kind of odd- layered stackihg sequence arises from the stacking faults of Si-O chains in pyroxenes in the process of phase transformation. According to different stacking faults of Si-O chains, the 13.5 A blocks could be arranged as Si(A) Si(B)Si(B`)Si(A)and Si(B)Si(A)Si(A)Si(B)，with the former belonging to the orthorhombic system while the latter to the monoclinic system. In orthopy- roxeae from anorthosite, the 13·5A, + 22.5A blocks were transformed to orthopyroxene+clinopyroxene, which is considered, to have resulted from the change of Si(A)<=>Si (B').