Sir please explain last 2 lines

unstable. In view of the stability characters of the minerals, in most of the sedimentary rocks we find the minerals like quartz and muscovite whereas the presence of unstable minerals like augite, hypersthene etc. is rarely found. In general, the sandy rocks consist largely of stabler primary minerals, while the fine grained clayey materials are constituted mostly by the ferromagnesian silicates and feldspars.