It is well known that minerals are in equilibrium with the environment in which they are formed and hence stable. Some are stable over a wide range of temperature-pressure conditions while others yield rather readily. Different silicate minerals of which most of the rocks are composed have different stability. The stability of minerals is approximately in the reverse order to that of their crystallisation from magma. It is therefore, commonly observed that rocks of basic and magnesium varieties breakdown much faster than the acid and ferruginous ones.
The mincralogical characters of the sedimentary rocks are therefore mostly governed by the stability-characteristic of the minerals.
The most stable minerals, which are found quite abundantly in the sedimentary rocks arc quartz followed by muscovite.
The various feldspar, along with zircon rutile, tourmaline, monazite, garnet etc. arc fairly stable.
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Biotite hornblende, apatite, ilmenite, magnetite, staurolite, kyanite, topoz, sphene etc. are minerals of the ‘least-stable’ category.
Mirerals like augite, hypersthere, diopside, actinolite, olivine etc. are highly 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.