The composition of sedimentary rocks is of two types; chemical composition and mineral composition. The chemical composition of a substance means the aggregate chemicals that constitute that substance, while the presence of different types of minerals illustrates the mineral composition of that substance. Here we will discuss the two types of compositions separately.
(1). Chemical Composition of the sedimentary rocks
The range in the chemical composition of sedimentary rocks is quite large and in this respect, they differ from igneous rocks. For example, a sandstone may contain as much as 99% silica, in banded iron formations the iron oxide content may be as high as 58%, in pure limestones the CaO content may reach 55%. This large range in composition, which is illustrated in the following figure is caused by weathering cycle. This cycle has a tendency to produce mechanical sediments, which are compositionally very different from chemical sediments.
(2). Mineral Composition of sedimentary rocks
The minerals of the sedimentary rocks can be divided into two major groups: (i) minerals that are resistant to weathering and (ii) minerals that are products of chemical weathering. The relative stability of minerals to weathering are shown in the following table;
This table shows that quartz is one of the most resistant minerals, whereas olivine is easily altered by chemical weathering. The position of minerals shown between quartz and olivine have intermediate stabilities.
Detrital sedimentary rocks consist mainly of the most resistant rock-forming minerals such as quartz, K-felspar, mica, and sometimes plagioclase. A small amount of garnet, zircon, and spinel may also occur. Sedimentary rocks, which are formed from the inorganic or organic precipitation of minerals frequently contain calcite, aragonite, gypsum, halite, hematite, siderite, and chert.
In addition to the detrital minerals and chemical precipitates, sedimentary rocks may contain clay minerals (Kaolinite, Montmorillonite, Illite) and chlorite. These minerals result from the weathering of earlier primary silicate, such as feldspars, olivine, and pyroxenes.
You may like to read:
- Sedimentary Rocks and their formation
- Classification of Sediments
- The texture of the Sedimentary Rocks
- Structural features of the sedimentary rocks