Sandstones belong to the arenaceous group of sedimentary rocks. They consist of detrital particles; usually but not necessarily rounded; of sand-grade (i.e., the size range is 1/16 to 2 mm) held together with a matrix or cement. The mineral composition of average sandstone’ is as follows:
Quartz 68%
Feldspar 12%
Mica 6%
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Carbonates 11%
Miscellaneous 4%
The classification of sandstones proposed by various geologists like Pettijohn, Krynine, Dott etc. indicates that chiefly there are three major groups of sandstone as:
1. Arkose.
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2. Graywacke.
3. Quartz-arenite.
1. Arkose:
The important characteristics of Arkose are as follows:
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(a) Mineral composition:
It is composed primarily of feldspar and quartz. Generally quartz exceeds feldspar. The feldspar is largely potash feldspar. Other constituents form 5 to 15% of the rock and are mainly micas. The composition of an arkose corresponds to that of granitic rocks from which it-is presumably derived.
(b) They are typically light pink or light gray in colour.
(c) Arkose commonly shows current-bedding and are terrestrial-shallow water deposit.
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(d) The cementing material is chemical and the detrital matrix is less than 15% or absent.
(e) It is a product of oxidising environment.
(f) When feldspar exceeds quartz in an arkose it is termed as feldspathic arkose.
(g) Of all the sandstones, arkose constitute only about 15%.
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2. Graywacke:
They show the following important characteristics:
(a) Mineral composition:
These rocks are composed mainly of rock-fragments. In this case feldspar exceeds quartz.
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(b) Graywacke is dark in colour.
(c) It commonly shows, graded bedding and is mostly sub- marine, deep water deposit.
(d) These are the sandstones with high detrital matrix content and no chemical cement.
(e) It is a product of reducing environment.
(f) The matrix has the composition of slate, and imparts both the distinctive colour and general toughness to the rock. Like slate, the matrix consists of a fine grained mixture of white mica, chlorite and quartz.
(g) Of all the sandstones, graywackes constitute about 50%.
(h) The composition of graywackes corresponds to the basic igneous rocks which are the source-rocks.
3. Quartz arenite:
These are also known as orthoquartzites, which are pure sandstones and show the following characteristics:
(а) They are composed of quartz to the extent of 95% or more.
(b) They are presumed to be associated with the peneplanation stage of geomorphic cycles.
(c) In this case the detrital matrix is absent or less than 15% at the most.
(d)Here the feldspar as well as the rock-fragments constitute even less than 5%.
(e)They may be derived from the lithic sandstones or from, arkosic sandstones. In the former case they are characterised by metamorphic quartz and chert particles; whereas in the latter case the chert is absent or nearly so (under 5%) and the quartz is igneous- quartz.