Question

Which of the following is an argillaceous rock?
A.Sandstone
B.Shale
C.Limestone
D.Slate

Answer 1

Hint: Argillaceous rocks contain a substantial amount of clay-grade particles and thus, are also called clastic sedimentary rocks. They are also characterised by the wide range of durability behaviour they display after excavation and in cut slopes. They can be classified on the basis of three components— the clastic framework, the fine-grained matrix and the cementing agent.

Complete answer: Option A: is incorrect. Sandstone is a sedimentary rock made up of sand-size grains of minerals and organic particles. Most sandstones contain silicates like quartz and feldspar but the cementing materials in it may be a matrix of silt or clay-size particles. This, however, does not make them argillaceous rocks.
Option B: is correct. Shale is a fine-grained clastic sedimentary rock and is made up of clay minerals like kaolinite, illite, chlorite and such and smaller fragments of other minerals like quartz and calcite. It is also referred to as mudstone and is the most widely found sedimentary rock. As it is classified as a clastic sedimentary rock, it is an argillaceous rock.
Option C: is incorrect. Limestone is classified under carbonate sedimentary rocks and it contains calcite and aragonite (both different forms of calcium carbonate). It is not an argillaceous rock.
Option D: is incorrect. Slate is a fine-grained, homogeneous metamorphic rock-forming smooth flat sheet of stone. Although they are derived from shale-type sedimentary rocks, they cannot be called an argillaceous rock.

Hence, The correct answer is Option C.

Note: Most argillaceous rocks contain aluminosilicates and particular clay minerals like kaolinite, illite, chlorite, etc. One very prominent example of argillaceous rocks is Claystone.