Sedimentary Rock Classification
How Do You tell One Sedimentary Rock from Another

Sedimentary rock is classified into two groups based on how they form. They are clastic and chemical.

Clastic sedimentary rock is formed as bits of weathered rock become cemented together. Because all kinds of rock are subject to weathering many different minerals can make up this group of rocks. Clays and quartz are the most common.

Classification of clastic sedimentary rocks is done according to the size of the sediments that make up the rock. The following table contains the major groups of clastic sedimentary rocks with their characteristics:

Clastic Sedimentary Rocks
Name of Rock
Sediment Type
Texture
Conglomerate
gravel - rounded fragments

course

over 2 mm

Breccia
gravel - angular fragments

course

over 2 mm

Sandstone
sand

medium

1/16 to 2mm

Siltstone
mud

fine

1/256 to 1/16 mm

Shale
mud

very fine

less than 1/256 mm



Chemical sedimentary rocks form from dissolved minerals that are precipitated or separated from water. This happens most frequently when water evaporates leaving the minerals behind. You can see this process taking place in your own home. The white deposits that form around the faucets in your bathroom or kitchen are from minerals left behind as water evaporates. Below is a list of chemical sedimentary rocks with some of their characteristics:

Chemical Sedimentary Rocks
Name of Rock
Composition
Texture
Crystalline Limestone
Calcite - CaCo3
corse to fine Crystalline
Fossiliferous Limestone
Calcite - CaCo3
visible fragments of shells
Chalk
Calcite - CaCo3
microscopic shells and clay
Chert
Quartz - SiO2
very fine crystalline
Gypsum
Gypsum - CaSO4-2H2O
fine to course crystalline
Rock Salt
Halite - NaCl
fine to course crystalline
Bituinous Coal
Organic Matter
fine


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Examples of Sedimentary Rock

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