Sedimentary rocks form from the compaction and cementation of sediments that have been eroded from other rock formations. These rock formations can be igneous, metamorphic or other sedimentary formations. But the minerals present in them all came from the same source – magma – and were brought to the surface through igneous rock formation processes. Therefore, the minerals present in sedimentary rocks are recycled minerals that were originally present in igneous rocks.
Mineral Composition of Sedimentary Rocks
The mineral composition and texture of a sedimentary rock depends on four main factors:
- Source rock and materials
- The erosion processes involved in the transport of the sediment
- The physical and chemical conditions at the site of deposition
- Post-depositional processes such as those that lead to compaction and cementation
The composition of the source rock determines what the composition of the sedimentary rock will be. The erosion processes involved in the transport of the sediment also affects the composition. For example, an igneous rock with iron-bearing minerals, such as olivine, is eroded by water processes. Elements within the water react to the olivine and biotite to create clay. These clay particles are transported downstream where they are deposited. Several thousands of years of compaction and cementation lead to the creation of a claystone formation.
Iron-bearing silicate minerals such as olivine, pyroxene, amphibole and biotite are the most susceptible to weathering, especially chemical weathering because of their reaction to elements present in water. Calcium and sodium-rich plagioclase are also very susceptible to weathering. Minerals susceptible to weathering are altered during the erosion process, so are less likely to be present in sedimentary deposits in their original form and are most often altered to clay grains. Potassium feldspar, quartz and muscovite are not as susceptible to weathering and therefore are more often found in sedimentary rock formations. Sandstone, for example, is made entirely of small grains of quartz crystals.
Textures in Sedimentary Rocks
When it comes to describing the texture of a sedimentary rock, it all depends on the grain sizes, also called clasts, of the rock. The range of clasts that make up sedimentary rocks is:
- Boulder - more than 256 mm in diameter
- Cobble - 64 to 256 mm in diameter
- Pebble - 2 to 64 mm in diameter
- Sand grain - 1/16 to 2 mm in diameter
- Silt grain -1/256 to 1/16 mm in diameter
- Clay particle - less than 1/256 mm in diameter
Boulders, cobbles and pebbles are compacted and cemented together to form conglomerates. The fine grained material in between the boulders, cobbles and pebbles - also called a matrix - acts as a bonding material to cement all the larger grains together. The matrix is usually made up of finer grained material such as sand and silt. Conglomerates are very easy to identify because you can easily see the different clasts that make up the rock. These clasts are pieces of other igenous, metamorphic and sedimentary rock. Sand grains are compacted and cemented to form sandstone. As are silt grains to from siltstone and clay particles to from claystone, mudstone or shale. These kinds of sedimentary rocks are referred to as clastic sedimentary rocks.
Sources:
Weathering and Sedimentary Rocks
Dietrich, Richard V. and Skinner, Brian J., Rocks and Rock Minerals. NY: John Wiley and Sons, 1979.
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