SEDIMENTARY ROCKS
There are two main classes of sedimentary rocks
1) Clastic sedimentary rocks consist of rock
and mineral grains derived from the chemical and mechanical breakdown (weathering)
of pre-existing rock. They contain rock fragments and more commonly, particles
of quartz and feldspar. Clastic rocks are further classified on the
basis of grain size. Next to each rock type, the Wentworth Scale of
particle sizes is shown.
- Conglomerates (> 2mm) consolidated gravel
- Boulder (>256mm)
- Cobble (65-256 mm)
- Pebble (4-64 mm)
- Granule (2-4 mm)
- Sandstones (0.062-2 mm) consolidated sand
- Very coarse (1.0 - 2.0 mm)
- Coarse (0.5 - 1 mm)
- Medium (0.25 - 0.5 mm)
- Fine (0.125 - 0.25 mm)
- Very fine (0.0625 - 0.125 mm)
- Shales (<0.0062 mm) consolidated mud, rich in organic matter.
- Silt (0.0039 - 0.0625 mm)
- Clay (0.0002 - 0.0039 mm)
Argillite: A sedimentary rock, composed of clay particles which have
been hardened and cemented.
Illite (muscovite): K2Al4(Si6Al2)O20(OH)4. is a sedimentary fine-grained
rock, equivalent to ordinari mica (muscovite).
2) Chemical sedimentary rocks are formed either
from minerals that precipitate directly from aqeous (water) solutions or
from the accumulation of fossilised remains of organisms which become limestone.
Gypsum (CaSO4.2H2O)
Anhydrite (CaSO4)
Halite (NaCl) salt
Limestone (CaCO3)