I. The second major type of
sedimentary rock is chemical sedimentary rock which usually forms from
the precipitation of dissolved material in water or from the biological
production of shell material. What are various types of chemical
sedimentary rocks?
- A. Limestone
- 1. It is composed of calcite (CaCO3).
- 2. Limestone of biological origin has shells and
skeletons of marine animals and algae.
- 3. Inorganic limestone is precipitated from seawater
- 4. Some varieties of limestone are chalk, travertine,
oolites, and skeletal limestone.
- B. Chert
- 1. It is composed primarily of silica (SiO2
or quartz)
- 2. Chert of biologic origin contains skeletons of
diatoms and radiolarians deposited, typically, in a marine environment
- 3. Inorganic chert forms chert nodules precipitated out
of groundwater.
- C. Evaporites form in areas in which standing bodies of
water experience high rates of evaporation
- 1. Gypsum is an evaporite mineral that is found in
plaster of Paris
- 2. Rock salt (halite) is found in salt flats and playa
lakes
- D. Coal
- 1. Coal forms when plant material is buried under
sediments before it has a chance to decompose, exposed to the
atmosphere.
- 2. Depending on how deeply the coal is buried, coal
will form at higher grades at greater burial depths. Some varieties of
coal material, by increasing grades, are peat, lignite, bituminus, and
anthracite. Anthracite is legitimately a metamorphic rock.
- 3. Coal often forms as beds between layers of other
clastic sedimentary rocks, in environments such as river swamps or
lakes.
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