SEDIMENTARY ROCKS

Part II: Chemical Precipitate Sedimentary Rocks

Classification of Chemical Sedimentary Rocks
Composition
Quartz (SiO2) Halite (NaCl) Gypsum Calcite (CaCO3) Aluminum-Rich
Rock NameChertRock SaltRock GypsumTravertineKaolin/Bauxite

CHERT

Chert is a chemical precipitate formed by groundwater. Chert can also form by biochemical accumulation of silica-rich fossils in deep marine environments; however, distinction requires microscopic examination.
  • Sediment type: chemical/biogenic
  • Composition: quartz
  • Grain Size: microscopic
  • Common Sedimentary Environments: groundwater precipitate or deep-marine biogenic accumulations
  • Other Characteristics: highly variable in color but easily recognized by its high hardness, very fine grain size and conchoidal fracture
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    ROCK SALT

    Rock salt is a chemical sedimentary rock formed by the evaporation of seawater and the precipitation of halite. Large enclosed bodies of seawater and desert (playa) lakes commonly form rock salt deposits.
  • Sediment type: chemical
  • Composition: halite
  • Grain Size: mostly fine grained
  • Common Sedimentary Environments: desert lakes and marine areas with high evaporation rates
  • Other Characteristics: halite composition, low hardness and soluble

    • ROCK GYPSUM

    Rock gypsum is a chemical precipitate formed by the evaporation of cencentrated solutions such as seawater. It ranges in texture from massive/granular to fibrous depending on the size and shape of the gypsum crystals.
  • Sediment type: chemical
  • Composition: gypsum
  • Grain Size: varible, commonly fibrous or granular and medium to fine grained
  • Common Sedimentary Environments: desert lakes and marine areas with high evaporation rates (often forms in same environment as rock salt)
  • Other Characteristics: low hardness and granular to fibrous textures

    • TRAVERTINE

    Travertine is calcite dripstone formed in caves. This chemical precipitate is recognized by its concentric layering and dripstone structure.
  • Sediment type: chemical
  • Composition: calcite
  • Grain Size: usually fine grained
  • Common Sedimentary Environments: caves
  • Other Characteristics: low hardness and reactivity to acid of calcite combined with the dripstone structure (stalagmites and stalactites) of the rock

    • KAOLIN

    Kaolin is a sedimentary rock composed of white kaolinite clay. This aluminum-rich rock is an aluminum ore commonly used in ceramics, paints, paper, and other products. Large valuable deposits of kaolin are located in central and southwest Georgia.
  • Sediment type: chemical/weathering product
  • Composition: aluminum-rich
  • Grain Size: very fine grained
  • Common Sedimentary Environments: areas of tropical or sub-tropical weathering and low energy deposition
  • Other Characteristics: white, slippery feel

    • BAUXITE

    Bauxite is a tropical or sub-tropical weathering product composed of several aluminum-rich clay minerals. A distinctive pisolitic texture (small equidimensional spheres) and common red-brown color distinguishes it from kaolin. It is the principle ore of aluminum.
  • Sediment type: chemical/weathering product
  • Composition: aluminum-rich
  • Grain Size: pisolites (gravel-size concretions)
  • Common Sedimentary Environments: areas of intense tropical or sub-tropical weathering
  • Other Characteristics: similar to conglomerate, but distinguished by the layered concretions (pisolites)

    • Part III: Biochemical (Biogenic) Sedimentary Rocks

    Classification of Biochemical Sedimentary Rocks
    Composition
    Calcite (CaCO3) Calcite (CaCO3) Carbonaceous
    Rock NameChalkFossiliferous LimestoneCoal

    CHALK

    Chalk is a biochemical sedimentary rock formed by the accumulation of microscopic marine calcite fossils. The fine-grained nature and white color of the calcite are used in the recognition of chalk.
  • Sediment type: biochemical
  • Composition: calcite
  • Grain Size: very fine grained
  • Common Sedimentary Environments: marine
  • Other Characteristics: low hardness and reactivity to acid of calcite combined with the white rock color. Individual shells are visible with a microscope.

    • FOSSILIFEROUS LIMESTONE

    Limestone is a biochemical rock formed by the accumulation and consolidation of macroscopic calcite fossils. Shell beds and reef deposits commonly form fossiliferous limestone deposits in areas of warm, shallow, clear, marine conditions.
  • Sediment type: biochemical
  • Composition: calcite
  • Grain Size: medium to coarse grained
  • Common Sedimentary Environments: warm, clear, shallow marine
  • Other Characteristics: low hardness and reactivity to acid of calcite combined with the presence of visible fossils

    • COAL

    Coal is a biochemical rock formed by the accumulation of plant material that is later buried and partially altered to a carbon-rich rock. Different grades of coal reflect differences in composition and degree of alteration.
  • Sediment type: biochemical
  • Composition: carbon-rich
  • Grain Size: very fine grained
  • Common Sedimentary Environments: swamps
  • Other Characteristics: black color, low density

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