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Geosciences LibreTexts

2.2: Sedimentary rocks

  • Page ID
    3173
  • Basic

    The sedimentary cycle is the second largest cycle in mineral and rock formation. Sedimentary rocks are formed by erosion, transport in rivers, ice etc. and involve the decay and disintegration of a preexisting rock mass. Usually, there are no new minerals formed, only found.

    When these particles eventually settle, they form alluvial gravels, sands or clays. When they are either cemented or compressed, then they form sedimentary rocks such as conglomerations, sandstones, and limestones.

    Chemical action in the environment leads to some material dissolving in water. Eventually, the water may evaporate and deposits of borax and other salt "evaporates" may form this way.

    Plant and animal remains are commonly incorporated among the rock fragments and these may be preserved as fossils.

    Many gemstones are found in "alluvial deposits". These deposits have their origin in the destruction of the original rocks and the resulting materials by rivers, floods, and glacial movement. During this movement, the heavier minerals tend to remain relatively close to the source, whilst lighter minerals are carried further away.

    The heavier and harder materials do not wear as much as the lighter ones and tend to retain more of their crystal shape. Stones such as sapphire and topaz do not show as much abrasion as softer minerals like quartz.

    However, due to the continuous grinding and tumbling over a period of time, a large number of gem minerals are found as rounded "water-worn" pebbles. The gem gravels in Sri Lanka contain a wide variety of such minerals.

    Because of their supreme hardness and density, many diamonds survive the sedimentary processes and are frequently found in alluvial deposits.

    Minerals found in sedimentary rocks:

    • Beryl
    • Opal
    • Quartz
    • Turquoise
    • Malachite
    • Azurite
    • Chrysoprase
    • Chrysocolla