Granite

Unweathered granite contains the following minerals:

  • Sodium Plagioclase feldspar (Na feldspar)
  • Potassium feldspar (K feldspar)
  • Quartz
  • Accessory biotite, amphibole, or muscovite

What happens when granite is weathered?

  • Na feldspar and K feldspar undergo hydrolysis to form kaolinite (clay) and Na + and K + ions.
  • The quartz (and muscovite if present) remain as residual minerals due to their high weathering resistance.
  • Biotite and/or amphibole undergo hydrolysis to form clay, and oxidation to form iron oxides.
  • The weathered rock fragments become constituents of the soil.

What happens next?

  • Quartz grains may then be eroded and become sediment. The quartz in granite is sand-sized and therefore forms quartz sand. This quartz sand will ultimately be transported to the sea (as bed load in a river) where it will accumulate to form beaches.
  • Clays will ultimately be eroded and washed out to sea by rivers. Clay is fine-grained and remains as suspended load in the water column so it may be deposited in quiet water.
  • Dissolved ions will be transported by rivers to the sea in the dissolved load, and will become part of the salts in the sea.

    granite.gif