Unweathered granite contains these minerals:
1. Na plagioclase feldspar (white)
2. K feldspar (pink, but may be white in other granites)
3. Quartz (gray)
4. Small amounts of biotite and/or amphibole (black)
5. and sometimes muscovite (not shown)
Here is what will happen to each of the mineral constituents in a granite under warm, humid weathering conditions:
1. The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions
2. The sodium and potassium ions will be removed through leaching and will be carried in solution in running water
3. The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides.
4. The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering.
2. The sodium and potassium ions will be removed through leaching and will be carried in solution in running water
3. The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides.
4. The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering.
Under warm, humid conditions, the granite bedrock will weather in place until the feldspars alter to soft clay. The weathered rock is called saprolite, a term meaning "rotten rock". In areas of the southeastern U.S. which are underlain by granite (and other igneous and metamorphic rocks), a thick soil zone of weathered rock or saprolite has developed. Where the bedock contained iron-bearing minerals (such as biotite, amphibole, or pyroxene) which weathered to iron oxides, the saprolite has been stained a deep red color. (This is the same principle by which one red sock in a load of laundry can stain all of the clothes red.) The red-stained clay in the saprolite has given rise to the famous Georgia Red Clay
What happens after the rock has been weathered to saprolite?
1.
The clays will be eroded and transported by running water to the sea. Clay is fine-grained and remains suspended in the water column. The clay may ultimately be deposited in deep quiet water far from shore.
2. As the soft clay is removed, the unweathered, residual quartz grains will be released from the saprolite by erosion. The quartz in granite is sand-sized, and it becomes quartz sand. The quartz sand is ultimately transported to the sea, where it accumulates to form beaches.
3. The dissolved ions (sodium and potassium) will be transported by rivers to the sea, and will become part of the salts in the sea.
2. As the soft clay is removed, the unweathered, residual quartz grains will be released from the saprolite by erosion. The quartz in granite is sand-sized, and it becomes quartz sand. The quartz sand is ultimately transported to the sea, where it accumulates to form beaches.
3. The dissolved ions (sodium and potassium) will be transported by rivers to the sea, and will become part of the salts in the sea.
