This reaction occurs when the rain  is slightly acidic. In this process water is required, it occurs rapidly if the temperature is high, and it is the first  stage in production of soil.
there are different  types of chemical weathering, some major processes are:
a) Removal of rock by acidic rain water. Limestone is weathered with the same process,
b) the breakdown of rocks by acidic water to form clay, and
c) the break down of rocks by oxygen and water to form iron rich rocks.