From Feldspar to Kaolin: Understanding Chemical Weathering and Kaolinite Formation

By Iain McLean, updated Aug 30, 2022

Feldspar: The Cornerstone of Granite

Feldspar makes up roughly 60 % of granite, monzonite, and syenite, imparting the classic porphyritic texture of these igneous rocks. It is an aluminosilicate mineral that exists in two primary variants: plagioclase (a sodium–calcium series) and orthoclase (potassium‑rich).

Plagioclase vs. Orthoclase

• Plagioclase – A sodium or calcium aluminosilicate. Under a polarizing microscope, plagioclase crystals exhibit triclinic symmetry and tend to persist longer during weathering.
• Orthoclase – A potassium‑bearing silicate that crystallizes in a monoclinic system. Orthoclase is more chemically reactive and weathers more rapidly.

Hydrolysis: The Key Weathering Mechanism

Feldspar is chemically stable deep underground but becomes vulnerable when exposed to the Earth's surface. Hydrolysis—the reaction of a water molecule with feldspar ions—releases hydrogen ions that subsequently combine with silica, producing the clay mineral kaolinite.

Kaolinite: The Resulting Clay Mineral

Kaolinite is an aluminum silicate hydroxide that appears as a white or gray clay. Its precise composition depends on the original feldspar type: sodium, calcium, or potassium ions are leached into solution and incorporated into the kaolinite structure.

Kaolin’s Historical and Industrial Significance

The name “kaolin” originates from a region in China where the mineral was first discovered. For over a millennium, this fine, white clay has been essential to porcelain and china manufacture. In addition, potassium feldspar serves as a key component in glass production.