Silicates: Kaolinite

Composition: Al₄Si₄O₁₀(OH)₈

Crystal system: Triclinic; usually as earthy masses

5.14.1
earthy kaolinite mass

Properties:

• Earthy to silky lustre
• White color; sometimes brownish
• White streak
• Fingernail scratches easily (Mohs Hardness 2.5)
• Average density
• Cleavage not usually apparent
• Smooth fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Occurrence:

Clay minerals including kaolinite form by the weathering or hydrothermal alteration of feldspars and other aluminium silicates and thus occur in weathering profiles and finegrained clastic sedimentary rocks. Kaolinite may also infill porosity in sedimentary rocks. Specialised characterisation may be required to distinguish among the clay minerals.

Kaolinite is an important industrial mineral used in the manufacture of ceramics, paper and paint and as a catalyst in other manufacturing and refining.