Silicates: Smectite

Composition: (1/2Ca,Na)_0.7(Al,Mg,Fe)₄(Si,AL)₈O₂₀(OH)₄. nH₂O

Crystal system: Monoclinic; usually as earthy masses

5.26.1
smectite aggregates
Rock River WY USA

Properties:

• Earthy to greasy lustre
• Variable color; grey, white, yellow, greenish or brownish
• White streak
• Fingernail scratches easily (Mohs Hardness 2-2.5)
• Below average to average density
• Cleavage not usually apparent
• Smooth fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl) but swells and becomes gelatinous in water

Occurrence:

Clay minerals including the smectites form by the weathering or hydrothermal alteration of feldspars and other aluminium silicates and thus occur in weathering profiles and finegrained clastic sedimentary rocks. The soapy feel of smectites and property of swelling in water are diagnostic; however, specialised characterisation may be required to distinguish among the clay minerals.

5.26.2
smectite aggregates
Rock River WY USA

The smectite clays are important industrial minerals used mainly in the production of drilling mud for the oil and mineral exploration industries.