Sauconite (reddish brown)
CategoryPhyllosilicate mineral
(repeating unit)
IMA symbolSau
Strunz classification9.EC.45
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
ColorReddish brown, brown, brownish yellow, mottled
Crystal habitClayey; as small micaceous plates in laminated to compact masses
CleavagePerfect on {001}
Mohs scale hardness1–2
Specific gravity2.45
Optical propertiesBiaxial (−)
Refractive indexnα = 1.550 – 1.580 nβ = 1.590 – 1.620 nγ = 1.590 – 1.620
Birefringenceδ = 0.040
Dispersionr > v strong

Sauconite is a complex phyllosilicate mineral of the smectite clay group, formula Na0.3Zn3(SiAl)4O10(OH)2·4H2O. It forms soft earthy bluish white to red-brown monoclinic crystals typically massive to micaceous in habit. It has a Mohs hardness of 1 to 2 and a specific gravity of 2.45. Optically it is biaxial positive with refractive index values of nα = 1.550 – 1.580, nβ = 1.590 – 1.620 and nγ = 1.590 – 1.620. It is found in vugs and seams in the oxidized zones of zinc and copper deposits. It occurs in association with hemimorphite, smithsonite, chrysocolla, coronadite and various iron oxides.

It was named for the Saucon Valley in the Lehigh Valley region of eastern Pennsylvania, where it was originally discovered in 1875.

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