Sauconite

Sauconite is a complex phyllosilicate mineral of the smectite clay group, formula Na_0.3Zn₃(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 n_α = 1.550 - 1.580, n_β = 1.590 - 1.620 and n_γ = 1.590 - 1.620.[1][3] 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.

Sauconite
Sauconite (reddish brown)
General
Category	Phyllosilicate mineral
Formula (repeating unit)	Na0.3Zn3(SiAl)4O10(OH)2·4H2O
Strunz classification	9.EC.45
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
Color	Reddish brown, brown, brownish yellow, mottled
Crystal habit	Clayey; as small micaceous plates in laminated to compact masses
Cleavage	Perfect on {001}
Mohs scale hardness	1 - 2
Luster	dull
Diaphaneity	Translucent
Specific gravity	2.45
Optical properties	Biaxial (-)
Refractive index	nα = 1.550 - 1.580 nβ = 1.590 - 1.620 nγ = 1.590 - 1.620
Birefringence	δ = 0.040
Dispersion	r > v strong
References	[1][2][3]

It was named for the Saucon Valley in Pennsylvania, where it was originally discovered in 1875.[2]