Fraipontite

Fraipontite is a zinc aluminium silicate mineral with a formula of (Zn,Al)₃(Si,Al)₂O₅(OH)₄.^[2][3]

Fraipontite
Pockets of minute pearly white fraipontite crystals with green smithsonite from Laurium, Greece (size: 1.4 x 1.0 x 0.9 cm)
General
Category	Phyllosilicate
Formula (repeating unit)	(Zn,Al)3(Si,Al)2O5(OH)4
IMA symbol	Fpt[1]
Strunz classification	9.ED.15
Crystal system	Monoclinic
Crystal class	Domatic (m) (same H-M symbol)
Space group	Cm
Unit cell	a = 5.34, b = 9.21 c = 14.12 [Å]; β = 93.2°; Z = 2
Identification
Color	Blueish, yellow white light green
Crystal habit	Fibrous to porcelaneous massive
Mohs scale hardness	3.5 – 4
Luster	Silky
Streak	White to pale green
Diaphaneity	Opaque to translucent
Specific gravity	3.08 – 3.10
Optical properties	Biaxial (-)
Refractive index	nα = 1.620 nβ = 1.624 nγ = 1.624
Birefringence	δ = 0.004
2V angle	Measured: 15° to 20°
References	[2][3][4]

It is a member of the kaolinite-serpentine mineral group and occurs as an oxidation product of zinc deposits. It occurs with smithsonite, gebhardite, willemite, cerussite and sauconite.^[2]

It was first described in 1927 for an occurrence in Vieille Montagne, Verviers, Liège Province, Belgium.^[3] It was named for Julien Jean Joseph Fraipont (1857–1910), and Charles de Fraipont, geologists of Liege, Belgium.^[4] In addition to the type locality in Belgium, it has been reported from Tsumeb, Namibia; Laurium, Greece; Swaledale, North Yorkshire, England; the Silver Bill mine, Cochise County, Arizona, the Blanchard Mine, Socorro County, New Mexico and the Mohawk mine, San Bernardino County, California in the US; and from the Ojuela mine, Mapimi, Durango, Mexico.^[2]

A synonym of the fraipontite is the zinalsite, which was reported in 1956 for an occurrence in Kazakhstan.^[5][6]