Legrandite

Legrandite is a rare zinc arsenate mineral, Zn₂(AsO₄)(OH)·(H₂O).

Legrandite
General
Category	Arsenate minerals
Formula	Zn2(AsO4)(OH)·(H2O)
IMA symbol	Leg[1]
Strunz classification	8.DC.10
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 12.805(2), b = 7.933(1) c = 10.215(2) [Å]; β = 104.23°; Z = 8
Identification
Color	Bright yellow, wax-yellow, colorless
Crystal habit	Crystalline, prismatic, typically in sprays or sheaflike aggregates
Cleavage	Imperfect, poor one {100}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	4.5–5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	3.98–4.01
Optical properties	Biaxial (+)
Refractive index	nα = 1.675 – 1.702 nβ = 1.690 – 1.709 nγ = 1.735 – 1.740
Birefringence	δ = 0.060
Pleochroism	X = Y = Colorless to yellow Z = Yellow
2V angle	Measured: 50°
References	[2][3][4]

It is an uncommon secondary mineral in the oxidized zone of arsenic bearing zinc deposits and occurs rarely in granite pegmatite. Associated minerals include: adamite, paradamite, köttigite, scorodite, smithsonite, leiteite, renierite, pharmacosiderite, aurichalcite, siderite, goethite and pyrite.^[2][3] It has been reported from Tsumeb, Namibia; the Ojuela mine in Durango, Mexico and at Sterling Hill, New Jersey, US.^[2]

It was first described in 1934 for an occurrence in the Flor de Peña Mine, Nuevo León, Mexico and named after Louis C.A. Legrand, a Belgian mining engineer.^[3]