Legrandite

Legrandite
Yellow radiating prismatic crystals on limonite, from Ojuela Mine, Mun. de Mapimí, Durango, Mexico. Large crystal is 1.4 cm. long.
General
Category	Arsenate minerals
Formula (repeating unit)	Zn2(AsO4)(OH)·(H2O)
Strunz classification	8.DC.10
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) H-M symbol: (2/m)
Space group	Monoclinic 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

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

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, kottigite, scorodite, smithsonite, leiteite, renierite, pharmacosiderite, aurichalcite, siderite, goethite and pyrite. It has been reported from Tsumeb, Namibia; the Ojuela mine in Durango, Mexico and at Sterling Hill, New Jersey, US.

It was first described in 1934 for an occurrence in the Flor de Peña Mine, Nuevo Leon, Mexico and named after M. Legrand, a Belgian mining engineer .

References

Radiating cluster of legrandite crystals to nearly 4 cm, on limonite matrix. Ojuela mine, Mun. de Mapimí, Durango. Size: 6 x 3.8 x 3 cm.