Legrandite

legrandite

leiteite

scorodite

kottigite

Images

Formula: Zn2(AsO4)(OH).H2O
Hydrated arsenate containing hydroxyl
Specific gravity: 3.98 to 4.01
Hardness: 4½
Streak: White
Colour: Yellow to colourless; pale yellow to colourless in transmitted light.
Environments

Pegmatites rarely
Hydrothermal environments

Legrandite is an uncommon secondary mineral occurring in the oxidised zones of arsenic and zinc bearing deposits, and, rarely, in granite pegmatites (Mindat, Webmin, HOM). Common associates include adamite, limonite, paradamite, scorodite and smithsonite (Mindat).

Localities

At the Inglaterra Mine, Francisco Portillo, West Camp, Santa Eulalia Mining District, Aquiles Serdán Municipality, Chihuahua, Mexico, legrandite crystals to 1.2 cm have been found partially coated by gypsum (R&M 95.4.367).

The Ojuela mine, Durango, Mexico, is the source of some of the world's finest legrandite specimens; the mineral may occur as isolated crystals, or associated with adamite, paradamite, köttigite, ojuelaite, parasymplesite, pharmacosiderite, scorodite and smithsonite. Legrandite occurs on a vuggy gossan matrix, commonly with tiny adamite crystals in the vugs; mixed iron oxide pseudomorphs after legrandite are common. (HOM, R&M 95.4.365-367). Goethite pseudomorphs after legrandite crystals to 12 cm long sprinkled with adamite or paradamite have been found, as well as adamite balls with hollow internal molds of legrandite crystals to 3 cm long, which have dissolved away (R&M 95.4.367).

At the type locality, Flor de Peña Mine, Nuevo Leon, Mexico, legrandite occurs in the oxidised zone of an arsenic-bearing zinc deposit, associated with pyrite, and with siderite and mimetite on massive sphalerite (Dana, Mindat). Legrandite crystals to 3.5 cm have been found on smithsonite, and microcrystals of anglesite, cerussite, aurichalcite and hemimorphite have been reported as associated species (R&M 95.4.362-365).

At Tsumeb, Namibia, legrandite is associated with leiteite, smithsonite adamite, paradamite and reinerite (HOM, R&M 95.4.367).

At Sterling Hill, New Jersey, USA, legrandite occurs in willemite-rich ore associated with köttigite, adamite, pharmacosiderite and scorodite (HOM, R&M 95.4.367).

Alteration

legrandite to adamite and H2O
Zn2AsO4(OH). H2O (s) ⇌ Zn2AsO4(OH) (s) + H20(l)
Legrandite is a very much rarer mineral than adamite and it is suggested that it is in fact metastable with respect to adamite at room temperature, although it may crystallise as the stable phase under appropriate conditions at temperatures between 0 and 25oC. It is of interest that legrandite has been found at several localities without adamite (MM 52.685).

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