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Formula: Zn2(AsO4)(OH).H2O
Hydrated arsenate containing hydroxyl
Crystal System: Monoclinic
Specific gravity: 3.98 to 4.01 measured, 4.015 calculated
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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