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Formula: Cu3O[AsO3(OH)]2.H2O
Hydrated arsenate of copper
Crystal System: Monoclinic
Specific gravity: About 4.3 measured, 4.385 calculated for the empirical formula, 4.397 calculated for the ideal formula
Streak: Yellowish green
Colour: Dark pistachio-green
Luminescence: No fluorescence under UV
Solubility: Soluble in dilute hydrochloric acid
Environments
Lapeyreite was approved in 2008, but to date (December 2025) it has been reported only from the type locality.
Localities
At the type locality, the Roua Mines, Daluis, Nice Arrondissement, Alpes-Maritimes, Provence-Alpes-Côte d'Azur, France,
the mineralisation is hosted by subvertical dolomite and
calcite veinlets containing accessory
aragonite, and consists of
native copper, cuprite,
domeykite, algodonite,
koutekite and native silver. The
thickness of the veinlets varies from a few millimetres to a few centimetres. The lapeyreite occurs in small geodes
of cuprite (0.5 mm across) in association with
trippkeite (always present),
olivenite, malachite,
gilmarite, cornubite,
connellite, theoparacelsite,
brochantite, native copper,
algodonite and domeykite. In this
paragenesis, the crystallisation sequence of theoparacelsite and
lapeyreite is probably as follows:
Oxidation of native copper produced
cuprite; then, reaction of copper with
solutions enriched in arsenous acid (H3AsO3) produced
trippkeite precipitated on the earlier-formed
cuprite. Subsequent oxidation of arsenous acid produced arsenic acid
(H3AsO4) that also reacted with copper, yielding crystals of
lapeyreite. In the material studied, olivenite is one of the earliest
secondary minerals to crystallise.
Olivenite crystals are sometimes
pseudomorphed by
theoparacelsite, but never by lapeyreite, which is a more hydrated
phase than is theoparacelsite. These paragenetic relations suggest that
theoparacelsite crystallised before lapeyreite.
Lapeyreite occurs as aggregates formed by perfect elongate rectangular crystals up to
0.2 × 0.05 × 0.01 mm3 in size, acicular fibrous crystals, and powdery masses. Macroscopically,
lapeyreite is dark pistachio-green with a vitreous to adamantine lustre. It is translucent in aggregates and
transparent in thin fragments. The streak is yellowish green. Lapeyreite is brittle and has conchoidal fracture,
one perfect cleavage and one good cleavage
(AM 95.1.171-176).
Lapeyreite from the Roua Mines - Image
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