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Formula: Cu6(SO4)(OH)10(H2O)4.H2O
Hydrated sulphate
Crystal System: Orthorhombic
Specific gravity: 3.00 measured, 2.946 calculated from empirical formula
Streak: Light blue
Colour: Blue
Environments
Isseleite is a relatively new mineral, approved in 2019 and to date (January 2023) reported only from the type
locality.
Localities
At the type locality, the Lagoscuro mine, Case Lagoscuro, Livellato, Ceranesi, Genoa, Liguria, Italy,
isselite occurs as a secondary phase in the
iron-nickel-cobalt
sulphide mineralisation. This mine operated from 1815 to 1825, mainly for magnesium sulphate. The
cobalt-rich pyrrhotite +
pentlandite +/- sphalerite
sulphide ore was hosted within serpentinite and
serpentinite breccia
in contact with basaltic rocks. It underwent multi-stage alteration
processes and hydrothermal mobilisation, which developed a mineral assemblage formed of
violarite + valleriite +
andradite +/- magnetite, along
with other nickel-cobalt
secondary minerals such as
pecoraite, heterogenite
and asbolane, and hydrated
magnesium-copper-(nickel) carbonates
and sulphates such as malachite,
brochantite, langite,
posnjakite and
hydromagnesite. Indications are that the first stage of hydrothermal
alteration occurred at less than 210°C under strongly acidic to neutral (1 < pH < 6) and reducing conditions.
Isselite was identified in only a few specimens as sprays of blue acicular crystals up to 0.1 mm long. It is a
late-stage, secondary mineral that crystallised from a
low-temperature, aqueous solution. In the holotype material, it is associated closely with
brochantite and posnjakite.
Other associated minerals are pyrrhotite,
pentlandite, allophane,
chrysocolla and langite. The
source of the copper in isselite may be
valleriite, along with minor
chalcopyrite, covellite and
digenite. These sulphides are widespread in the ore deposit, though not in
close association with isselite in the samples studied
(MM, 84.5.653-661).
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