Formula: Cu6(SO4)(OH)10(H2O)4.H2O
Hydrated sulphate
Specific gravity: 3.00 measured, 2.946 calculated from empirical formula
Streak: Light blue
Colour: Blue

Hydrothermal environments

Isseleite is a relatively new mineral, approved in 2019 and to date (May 2022) reported only from the type locality.


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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