garnierite

Garnierite

Garnierite is a general name for a green nickel ore which is found in pockets and veins within weathered and serpentinized ultramafic rocks. The name was given by Jules Garnier who first described it 1864 for an occurrence in New Caledonia. It forms by lateritic weathering of ultramafic rocks and occurs in many nickel laterite deposits in the world.

Garnierite is not a single mineral but a mixture of the Ni-Mg-hydrosilicates serpentine, talc, sepiolite, chlorite and smectite. These minerals occur in garnierite ores individually and in intimate mixtures.

A detailed study of the garnierite ores from Falcondo Mine in the Loma Caribe serpentinized peridotites of the Dominican Republic showed that the garnierites have compositions within the three series:^[1]

• Ni-bearing talc - willemseite (up to 25 wt% Ni)
• Ni-lizardite - népouite (up to 34 wt% Ni)
• Ni-sepiolite - falcondoite (up to 24 wt% Ni)

The lateritization of ultramafic rocks gives rise to a strong dissolution and removal of magnesium and silicium which leads to a strong residual concentration of iron and nickel in a goethite-rich surface layer (nickel limonite ore). A portion of the nickel is leached downwards and finally fixed in the underlying decomposed ultramafic rock. This process gives rise on the one hand to a moderate nickel increase of the total decomposed rock (nickel silicate, nickel saprolite); on the other hand relatively small amounts of nickel-rich garnierite ore are precipitated in hollow spaces.

References

• Brindley, G.W. and P. T. Hang (1973) The nature of garnierite. structure, chemical composition and color characteristics, Clays and Clay Mineral. 21, 27-40.