Garnierite is the name for a green nickel ore which is found in pockets and fissures of weathered ultramafic rocks (serpentinite, dunite, peridotite). The name was given by Jules Garnier who first discovered it 1864 in New Caledonia. It forms by lateritic weathering of ultramafic rocks and occurs in many nickel laterite deposits in the world.
Garnierite consists of the Mg-rich phyllosilicates serpentine, talc, chlorite and smectite in which a high percentage of magnesium is substituted by nickel. These minerals occur in garnierite ores as well individually as in intimate mixtures. They contain mostly 20-40 % 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 (formation of nickel silicate ore); on the other hand relatively small amounts of nickel-rich garnierite ore are precipitated in hollow spaces. Today, nickel silicate ores with high garnierite amounts are depleted; the economic relevance of garnierite is therefore relatively small.