Garnierite

Garnierite is not a single mineral species but a field and trade term for a family of soft, green, hydrous nickel-magnesium silicates that occur intermixed in nickel laterite deposits. Members include népouite, pimelite, nepouite and nickel-rich serpentines and talcs.

Its striking apple- to emerald-green color comes directly from nickel substituting for magnesium in serpentine- and talc-group structures. The material is usually massive, earthy, or claylike rather than well-crystallized.

First described from New Caledonia, garnierite remains one of the world's most economically important sources of nickel, particularly from tropical weathering profiles.

Garnierite forms by the deep chemical weathering (lateritization) of nickel-bearing ultramafic rocks such as peridotite and dunite in warm, humid, tropical climates. As rainwater percolates through the rock, magnesium and silica are leached and nickel is concentrated in a lower zone where it precipitates as hydrous nickel silicates.

These supergene deposits, called nickel laterites or saprolite ore, develop over long timescales and can be tens of meters thick. Major producing regions include New Caledonia, Indonesia, the Philippines, Cuba, the Dominican Republic, and Brazil.

Because it is a mixture, garnierite typically fills cracks and coats serpentinite as green veinlets, crusts, and earthy masses.

Look for an intense apple-green to emerald-green color, soft enough to be scratched by a knife or even a fingernail (Mohs 2-4). It is usually massive, earthy, or claylike, often as green veins and coatings on dark serpentinite.

Streak is pale green to whitish. Luster is dull, earthy, to faintly waxy or pearly. It is greasy or soapy to the touch and may cling to the tongue when dry.

Look-alikes include chrysocolla (harder, more blue), malachite (effervesces in acid, banded), and green serpentine (harder, less vivid). The association with weathered ultramafic rock and very low hardness helps confirm garnierite.

Garnierite's main importance is as an ore of nickel, supplying metal for stainless steel, alloys, plating, and rechargeable batteries. Nickel laterite ores containing garnierite account for a large share of global nickel production.

Because it is soft and not durable, garnierite is rarely used in jewelry, though attractive green specimens are collected and occasionally cut into cabochons or carved. It is sometimes marketed under trade names within the lapidary world.

Metaphysically it is associated with heart-centered healing and growth, though such claims are not scientific. Its chief value remains industrial and as a mineralogical curiosity.