Annabergite: Mineral Information & Facts
Crystals are minute and capillary and rarely met with, the mineral
occurring usually as soft earthy masses and encrustations. A fine
apple-green color is its characteristic feature.
Annabergite has a wonderful, bright green color. This characteristic color is easily noticeable and was used to spot veins of nickel-bearing ore. Annabergite, or "Nickel Bloom" as it is called by miners, is a weathering product of nickel-containing minerals such as niccolite, NiAs.
Where weathered cobalt and nickel ores are found, both erythrite, and annabergite are important markers. Erythrite, called "Cobalt Bloom", is bright red-purple and is isostructural with annabergite. Isostructural means that the two minerals have the same structure but different chemistries. The two minerals are actually in a series where the nickel and cobalt ions can substitute freely for each other. When the two ions are in near-equal concentrations, the resulting color is gray or off-white.
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| Radial Annabergite Crystals, From: Lavrio; Greece Photo Credit: László Tóth |
Where weathered cobalt and nickel ores are found, both erythrite, and annabergite are important markers. Erythrite, called "Cobalt Bloom", is bright red-purple and is isostructural with annabergite. Isostructural means that the two minerals have the same structure but different chemistries. The two minerals are actually in a series where the nickel and cobalt ions can substitute freely for each other. When the two ions are in near-equal concentrations, the resulting color is gray or off-white.
A third mineral is also isostructural with annabergite and is called kottigite or koettigite, However, the series is incomplete, meaning that the zinc and nickel ions only substitute for each other on a very limited basis. Most annabergite is found in a crust or powder form, but a few specimens from Greece show small, well-shaped crystals. This mineral, when found in fine green crystals, can be rather attractive.
Properties of Annabergite
- Formula: Ni3(AsO4)2·8H2O
- Color is bright apple green to pale green or gray in massive and thin crust forms.
- Luster is vitreous to adamantine to dull in massive or powdered forms.
- Transparency: Crystals are transparent to translucent.
- Crystal System is monoclinic; 2/m
- Crystal Habits include flattened striated blades or radiating accicular crystals, but large crystals are extremely rare. More commonly as crusts or powdery masses.
- Cleavage is perfect in one direction.
- Fracture is flaky.
- Hardness is 1.5 - 2.5
- Specific Gravity is approximately 3.0 - 3.1 (average for translucent minerals)
- Streak is pale green or gray, in gray specimens.
- Associated Minerals are niccolite, skutterudite, gersdorffite and erythrite.
- Other Characteristics: Thin flakes are flexible.\
- Notable Occurrences include Cobalt, Ontario; Lavrion, Greece; Humboldt, Nevada, USA; Sierra Cabrera, Spain and Germany.
- Best Field Indicators are color, softness, associations, and flexible crystals.