Cabrerite
Chemical formula: Ni<sup>2+</sup>Mg<sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>·8H<sub>2</sub>O
Cabrerite is a magnesian variety of annabergite, a rare nickel and magnesium arsenate, forming characteristic green coatings and crusts.
Properties
- Mohs hardness
- 1.5-2.5
- Luster
- Vitreous, Pearly, Earthy
- Streak
- White to pale green
- Density
- 3.0-3.3
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Transparent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Key diagnostic features include its apple-green color combined with very low hardness and occurrence as coatings or fine acicular crystals in the oxidation zones of nickel deposits. It is often accompanied by other arsenates. ## Distinguishing from Similar Minerals It is impossible to distinguish from annabergite without chemical analysis confirming a significant magnesium content. It is distinguished from erythrite (pink to purple) by its color. Malachite, although sometimes green, is harder and reacts vigorously with hydrochloric acid, unlike cabrerite. Other green secondary minerals, such as garnierite, have different formation environments and are usually harder. ## Crystal Forms Crystals, if well-formed, are monoclinic, very small, and acicular or fibrous. They usually form radial or tangled aggregates, and most commonly occur as earthy and powdery coatings and crusts.
Geological environment
## Genesis Cabrerite is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal ore deposits rich in nickel and arsenic minerals, such as niccolite or gersdorffite. Its formation requires the presence of magnesium in the surrounding rocks of the deposit, for example, in dolomites or serpentinites, from which it is leached by circulating waters in the rock mass. ## Mineral Associations It most commonly co-occurs with annabergite, erythrite (the cobalt analogue of annabergite), pharmacolite, gypsum, calcite, as well as primary nickel and arsenic ore minerals. ## Localities The most important localities for specimens historically identified as cabrerite are Sierra Cabrera in Spain and mines in the Laurion area in Greece, where magnesian varieties of annabergite occurred. It is also found in some nickel deposits in Germany (Saxony) and Morocco.
Rarity
Rare
For collectors
## Quality Criteria Specimens with well-formed, even microscopic, crystals forming fluffy or radial aggregates are most valued by collectors. An intense, apple-green color is a key factor enhancing attractiveness. Rich crusts on contrasting rock matrix and associations with other rare secondary minerals, e.g., erythrite, are also highly regarded. ## Popular Localities The classic localities from which the best specimens of magnesian annabergite (cabrerite) originate are considered to be Laurion, Greece, and, to a lesser extent, the historical locality in Sierra Cabrera, Spain. Specimens from these locations have the greatest value for collectors due due to their historical significance and quality.
Care and storage
## Cleaning Cabrerite specimens are extremely delicate and sensitive. They should only be cleaned dry, using a very soft brush to remove dust. Blowing off dust with compressed air from a distance is also permissible. Absolutely avoid contact with water and any chemical agents. ## What to Avoid This mineral is sensitive to moisture, which can lead to its slow decomposition. Acids and detergents should be avoided. As a hydrated arsenate, it is also sensitive to high temperatures, which can cause water loss and structural damage. Due to its arsenic content, it is toxic – avoid inhaling dust and wash hands after any contact with the specimen. ## Storage It is best to store specimens in closed, padded boxes or display cases to protect them from mechanical damage, dust, and sudden changes in humidity. Storing at a stable room temperature, away from direct sunlight, will ensure the preservation of its form and color.