Ferrarisite
Chemical formula: Ca<sub>5</sub>(As<sup>5+</sup>O<sub>3</sub>OH)<sub>2</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>2</sub>·9H<sub>2</sub>O
Ferrarisite is a very rare, hydrated calcium arsenate, forming colorless, tabular crystals and aggregates, found in the oxidation zones of arsenic deposits.
Properties
- Mohs hardness
- 2
- Luster
- Vitreous
- Streak
- White
- Density
- 2.56
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Amateur identification of ferrarisite is practically impossible. It requires advanced analytical methods, such as X-ray diffraction (XRD) or Raman spectroscopy. Visually, it can be suspected based on characteristic, fan-shaped aggregates of tiny, colorless crystals, if it occurs in association with other rare arsenates. ## Distinguishing from similar minerals Ferrarisite is difficult to distinguish from many other secondary arsenates, such as pharmacolite, picropharmakolite, or guerinite, with which it often coexists. Guerinite is a product of its dehydration and can form pseudomorphs after ferrarisite. Definitive differentiation is only possible using laboratory methods. ## Crystal forms Crystals are tabular, elongated, and flattened. They usually form fan-shaped, radial, or spherulitic aggregates. Less commonly, they occur as rosettes or isolated, bladed crystals on the surface of the host rock.
Geological environment
## Genesis Ferrarisite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits rich in arsenic, especially in the presence of carbonate rocks (limestones, dolomites), which provide calcium. It forms as a result of the reaction of solutions rich in arsenic acid with calcium minerals under low-temperature conditions. ## Mineral associations It most often coexists with other arsenates, such as pharmacolite, picropharmakolite, sainfeldite, rauenthalite, guerinite, as well as with gypsum, calcite, and native arsenic. ## Localities The most important and well-documented ferrarisite localities worldwide include: - Gabe Gottes mine, Sainte-Marie-aux-Mines, Grand Est, France (type locality). - Anton mine, Heubach, Schiltach, Black Forest, Germany. - Jáchymov (St. Joachimsthal) in the Czech Republic.
Rarity
Very rare
For collectors
## Quality criteria The most prized ferrarisite specimens are those that possess well-formed, distinct fan-shaped or radial aggregates, deposited on a contrasting rock matrix. Due to the mineral's colorless nature, the form and structure of the aggregates are crucial. The size of the crystals, even if they only reach 1-2 mm, significantly increases the value of the specimen. Association with other rare, colored arsenates is also important. ## Popular localities Specimens from the type locality (Sainte-Marie-aux-Mines) and from the mine in the Black Forest (Germany) are most sought after by collectors specializing in rare minerals or systematics.
Care and storage
## Cleaning Ferrarisite specimens should be cleaned with utmost care, using only a soft brush to remove dust. The use of water, and especially ultrasonics, is absolutely inadvisable due to its solubility and delicacy. ## What to avoid Avoid contact with water and all chemicals. The mineral is unstable and can undergo dehydration (loss of water) in dry air, leading to its disintegration or transformation into guerinite. It should be protected from changes in humidity, high temperatures, and direct sunlight. ## Storage It is recommended to store specimens in tightly sealed containers (e.g., "membrane box" type or in humidity-controlled boxes) to prevent dehydration. Store in stable temperature conditions, away from heat sources and light.