Sainfeldite
Chemical formula: Ca₅(As⁵⁺O₄)₂(As⁵⁺O₃OH)₂·4H₂O
Sainfeldite is a rare, hydrated calcium arsenate, forming colorless or pale pink crystals, valued by collectors of rare minerals.
Properties
- Mohs hardness
- 4
- Luster
- Vitreous
- Streak
- White
- Density
- 3.04
- Fracture
- Irregular/Uneven
- Transparency
- Transparent,Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Sainfeldite can be identified by its characteristic, colorless or pale pink, prismatic crystals, often forming radial aggregates. Its relatively low hardness (4 on the Mohs scale), vitreous luster, and white streak are helpful diagnostic features. Its occurrence in association with other arsenates in the oxidation zones of ore deposits is also an important clue. ## Distinguishing from Similar Minerals It can be confused with other colorless secondary minerals, such as pharmacolite or gypsum. It is distinguished from gypsum by its significantly higher density and hardness (gypsum has a hardness of 2). Distinguishing it from pharmacolite, with which it often co-occurs, can be difficult without advanced analysis, although pharmacolite more often forms silky, fibrous aggregates. ## Crystal Forms Sainfeldite crystals are typically prismatic, elongated, or short-columnar, often terminated by flat faces. They commonly occur in radial or stellate aggregates, as well as crusts of fine, acicular crystals on the matrix rock.
Geological environment
## Genesis Sainfeldite is a secondary mineral, formed in the oxidation (weathering) zones of hydrothermal ore deposits that are rich in arsenic. It forms as a result of reactions between arsenic-rich solutions and calcium minerals or carbonate rocks. ## Mineral Associations This mineral often co-occurs with other arsenates, such as pharmacolite, picropharmacolite, rauenthalite, as well as with calcite, gypsum, and iron oxides (e.g., goethite). ## Localities The most important and classic locality for sainfeldite is the Gabe Gottes mine in Sainte-Marie-aux-Mines in the Grand Est region, France. It is also known from other localities worldwide, including the Tsumeb mine in Namibia, the Bou Azzer region in Morocco, and several places in Germany (e.g., in the Black Forest and the Ore Mountains).
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp, and transparent crystals, forming aesthetic, radial aggregates. A distinct, pale pink color enhances the attractiveness of the specimen, although it is rarer than the colorless variant. Contrast with the rock matrix and the absence of damage to the delicate crystals are also important. ## Popular Localities Specimens from the type locality – Sainte-Marie-aux-Mines, France – have particular historical value. High-quality crystals also come from the Tsumeb mine in Namibia, which was famous for its exceptional secondary minerals.
Care and storage
## Cleaning Sainfeldite specimens should be cleaned with the utmost care. For dust removal, it is best to use a soft brush or compressed air from a safe distance. If necessary, distilled water can be used, but prolonged soaking should be avoided. After wet cleaning, the specimen must be thoroughly dried. ## What to Avoid As an arsenate, sainfeldite is potentially toxic if ingested or if dust is inhaled, so situations leading to dusting should be avoided, and hands should always be washed after contact with the mineral. Contact with acids and other chemicals should be avoided. It is a relatively soft and brittle mineral, susceptible to mechanical damage, scratches, and impacts. ## Storage It is recommended to store sainfeldite in a separate, closed display box to protect it from dust, damage, and contact with other minerals. Exposure to moisture and sudden temperature changes should be avoided.