Coquandite
Chemical formula: Sb³⁺₆+xO₈+x(S⁶⁺O₄)(OH)x(H₂O)₁-x (x = 0.3)
Coquandite is a rare, secondary antimony mineral, forming small, platy crystals with a pearly luster, found in the oxidized zones of antimony deposits.
Properties
- Mohs hardness
- 3-4
- Color
- Colorless
- Luster
- Pearly
- Streak
- White
- Density
- 0
- Cleavage
- Perfect on {0001}
- Fracture
- Uneven
- Transparency
- Transparent,Translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification Field identification of coquandite is practically impossible due to its rarity and small crystal size. Recognition requires advanced laboratory methods such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). In a collection, it can be preliminarily identified based on the characteristic form of small, colorless, platy crystals with a pearly luster, occurring in association with other secondary antimony minerals. ## Distinguishing from Similar Minerals Coquandite can be confused with other secondary antimony minerals, such as senarmontite, valentinite, or peretaite, with which it often co-occurs. Distinguishing it from these minerals without specialized analysis is extremely difficult. Peretaite forms similar aggregates, but its crystals are usually acicular, not platy. ## Crystal Forms Coquandite crystallizes as very small, thin, platy crystals with a hexagonal outline. These crystals often form rosette-like or radial aggregates, as well as thin coatings on the surface of the host rock.
Geological environment
## Genesis Coquandite is a secondary mineral, formed in the oxidation zones of hydrothermal antimony ore deposits. It forms as a result of the weathering of primary antimony sulfides, mainly stibnite (Sb₂S₃), in the presence of sulfate-rich waters. ## Mineral Associations This mineral co-occurs with other secondary antimony minerals. In the type locality (Cetine di Cotorniano), it was found in association with valentinite, senarmontite, peretaite, klebelsbergite, and stibnite. ## Localities The most important and type locality for coquandite is the Cetine di Cotorniano mine in Tuscany, Italy. This is the only confirmed and well-documented locality for this mineral in the world.
Rarity
Very rare
For collectors
## Quality Criteria The collector appeal of coquandite specimens primarily depends on the richness and aesthetics of the crystal aggregates. Samples with well-formed, sharp crystals forming rosettes or dense coatings on a small rock matrix are most valued. Due to the microscopic size of the crystals, their abundance and good contrast with the substrate are crucial. The clarity and transparency of individual crystals, visible under a microscope, also increase the specimen's value. ## Popular Localities The only source of collector specimens is the Cetine di Cotorniano mine in Italy. Samples from this locality are the standard for this mineral.
Care and storage
## Cleaning Coquandite specimens should be cleaned with the utmost care, using compressed air to remove dust. Due to its fragility and small crystal size, contact with water and brushes, which could mechanically damage the delicate aggregates, should be avoided. ## What to Avoid All chemicals, especially acids and strong bases, should be avoided. The mineral is sensitive to shocks and abrasions. It should not be exposed to ultrasound or sudden temperature changes. ## Storage Coquandite specimens are best stored in sealed, padded "micromount" boxes to protect them from dust, mechanical damage, and direct contact. Display should be in stable conditions, away from vibrations.