Dacostaite
Chemical formula: K(Mg₂Al)[Mg(H₂O)₆]₂(As⁵⁺O₄)₂F₆·2H₂O
Dacostaite is an extremely rare arsenate, forming colorless, tabular crystals, discovered in the Arsenatnaya mine in Russia.
Properties
- Mohs hardness
- 2.5
- Color
- colourless to white
- Luster
- Vitreous
- Streak
- white
- Density
- 2.162
- Cleavage
- {001}
- Fracture
- Uneven
- Transparency
- Transparent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Identification of dacostaite under amateur conditions is impossible. It requires advanced analytical techniques, such as Raman spectroscopy and chemical analysis using an electron microprobe (EDS/WDS) to confirm its complex chemical composition. Visually, it can be suspected based on the characteristic crystal form and co-occurrence with other rare arsenates in its known locality. ## Distinguishing from Similar Minerals It can be confused with other colorless secondary minerals occurring in the same paragenesis, such as pharmacosiderite or arseniopleite. Definitive distinction is only possible based on structural (X-ray diffraction) and chemical analysis. ## Crystal Forms Dacostaite forms thin, tabular crystals with a hexagonal outline, often with truncated edges. They occur as single, isolated crystals or small, loose aggregates on the surface of the host rock (matrix).
Geological environment
## Genesis Dacostaite is a secondary mineral, formed as a result of hydrothermal processes occurring in the oxidation zone (gossan) of an arsenic deposit. It crystallizes in voids and fractures within arsenic-rich rocks, mainly in hematite-quartz aggregates. ## Mineral Associations This mineral co-occurs with other rare arsenates and sulfates. In its type locality (Arsenatnaya mine), it has been found in association with pharmacosiderite, arseniopleite, svabite, talmessite, erythrite, annabergite, gypsum, and iron oxides (hematite, goethite), among others. ## Localities The only confirmed occurrence of dacostaite in the world is its type locality – the Arsenatnaya fumarole on Tolbachik volcano in Kamchatka, Russia. This is a unique geological environment, known for the occurrence of many extremely rare minerals that formed as a result of the interaction of volcanic gases with surrounding rocks.
Rarity
Extremely rare
For collectors
## Quality Criteria As a "micromount" mineral, specimens with sharply defined, well-formed, and transparent crystals that are clearly visible on a contrasting matrix are most highly valued. Crystal size, although always microscopic, is a key factor – specimens with crystals approaching the maximum recorded size (0.3 mm) are most desirable. The aesthetics of the entire sample and the presence of rare associated minerals are also important. ## Popular Localities All known dacostaite specimens originate from a single location in the world: the Arsenatnaya fumarole on Tolbachik volcano in Kamchatka, Russia. Material from this locality is extremely difficult to obtain.
Care and storage
## Cleaning Dacostaite specimens, being microscopic crystals on a matrix, should not be cleaned mechanically or chemically. Any attempts at cleaning can irreversibly damage the delicate crystals. If dust removal is necessary, a gentle stream of compressed air can be used from a safe distance. ## What to Avoid Avoid contact with water, acids, solvents, and any chemical agents. The mineral is sensitive to changes in temperature and humidity. It should not be heated or exposed to direct sunlight. ## Storage Specimens should be stored exclusively in stable conditions, in specialized "micromount" boxes that protect them from mechanical damage, dust, and humidity changes. Store in a dry place, away from heat and light sources.