Erythrosiderite
Chemical formula: K<sub>2</sub>Fe<sup>3+</sup>Cl<sub>5</sub>·H<sub>2</sub>O
Erythrosiderite is a rare iron and potassium halide, forming small, hygroscopic crystals with a characteristic red or orange-red color.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous
- Streak
- Orange-yellow
- Density
- 2.46
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Erythrosiderite is identified primarily by its intense red or orange color, its occurrence (volcanic exhalation products), and its physical properties. A key diagnostic feature is its strong hygroscopicity and water solubility – a characteristic easy to observe but destructive to the specimen. It reacts with water to form a yellow solution. ## Distinguishing from Similar Minerals A mineral with a similar appearance and genesis is kremersite, which is also a hydrated iron and potassium chloride, but with a different formula and structure. Kremersite often forms crystals of a different habit (isometric) and can have similar colors. Definitive differentiation requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Erythrosiderite crystallizes in the orthorhombic system. It most often forms small, tabular or short, prismatic crystals. It also occurs as fine-grained coatings and crusts on volcanic rocks, often in association with other chlorides.
Geological environment
## Genesis Erythrosiderite is a secondary mineral, formed under specific conditions. It forms as a sublimation product from volcanic gases (exhalations) in fumaroles, where hot, chlorine- and iron-rich gases react with the environment. It forms at temperatures from 100 to approximately 600°C. ## Mineral Associations This mineral often co-occurs with other halides and sulfates formed in a similar environment. The most common associated minerals include: halite, sylvite, kremersite, molysite, and also hematite. ## Localities The most important and classic locality for erythrosiderite is the Vesuvius-Somma volcanic complex in Campania, Italy, where it was discovered. It is also found in fumarole products on Mount Etna in Sicily (Italy) and on Fogo volcano in Cape Verde.
Rarity
Rare
For collectors
## Quality Criteria Specimens with well-formed, sharp crystals of intense red color, set on a contrasting matrix, are most valued by collectors. Due to the rarity and instability of the mineral, even small but distinct crystals are considered valuable. Crystal size rarely exceeds a few millimeters, so specimens with larger crystals are exceptionally desirable. Association with other rare fumarolic minerals is also important. ## Popular Localities By far the most classic and prized erythrosiderite specimens come from its type locality – Vesuvius in Italy. Specimens from this location have historical significance and are the standard for this mineral species.
Care and storage
## Cleaning The mineral is water-soluble, so it must not be cleaned wet. Any contact with water, even distilled, will cause damage or complete dissolution. Cleaning can only be done dry, very carefully removing dust with a soft brush or a stream of compressed air from a distance. ## What to Avoid Above all, moisture and water should be avoided. The mineral is highly hygroscopic and absorbs water vapor from the environment, leading to its degradation. It should not be stored in damp rooms. It must also be protected from all chemicals and sudden temperature changes. It is very soft and brittle, so it requires delicate handling. ## Storage Erythrosiderite specimens must be stored in conditions of controlled, low humidity. The best solution is a sealed, airtight container (e.g., a "perky box") with a desiccant such as silica gel. The container should protect the specimen from dust and mechanical damage.