Eggletonite
Chemical formula: (Na,K,Ca)₁-₂Mn²⁺₆(Si,Al)₁₀O₂₄(OH)₄·7-11H₂O
Eggletonite is a rare, brown silicate mineral, forming acicular or fibrous aggregates, discovered in the Big Creek mine in Arkansas, USA.
Properties
- Mohs hardness
- 3-4
- Color
- Dark to golden brown, red-brown
- Luster
- Vitreous
- Streak
- Light Brown
- Density
- 2.76
- Cleavage
- [001] Perfect
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Eggletonite can be identified by its characteristic radial or fibrous aggregates with a brown color and silky luster. It occurs in a specific geological environment – in voids and fissures within volcanic tuffs. Due to the small size of the crystals, definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other fibrous manganese minerals, such as piemontite or some zeolites. It is distinguished from them by its specific paragenesis (co-occurrence with aegirine, analcime, natrolite) and its characteristic silky luster. Unlike many zeolites, its fibers are usually brown. ## Crystal Forms It forms very fine, acicular or bladed crystals, elongated along the crystallographic axis. These crystals almost always occur as radial, spherulitic, or fibrous aggregates, forming spherical or hemispherical shapes in rock fissures.
Geological environment
## Genesis Eggletonite is a hydrothermal mineral. It forms in the late stage of crystallization in voids and fissures within phonolitic volcanic tuffs. It crystallizes from solutions rich in silica, manganese, and alkalis at relatively low temperatures. ## Mineral Associations It most commonly co-occurs with minerals such as aegirine, analcime, natrolite, microcline, nepheline, as well as other rare manganese minerals like sugilite or rhodochrosite. In the type locality (Big Creek, Arkansas), it is also accompanied by, among others, labuntsovite-Mn and zorite. ## Localities The most important and classic occurrence of eggletonite is its type locality: the Big Creek mine in Lane County, Arkansas, USA. This is the only place in the world where specimens of this mineral have been obtained in significant quantities. Additionally, its presence has been noted in the Murun complex on the Aldan Shield in Siberia, Russia.
Rarity
Very rare
For collectors
## Quality Criteria The most prized eggletonite specimens are those that exhibit well-formed, spherical or radial aggregates with an intense brown color and distinct silky luster. Contrast with the light matrix rock (most often analcime or natrolite) is important. Due to the microscopic nature of the mineral, the size of the aggregates (even a few millimeters in diameter) significantly increases the value of the specimen. Undamaged specimens are rare. ## Popular Localities The only source of valuable collector specimens is the Big Creek mine in Arkansas, USA. Specimens from this locality are considered classic and most desired by collectors specializing in rare minerals and micromounts.
Care and storage
## Cleaning Eggletonite specimens are extremely delicate and fragile. They should be cleaned only with great care, using compressed air to remove dust. Any contact with water or a brush can irrevocably damage the tiny, acicular crystals. ## What to Avoid Avoid contact with water, chemicals, and ultrasound. Avoid vibrations and impacts that could cause the delicate aggregates to disintegrate. It should not be heated or exposed to rapid temperature changes. ## Storage It is recommended to store eggletonite specimens exclusively in specialized "micromount" boxes, which protect them from dust, vibrations, and mechanical damage. Avoid exposure to open air.