Alstonite
Chemical formula: BaCa(CO<sub>3</sub>)<sub>2</sub>
A rare barium and calcium carbonate, forming characteristic, complex crystals with a double pyramid shape.
Properties
- Mohs hardness
- 4-4.5
- Luster
- Vitreous
- Streak
- White
- Density
- 3.69-3.71
- Cleavage
- Poor on {010}
- Fracture
- Uneven to conchoidal
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
Diagnostic features
## Identification The key diagnostic feature of alstonite is its characteristic, steep, pseudo-hexagonal dipyramidal crystals. Reaction with dilute hydrochloric acid (effervesces) confirms it is a carbonate. Its relatively high density is also helpful in identification. It sometimes exhibits fluorescence under UV light. ## Distinguishing from Similar Minerals Alstonite is sometimes confused with witherite (BaCO₃) and barytocalcite (BaCa(CO₃)₂), which is its monoclinic polymorph. Witherite has a similar composition but crystallizes in the orthorhombic system and forms different crystal habits, often lenticular or prismatic. Barytocalcite forms crystals with a different habit, often prismatic or tabular. Final distinction of these three minerals often requires X-ray diffraction (XRD) analysis. ## Crystal Forms Typically forms steep, pseudo-hexagonal dipyramidal crystals, which are a result of complex twinning of triclinic crystals. It also occurs in radial, fibrous, spherulitic aggregates, and massive forms.
Geological environment
## Genesis Alstonite is a low-temperature mineral, forming in the final stages of hydrothermal processes. It forms in ore veins, usually lead and zinc, cutting through carbonate rocks such as limestones and dolomites. It can also crystallize in voids and fissures within these rocks. ## Mineral Associations It most commonly co-occurs with other hydrothermal vein minerals, such as calcite, barite, witherite, galena, sphalerite, ankerite, and siderite. In the classic locality in England, it was also associated with barytocalcite. ## Localities The most important and historical locality, from which the world's best specimens originate, is the Fallowfield mine (also known as Brownley Hill Mine) near Alston in Cumbria, England. This mineral has also been found in several other places in England (e.g., Durham, Northumberland). Outside the UK, its occurrences have been reported in Russia (Siberia) and also in the United States (Illinois, Colorado).
Rarity
Rare
For collectors
## Quality Criteria The most desired by collectors are sharp, well-formed, and undamaged crystals with the characteristic, pseudo-hexagonal shape. Specimens from the historical type locality - the Fallowfield mine in England - are particularly valuable. The size of the crystals, their transparency, and aesthetic placement on the rock matrix significantly increase the value of the specimen. Crystal groups are more valuable than single crystals. ## Popular Localities By far the most prized and sought-after specimens come from the Fallowfield mine in Cumbria, England. Specimens from this now-closed mine are considered classics and fetch high prices in the collector's market. Other localities provide specimens of significantly lesser value and quality.
Care and storage
## Cleaning Alstonite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. The specimen should be thoroughly dried after cleaning. ## What to Avoid Alstonite, like all carbonates, is sensitive to acids – even weak acids like vinegar can damage it, causing effervescence. Ultrasonic cleaners and steam cleaning should be avoided. The mineral is brittle, so it should be protected from impacts and falls. ## Storage Specimens should be stored in separate, padded boxes to prevent scratches and mechanical damage. It is not sensitive to sunlight, but it is recommended to protect it from dust and sudden temperature changes.