Para-alumohydrocalcite
Chemical formula: CaAl<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>4</sub>·6H<sub>2</sub>O
Para-alumohydrocalcite is a rare, hydrated calcium aluminum carbonate, forming fine, white aggregates with a silky or pearly luster.
Properties
- Mohs hardness
- 2.5
- Luster
- Pearly or Silky
- Streak
- White
- Density
- 2.23-2.26
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Identification of para-alumohydrocalcite is based on its characteristic mode of occurrence – as white, spherulitic or radial aggregates with a silky luster. Key features include extreme softness, low density, and reaction with hydrochloric acid. However, definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD). ## Distinguishing from similar minerals It can be confused with other secondary, white carbonates or sulfates, such as alumohydrocalcite, hydrozincite, huntite, or gypsum. It is distinguished from gypsum by its reaction with acid. Differentiation from alumohydrocalcite and other rare aluminum carbonates is impossible without specialized testing. ## Crystal forms It forms very fine, elongated, bladed crystals that combine into spherulitic, radial, or fibrous aggregates. It also occurs as earthy masses and chalk-like coatings.
Geological environment
## Genesis Para-alumohydrocalcite is a secondary mineral, forming in weathering zones of rocks rich in aluminum and calcium. It forms at low temperatures as a result of carbon dioxide-rich waters acting on rock-forming minerals. It is found in cracks and fissures in sedimentary rocks, as well as in coal mines. ## Mineral associations It often co-occurs with other secondary minerals, such as calcite, dolomite, gypsum, allophane, and especially with the very similar alumohydrocalcite. ## Localities The most important localities for this rare mineral include the Santa Barbara mine in Tuscany (Italy), where it was discovered; the Klöch quarry in Styria (Austria); the Clara mine in the Black Forest (Germany); and some localities in Russia (North Caucasus) and Hungary.
Rarity
Very rare
For collectors
## Quality criteria The collector appeal of para-alumohydrocalcite specimens primarily depends on the richness and aesthetics of the aggregates. The most valued specimens are those with well-formed, distinct spherulites with a silky luster, contrasting with the color of the matrix rock. Due to the microscopic size of the crystals, their individual forms are not a criterion for evaluation. Association with other rare minerals is also important. ## Popular localities Specimens from the type locality, the Santa Barbara mine in Italy, are of particular value to systematic collectors. Good quality spherulitic aggregates also come from the Klöch quarry in Austria.
Care and storage
## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to its extreme softness and fragility, contact with water and any mechanical tools should be avoided. ## What to avoid Contact with water, acids (it reacts with the release of CO₂), and other chemicals must be absolutely avoided. The mineral is very soft and susceptible to mechanical damage, scratches, and abrasion. It should be protected from high temperatures, which can cause the loss of water from its structure. ## Storage Specimens of para-alumohydrocalcite should be stored in closed, padded collector boxes to protect them from dust, moisture, and physical damage. It is not suitable for display in open showcases.