Aragonite Vs Calcite Cave Speleothem

Buchanan (2025)

 

Aragonite and calcite are both polymorphs of calcium carbonate (CaCO₃), meaning they have the same chemical composition but different crystal structures. Aragonite is commonly found in marine shells, coral skeletons, and cave formations, while calcite is more widespread in sedimentary rocks like limestone, dolomite. In caves, aragonite and calcite both play significant roles in speleothem (cave formation) development, they form under different conditions and exhibit distinct crystal habits.

Picture: - Aragonite sprays- Wolkburg Caves, Limpopo, South Africa, Lake chamber (Mike Buchanan)

Crystallography of Aragonite

·         Crystal System: Orthorhombic

·         Habit: Prismatic, needle-like, or columnar crystals, often forming fibrous or radiating aggregates

·         Cleavage: Distinct, but not as perfect as calcite

·         Symmetry: Lower symmetry compared to calcite

·         Density: Slightly higher than calcite due to its tighter packing

Crystallography of Calcite

·         Crystal System: Trigonal (rhombohedral)

·         Habit: Commonly rhombohedral, scalenohedral (dog-tooth spar), or massive/granular forms

·         Cleavage: Perfect rhombohedral cleavage

·         Symmetry: Higher symmetry compared to aragonite

·         Twinning: Common, often seen as polysynthetic twinning

Key Differences

1. Crystal System: Aragonite is orthorhombic, while calcite is trigonal.

2. Habit: Aragonite tends to form elongated or needle-like crystals, while calcite often forms rhombohedral or scalenohedral crystals.

3. Cleavage: Calcite has perfect rhombohedral cleavage, whereas aragonite has less distinct cleavage.

4. Stability: Aragonite is metastable under Earth's surface conditions and slowly transforms into calcite over geological time.

5. Density: Aragonite is slightly denser due to its more compact structure.

 

Close-up of a white crystal AI-generated content may be incorrect.

Picture: - Calcite crystals (Dog Tooth Spar) – CHK Cave, Gauteng, South Africa (Mike Buchanan)

Battery AA removed- demonstrates size

  Calcite in Caves

·         The most common mineral in speleothems.

·         Forms classic stalactites, stalagmites, flowstones, and draperies.

·         Precipitation occurs when calcium bicarbonate-rich water (Ca(HCO₃)₂) loses CO₂, causing calcite (CaCO₃) to crystallize.

·         Typically forms smooth, rhombohedral, or scalenohedral crystals in large deposits.

Aragonite in Caves

·         Less common but forms striking needle-like sprays or fibrous crystals.

·         Often found as helictites, which are irregular, twisting formations growing in unpredictable directions due to capillary action and evaporation.

·         Forms in drier cave environments with high Mg²⁺ concentrations or when water is supersaturated with CaCO₃ but lacks the conditions for calcite growth.

·         Can also appear in frostwork, delicate white, feathery formations resembling frost.

·         Key Differences in Cave Formations

Over time, aragonite can transform into calcite, but its delicate structures often remain intact in caves, making it an important feature of cave mineralogy.

Moonmilk and Aragonite

Moonmilk is a soft, white, creamy speleothem found in caves, composed of microscopic mineral crystals suspended in water. It has a pasty or cheese-like consistency when wet but can dry into a powdery form. Aragonite moonmilk is less common than calcite-based moonmilk but occurs in caves where conditions favour aragonite formation, such as high magnesium content, low CO₂ conditions, and slow evaporation. Microorganisms, such as bacteria and fungi, may play a role by influencing calcium carbonate precipitation. Moonmilk has been historically used in folk medicine due to its perceived healing properties, though it is primarily of geological and microbiological interest today.

Formation of Aragonite Moonmilk

·         Forms in humid cave environments where water slowly seeps through rock, depositing tiny crystals.

·         The fibrous, needle-like nature of aragonite contributes to the pasty texture of moonmilk.

Key Differences from Other Speleothem

Unlike hard formations like stalactites or helictites, moonmilk remains soft.

It doesn’t crystallise into large, structured forms but instead appears as a creamy, powdery deposit on cave walls and ceilings and previously established calcite and aragonite crystals.           

 


                                                                    Aragonite helictite amongst sprays (Buchanan)

                                                                                                                 

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