WILLIAM PENGELLY CAVE STUDIES TRUST

Aragonite in Caves

Click on display for more detail.

Crystalline Structure

Crystalline calcium carbonate occurs naturally as CALCITE, ARAGONITE and a very rare VATERITE.

Aragonite is named after the locality in Spain where it was first identified. Although it has the same composition as Calcite ( CaC03) its atoms are arranged differently. For this reason the two minerals differ in their properties. One major difference is that Aragonite is about 16% more soluable than Calcite in water at normal cave temperature. Aragonite would never form in caves if the water were not so highly supersaturated with Calcite that it can no longer even dissolve Aragonite.

Click pictures to see larger versions.

The following factors encourage the formation of Aragonite, rather than Calcite, in caves:

SUPERSATURATION with respect to both Calcite and Aragonite. This is affected by temperature, rate of supply of solution, loss of carbon di-oxide from solution, concentration of calcium and rate of precipitation.

TEMPERATURE changes can affect the rate of reactions and diffusions in the crystallisation process. Aragonite is usually at higher cave temperatures.

STRONTIUM & MAGNESIUM IONS can provide carbonates as a nuclei upon which Aragonite will grow.

ORGANIC MOLECULES derived from the soil and present in cave water may accelerate the growth of Aragonite.

Aragonite Flowers

In its finest development Aragonite displays clusters of branching needle-like crystals known as anthrodite "flowers". It also occurs as compact masses of pure white fibrous crystals of "flos-ferri". It may also form stalagmites, stalactites and helictites. The mineral specimens and photographs shown here are from Napps Cave near Ilfracombe in North Devon.

Click pictures to see larger versions.

Photographs by Felton Fowler