Above we can see an aciculate aragonite that comes from Toledo. Its particular acicular form is determined by the ordering of its atoms, which tend to grow faster in the radial direction than in the others. This provides us needle-like structures all coming from a crystal nucleus.
Interestingly, its chemical formula is CaCO3, the same one as the calcite. Yet they are considered polymorphic
species due to their different crystalline structure. In fact, the crystalline system of the aragonite is orthorhombic, which provides the crystal more stress and instability than the trigonal of the calcite. This instability shows up at standard conditions, when it will slowly degrade to calcite. That is why we usually find them together in stalactites.
In the meantime, we shall enjoy the beautiful images that the aragonite leaves us. Like a sun trapped inside a crystal, resisting the slow pace of its degradation.
Author: Sergi Batlle
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