Story by Bob Jones
Aragonite
The mostcommon carbonate is calcite, calcium carbonate. It has acompanion in aragonite, which iscomposed of the same chemistry. Since both have exactly the same chemical molecules in the same ratios, the two are called paramorphs. The diTherence in paramorphs is internal, with each presenting diTherent crystal structures. Calcite forms six-sided hexagonal crystals while aragonite forms four-sided orthorhombic beauties. To confuse things further, aragonite has a great penchant for forming twins that look exactly like hexagonal crystals.