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.

Itcommonly forms in twins that join to form a six-sided crystal called a sixling, which may look like calcite. These aragonite sixling twins are pseudohexagonal. The key is pseudo, which means false. Aragonite is lesscommon than calcite, but in the organic world it is verycommon and very important because it is an integral part of most seashells, pearls, and ancient fossils, especially ancient ammonites. Aragonite and calcite form under low-temperature conditions, but Aragonite is a little less stable. After it forms, it does not take much for it to rearrange its structure and become calcite. For example, banded onyx, which iscommon around hot springs, oThen forms as aragonite but then tends to alter into calcite once cooled. This explains why we use the term calcite onyx.

Aragonite is a wonderful collector mineral because it iscommon enough to be found in quantity. It develops in a variety of attractive crystals.

Some are long, tapered beauties that approach needle-like form.prism faces of each crystal are very short and hardly visible. These hexagonal discs are seldom more than an inch high. The other type of aragonitecommon here is lovely, nearly transparent, individual pseudohexagonal crystals that measure up to an inch across and more than an inch long. They develop in nice sub-parallel clusters of high luster and sharp terminations.