Aragonite

Aragonite is the second most common polymorph of natural calcium carbonate. The most common is Calcite. It is significantly less common than Calcite and is formed under a much narrower range of geological conditions. It is an unstable formation compared to calcite and may actually convert to calcite with changes in its environment. It is nearly always a low-temperature, near the surface mineral. It was named after the village of Molina de Aragón, Spain where it was originally found.

Aragonite is a polymorph of calcite, which means that it has the same chemistry as calcite but it has a different structure, and more importantly, different symmetry and crystal shapes. Aragonite’s Crystal Habits include twinned hexagonal prismatic crystals as well as a diverse assortment of thin elongated prismatic, curved bladed, steep pyramidal (spiked) and chisel shaped crystals. A branching tree, coral or worm-like delicate form is called "flos ferri". Aragonite is a mineral found in many species' shell structures. A layered sedimentary marble like formation with Aragonite is called Mexican Onyx and is used for carvings and ornamental purposes. Calcite pseudomorphs of aragonite crystals and formations are common.