Aragonite

Aragonite is a carbonate mineral, calcium carbonate (CaCO₃), and a polymorph of calcite that shares the same chemistry but a different crystal structure. Aragonite has the same chemical formula as calcite (CaCO₃) but a different crystal structure - they're polymorphs. Aragonite forms under higher pressure or lower temperature conditions than calcite, and in the presence of certain ions (especially magnesium and strontium) that inhibit calcite growth.

The most iconic aragonite form is the 'star cluster' or 'sputnik' variety from Morocco - radiating clusters of prismatic crystals that look like geometric sea urchins. These form in clay sediments where aragonite crystallizes from groundwater solutions. The clusters grow outward from a central point, creating the distinctive starburst shape.

Aragonite is metastable at Earth's surface conditions - over geological time, it slowly converts to the more stable calcite form. This means all aragonite specimens are relatively young in geological terms. Aragonite is also biologically important: it's the primary mineral in pearls, mother of pearl, and many marine shells.

Aragonite is identified by its orthorhombic crystal habit (often as pseudo-hexagonal twins or radiating clusters), hardness of 3.5-4, and effervescence in dilute acid (confirming carbonate composition). The Moroccan star clusters are immediately recognizable.

Distinguish from calcite (different crystal system, rhombohedral cleavage), gypsum (softer, no acid reaction), and siderite (iron carbonate, higher SG). Aragonite's characteristic cyclic twinning produces pseudo-hexagonal forms that look hexagonal but have different cleavage.