mineral

Aragonite Star Cluster (or Aragonite Sputnik)

Aragonite (Orthorhombic Calcium Carbonate, CaCO3)

Hardness: 3.5-4 on Mohs scale; Color: Reddish-brown, honey, or orange; Luster: Vitreous to resinous; Crystal structure: Orthorhombic (cyclic twinning forms hexagonal-looking prisms); Cleavage: Distinct in one direction; Specific gravity: 2.95.

Hardness: 3
Color: Reddish-brown, honey, or orange
Luster: Vitreous to resinous

Identified March 18, 2026More mineral →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed at low temperatures near the Earth's surface in sedimentary environments, evaporite deposits, or around thermal hot springs. Many reddish-brown cluster specimens originate from the Pliocene to Pleistocene eras (approx. 2 million years old) in areas of Spain and Morocco.

Uses & applications

Primary use is as a collector's mineral and in metaphysical practices. Industrially, aragonite is used in the manufacture of cement, as a soil conditioner, and in aquarium filtration systems to maintain pH levels.

Geological facts

Aragonite is a polymorph of calcite, meaning they have the same chemical formula but different structures. It is also the primary material found in mother-of-pearl and pearls. The 'star cluster' formation is actually a result of cyclic twinning, where three or more crystals grow across each other.

Field identification & locations

Identify by its distinctive radiating 'Sputnik' shape with hexagonal-ended prisms and its characteristic reddish-orange to brown coloring. It will react and bubble (effervesce) if it comes into contact with even weak acids like vinegar. Common locations include Cuenca, Spain and the Midelt province in Morocco.

Other mineral specimens

See all

Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral

Epidote

Epidote - Ca2(Al2,Fe3+)3(SiO4)3(OH)

mineral