Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe3(SO4)2(OH)6.

This indicates that there is a wide miscibility gap between the two end members,[5] and it is doubtful whether a complete series exists between jarosite and natrojarosite.

The name jarosite is directly derived from "jara", the Spanish name of a yellow flower that belongs to the genus Cistus and grows in the sierra.

Mysterious spheres of clay, 1.5 to 5 inches (40 to 125 mm) in diameter and covered with jarosite, have been found beneath the Temple of the Feathered Serpent, an ancient six-level stepped pyramid 30 miles (50 km) from Mexico City.

In May 2009, the Spirit rover became stuck when it drove over a patch of soft ferric sulfate that had been hidden under a veneer of normal-looking soil.

On Earth, jarosite is mainly associated with the ultimate stage of pyrite oxidation in clay environment, and can also be found in mine tailings waste where acidic conditions prevail.

Against all expectations, jarosite has also been fortuitously discovered in minute quantities in the form of small dust particles in ice cores recovered from a deep borehole in Antarctica.

Jarosite is also a more generic term denoting an extensive family of compounds of the form AM3(OH)6(SO4)2, where A+ = Na, K, Rb, NH4, H3O, Ag, Tl and M3+ = Fe, Cr, V. In condensed matter physics and materials science they are renowned for containing layers with kagome lattice structure, relating to geometrically frustrated magnets.