The alunite supergroup includes the alunite, jarosite, beudantite, crandallite and florencite subgroups. The alunite supergroup minerals are isostructural with each other and substitution between them occurs, resulting in several solid solution series. The alunite supergroup has the general formula AB326. In the alunite subgroup B is Al, and in the jarosite subgroup B is Fe3+. The beudantite subgroup has the general formula AB36, the crandallite subgroup AB325•H2O and the florencite subgroup AB325 or 6. In the jarosite-alunite series Al may substitute for Fe and a complete solid solution series between jarosite and alunite, KAl326, probably exists, but intermediate members are rare. The material from Kopec, Czech Republic, has about equal Fe and Al, but the amount of Al in jarosite is usually small. In the jarosite-natrojarosite series Na substitutes for K to at least Na/K = 1:2.4 but the pure sodium end member NaFe3+326 is not known in nature. Minerals with Na > K are known as natrojarosite. End member formation is favoured by a low temperature environment, less than 100 °C, and is illustrated by the oscillatory zoning of jarosite and natrojarosite found in samples from the Apex Mine, Arizona, and Gold Hill, Utah. This indicates that there is a wide miscibility gap between the two end members, and it is doubtful whether a complete series exists between jarosite and natrojarosite. In hydroniumjarosite the hydronium ion H3O+ can also substitute for K+, with increased hydronium ion content causing a marked decrease in the lattice parameter c, although there is little change in a. Hydroniumjarosite will only form from alkali-deficient solutions, as alkali-rich jarosite forms preferentially. Divalent cations may also substitute for the monovalent cation K+ in the A site. Charge balance may be achieved in three ways.
History
Jarosite was first described in 1852 by August Breithaupt in the Barranco del Jaroso in the Sierra Almagrera. The name jarosite is also directly derived from Jara, the Spanish name of a yellow flower that belongs to the genus Cistus and grows in this sierra. The mineral and the flower have the same color. Mysterious spheres of clay 1.5 to 5 inches in diameter and covered with jarosite have been found beneath the Temple of the Feathered Serpent an ancient six level stepped pyramid 30 miles from Mexico City.
Ferric sulfate and jarosite have been detected by the two martian roversSpirit and Opportunity. These substances are indicative of strongly oxidizing conditions prevailing at the surface of Mars. 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. Because iron sulfate has very little cohesion, the rover's wheels could not gain sufficient traction to pull the body of the rover out of the iron sulfate patch. Multiple techniques were attempted to extricate the rover, but the wheels eventually sank so deeply into the iron sulfate that the body of the rover came to rest on the martian surface, preventing the wheels from exerting any force on the material below them. As the JPL team failed to recover the mobility of Spirit, it signified the end of the journey for the rover.
Jarosite is also a more generic term denoting an extensive family of compounds of the form AM362, 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.
