Jarosite is a basic hydrous sulfate of potassium and iron with a chemical formula of KFe3+3(OH)6(SO4)2. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct during the purification and refining of zinc and is also commonly associated with acid mine drainage environments.
Jarosite has a hexagonal crystal structure and is brittle, with basal cleavage, a hardness of 2.5-3.5, and a specific gravity of 3.15-3.26. It is translucent to opaque with a vitreous to dull lustre, and is coloured dark yellow to yellowish-brown. It can sometimes be confused with limonite or goethite with which it commonly occurs in the gossan (oxidized cap over an ore body). Jarosite is an iron analogue of the potassium aluminium sulfate, alunite.
Solid solution series
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 AB3(TO4)2(OH)6. In the alunite subgroup B is Al, and in the jarosite subgroup B is Fe3+. The beudantite subgroup has the general formula AB3(XO4)(SO4)(OH)6, the crandallite subgroup AB3(TO4)2(OH)5.H2O and the florencite subgroup AB3(TO4)2(OH)5or6.
In the jarosite-alunite series Al may substitute for Fe and a complete solid solution series between jarosite and alunite, KAl3(SO4)2(OH)6, 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+3(SO4)2(OH)6 is not known in nature. Minerals with Na > K are known as natrojarosite. End member formation (jarosite and natrojarosite) 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.
Jarosite was first described in 1852 by August Breithaupt in the Barranco del Jaroso in the Sierra Almagrera (near Los Lobos, Cuevas del Almanzora, Almería, Spain). The name jarosite is also directly derived from Jara, the Spanish name of a yellow flower growing in this sierra. The mineral and the flower have the same color.
In 2004 jarosite was detected on Mars by a Mössbauer spectrometer on the MER-B rover, which has been interpreted as strong evidence that Mars once possessed large amounts of liquid water.
Use in materials science
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.
1. ^ Gaines et al (1997) Dana's New Mineralogy Eighth Edition, Wiley
* Palache, C., H. Berman, and C. Frondel (1951) Dana’s system of mineralogy, (7th edition), v. II, 560–562
* Further information about the Jaros Hydrothermal system