Jarosite
A sulfate mineral linking Earth's geology to Mars exploration.
Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with the chemical formula KFe3(SO4)2(OH)6. This mineral forms in ore deposits through the oxidation of iron sulfides and is a common byproduct during zinc purification and refining. It is also frequently found in environments affected by acid mine drainage and acid sulfate soils. Jarosite has a trigonal crystal structure and is brittle, exhibiting basal cleavage, a hardness between 2.5 and 3.5, and a specific gravity ranging from 3.15 to 3.26. It appears translucent to opaque with a vitreous to dull luster, colored dark yellow to yellowish-brown, and can be mistaken for limonite or goethite, with which it often occurs in gossan. It is the iron analogue of the potassium aluminium sulfate mineral alunite. Within the alunite supergroup, jarosite belongs to a subgroup where iron (Fe3+) occupies the B site. A complete solid solution series likely exists between jarosite and alunite through aluminum substitution for iron, though intermediate members are rare. Sodium can substitute for potassium in the jarosite-natrojarosite series, but the pure sodium end member is not found in nature; minerals with more sodium than potassium are called natrojarosite. The hydronium ion can also replace potassium, with increased hydronium content reducing the lattice parameter c. Jarosite was first described in 1852 by August Breithaupt from the Barranco del Jaroso in Spain, its name derived from the Spanish word "jara" for a yellow flower of the same color. On Mars, ferric sulfate and jarosite have been detected by the rovers Spirit, Opportunity, and Curiosity, indicating strongly oxidizing surface conditions. The Spirit rover became permanently stuck after driving over a patch of soft ferric sulfate. Unexpectedly, minute jarosite dust particles have also been discovered in ice cores from a deep Antarctic borehole.
- chemical_formula
- KFe3(SO4)2(OH)6
- crystal_system
- Trigonal
- hardness
- 2.5–3.5
- specific_gravity
- 3.15–3.26
- color
- Dark yellow to yellowish-brown
- luster
- Vitreous to dull
- cleavage
- Basal
Lore & Background
Its name derives from 'jara,' the Spanish name for a yellow flower of the genus Cistus that shares the mineral's color. Mysterious spheres of clay, 1.5 to 5 inches in diameter and covered with jarosite, have been found at Teotihuacan, though not specifically beneath the Temple of the Feathered Serpent. On Mars, jarosite has been detected by the Opportunity and Curiosity rovers, while Spirit detected ferric sulfates but not jarosite specifically. The iron sulfate's lack of cohesion prevented traction, and the rover's wheels eventually sank so deeply that the body rested on the surface, ending Opportunity's journey—not Spirit's, which ended due to an inability to recharge after winter. On Earth, jarosite is mainly associated with pyrite oxidation in clay environments and mine tailings. Surprisingly, it was discovered in minute quantities as dust particles in ice cores from a deep borehole in Antarctica. Geologists speculate jarosite dust could accumulate in glaciers on Mars, though this hypothesis is controversial because Martian jarosite deposits can be up to 10 meters thick.
Reader's Guide
Jarosite holds significance across multiple fields. In mineralogy, it is the iron analogue of alunite and part of the alunite supergroup, which includes solid solution series with alunite, natrojarosite, and hydroniumjarosite. Its formation from pyrite oxidation in clays, with potassium sourced from illite or K-feldspar, provides insights into geochemical processes. In planetary science, jarosite's detection on Mars by three rovers indicates strongly oxidizing surface conditions, and the Spirit rover's entrapment in ferric sulfate highlighted the material's physical properties. The unexpected discovery of jarosite in Antarctic ice cores raises questions about its transport and preservation, with implications for Martian glacial environments. In materials science, jarosite-family compounds feature kagome lattice layers, relevant to geometrically frustrated magnets.
Did You Know?
- Mysterious clay spheres covered with jarosite were found at Teotihuacan, though not specifically beneath the Temple of the Feathered Serpent.
- Jarosite dust has been discovered in ice cores from a deep borehole in Antarctica.
Frequently Asked Questions
What is Jarosite?
Jarosite is a basic hydrous sulfate mineral made up of potassium and ferric (Fe-III) iron, carrying the formula KFe3(SO4)2(OH)6. It crystallizes in the trigonal system and typically presents in dark yellow to yellowish-brown hues.
How hard is Jarosite and what does its surface look like?
It sits between 2.5 and 3.5 on the Mohs scale, so it's fairly soft, and its surface shows a vitreous-to-dull luster. Its specific gravity falls in the 3.15–3.26 range, giving it a noticeable heft for its size.
Where does Jarosite form in nature?
It develops when iron sulfides undergo oxidation within ore deposits and is frequently encountered as a byproduct of zinc refining. It also thrives in acid mine drainage settings and acid sulfate soil environments.
Why do planetary scientists care about Jarosite?
Finding Jarosite on Mars is a strong indicator that the planet once experienced intensely oxidizing conditions, making it a key geochemical fingerprint for reconstructing Martian surface history. It effectively bridges terrestrial ore geology with extraterrestrial exploration.
What is Jarosite's crystal system and typical color?
It belongs to the trigonal crystal system and most commonly appears in shades from dark yellow to yellowish-brown. Its luster can range from glassy (vitreous) to quite dull depending on the specimen.
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