Jarosite
Jarosite on quartz from the Arabia District, Pershing County, Nevada
General
Category	Sulfate minerals
Chemical formula	KFe3+3(OH)6(SO4)2
Strunz classification	07.BC.10
Dana classification	30.2.5.1
Crystal symmetry	Trigonal 3 2/m hexagonal scalenohedral
Unit cell	a = 7.304 Å, c = 17.268 Å; Z=3
Identification
Molar mass	500.8 g
Color	Amber yellow or dark brown
Crystal habit	Crystals are usually pseudocubic or tabular, also as granular crusts, nodules, fibrous masses or concretionary.
Crystal system	Trigonal
Cleavage	Distinct on {0001}
Fracture	Uneven to conchoidal
Tenacity	Brittle
Mohs scale hardness	2.5 - 3.5
Luster	Subadamantine to vitreous, resinous on fractures
Streak	light yellow
Diaphaneity	Transparent to translucent
Specific gravity	2.9 to 3.3
Optical properties	Uniaxial (-), usually anomalously biaxial with very small 2V
Refractive index	nω = 1.815 to 1.820; nε = 1.713 to 1.715
Birefringence	0.102 to 0.105
Pleochroism	E colorless, very pale yellow, or pale greenish yellow, O deep golden yellow or reddish brown
Solubility	Insoluble in water. Soluble in HCl.
Other characteristics	Strongly pyroelectric. Non-fluorescent. Barely detectable radioactivity
References	[1][2][3]

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 and acid sulfate soil environments.

  Physical properties

  Jarosite crystals from Sierra Peña Blanca, Aldama, Chihuahua, Mexico (5.6 x 3.1 x 1.6 cm)

Jarosite has a trigonal 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 colored 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,^[4] and it is doubtful whether a complete series exists between jarosite and natrojarosite.

In hydroniumjarosite^[5] 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.^[6] 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.^[7] Charge balance may be achieved in three ways.

Firstly by replacing two monovalent cations by one divalent cation, and leaving an A site vacancy, as in plumbogummite, Pb²⁺Al₃(PO₄)₂(OH)₅.H₂O, which is a member of the crandallite subgroup.

Secondly by incorporating divalent ions in the B sites, as in osarizawaite, Pb²⁺Cu²⁺Al₂(SO₄)₂(OH)₆, alunite subgroup, and beaverite, Pb²⁺Cu²⁺(Fe³⁺,Al)₂(SO₄)₂(OH)₆, jarosite subgroup.

Thirdly by replacing divalent anions with trivalent anions, as in beudantite, PbFe³⁺₃(AsO₄)^3-(SO₄)(OH)₆, beudantite subgroup.

  History

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 that belongs to the genus cistus and grows 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.^[8][9]

  See also

• Alunite
• Iron(III) sulfate
• Spirit Mars rover final embedding event

  References

• Palache C., Berman H., and Frondel C. (1951) Dana’s system of mineralogy, (7th edition), v. II, 560–562.
• Webmineral data
• Cornell University (2004) How an obscure mineral provided a vital clue to Martian water.

  External links

• Further information about the Jaros Hydrothermal system