Jarosite
| Jarosite | |
|---|---|
Jarosite on quartz from the Arabia District, Pershing County, Nevada
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| General | |
| Category | Sulfate minerals |
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Formula (repeating unit) |
KFe3+3(OH)6(SO4)2 |
| Strunz classification | 7.BC.10 |
| Dana classification | 30.2.5.1 |
| Crystal system | Trigonal |
| Crystal class | Rhombohedral (3m) H-M symbol: (3m) |
| Space group | R3m |
| Unit cell | a = 7.304 Å, c = 17.268 Å; Z = 3 |
| Identification | |
| Formula mass | 500.8 g/mol |
| Color | Amber yellow or dark brown |
| Crystal habit | Crystals are usually pseudocubic or tabular, also as granular crusts, nodules, fibrous masses or concretionary. |
| 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 | |
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.
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 luster, 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.
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.
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