Jarosite

Jarosite is a basic hydrous sulfate of potassium and iron with a chemical formula of KFe3+3(OH)6(SO4)2. This 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 AMD (acid mine drainage) environments.
See more at Wikipedia.org...

(n.)
An ocher-yellow mineral occurring on minute rhombohedral crystals. It is a hydrous sulphate of iron and potash.
  

General Information:
 Chemical Formula:
KFe+++3(SO4)2(OH)6
 Composition:
(Molecular Weight = 500.81 gm)
Potassium 7.81 % K 
Iron 33.45 % Fe
Hydrogen 1.21 % H 
Sulfur 12.81 % S 
Oxygen 44.73 % O 
 Empirical Formula:
KFe2+3(SO4)2(OH)6
 Environment:
Secondary mineral, forming under conditions of weathering in arid climates.
 Locality:
Barranco Jaroso in southern Spain.
 Name Origin:
Named after its locality.
Physical Properties:
 Cleavage:
[0001] Distinct
 Color:
brown, yellow, yellow brown, or light yellow.
 Density:
2.9 - 3.3, Average = 3.09
 Diaphaniety:
Translucent
 Habits:
Crystalline - Fine - Occurs as well-formed fine sized crystals., Fibrous - Crystals made up of fibers., Massive - Uniformly indistinguishable crystals forming large masses.
 Hardness:
2.5-3.5 - Finger Nail-Copper Penny
 Luminescence:
None.
 Luster:
Vitreous (Glassy)
 Streak:
yellow
More details...

General Information:
 Chemical Formula:
(H3O)Fe+++3(SO4)2(OH)6
 Composition:
(Molecular Weight = 480.74 gm)
Iron 34.85 % Fe
Hydrogen 1.89 % H 
Sulfur 13.34 % S 
Oxygen 49.92 % O 
 Empirical Formula:
(H3O)Fe3+3(SO4)2(OH)6
 IMA Status:
Approved IMA 1960
Physical Properties:
 Color:
yellow, dark straw yellow, reddish brown, or brown.
 Density:
2.496 - 2.905, Average = 2.7
 Diaphaniety:
Transparent to Translucent
 Hardness:
4-4.5 - Between Fluorite and Apatite
 Luster:
Vitreous - Resinous
More details...