JAROSITE

Jarosite -KFe3+3(OH)6(SO4)2 – is a very common sulfate, a secondary mineral that is sometimes abundant. It only occasionally has economic importance (Pb, Ag), but in Materials Science it is known for exhibiting a special type of structure. It has been used for dating (K-Ar) of weathering processes.

Jarosite is the Fe analogue of alunite. It forms a series with natrojarosite. It may contain Mo, Na, NH3, Pb or Ag; in the latter two cases it may be of economic interest. Because of this, the recognition and analysis of jarosite are important.

It has detectable radioactivity and is strongly pyroelectric. There is a variety with Ca and another poor in Fe3+.

1. Characteristics

Crystal system: Trigonal.          

Color: Amber yellow or dark brown.     

Habit: Powdery, earthy, granular crusts, fibrous or concretionary masses. Crystals are rare. When they occur, they are pseudocubic rhombohedral according to {01-12} or tabular according to {0001}. The largest jarosite crystals found reached 25 cm.       

Cleavage: {0001} distinct, generally not perceptible.       

Tenacity: Brittle.        

Twinning: No.       

Fracture: Irregular to subconchoidal.       

Mohs Hardness: 2.5 – 3.5

Parting: No.         

Streak: Light yellow.         

Lustre: Subadamantine to vitreous, in resinous fractures.          

Diaphaneity: Transparent to translucent.           

Density (g/cm³): 2.9 – 3.3

           

2. Geology and Deposits

Jarosite forms in iron sulfide deposits (pyrite, marcasite, pyrrhotite) through oxidation. Typically, it develops from the oxidation of pyrite, a process that generates sulfuric acid. This sulfuric acid, contained in dilute form in groundwater, reacts with minerals in the gangue and host rocks to form jarosite.

Jarosite is a very abundant mineral in the “gossans” (“iron caps”) of pyrite-containing sulfide deposits. Because it occurs associated with limonite (goethite), it easily goes unnoticed. In arid climates, the presence of jarosite is especially noticeable. Jarosite is frequent in rhythmic layers of goethite that develop over pyrite and chalcopyrite.

Less commonly, it forms as a low-temperature hydrothermal primary mineral, including deposits around hot springs. It occurs as an alteration product of submarine hydrothermal sulfides. It also forms as a by-product during zinc purification and refining and is commonly associated with acid mine drainage and acidic soil environments with sulfates. It has been detected on Mars.

 

3. Mineral Associations

It is associated with many minerals, including quartz, goethite, gypsum, hematite, pyrite, chalcopyrite, gold, barite, brochantite, scorodite, rhodalkylarite, alunite, and natrojarosite.

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 1,815 – 1,820     nε: 1,713 – 1,715

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Discreet pleochroism exists between X = colorless, very pale yellow, or pale greenish-yellow. Y = deep golden yellow or reddish-brown. Due to the generally very small size of the crystals, pleochroism is difficult to observe.

Relief: High.           

Cleavage:  No.          

Habits: Earthy, yellow masses, very rarely pseudocubic crystals. As it is a product of alteration, it can form pseudomorphs on other minerals.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence between 0.102 and 0.105: high-order colors, masked by the intense color of the mineral itself.

Extinction: No information available. 

Elongation sign: No information available.  

Twins: No. 

Zoning: No.  

CONVERGENT LIGHT

Character: U(-), can be anomalously biaxial with a very small 2V angle.

2V angle: Small, if present.         

Alterations:  It can alter to a mixture of hematite and goethite.

May be confused with: Yellow clay minerals or goethite.         

Jarosite 01: In PPL, jarosite in a volcanic rock (basalt), with typical strong yellow colors and a weak pleochroism, difficult to see.
Jarosite 01: The same section as of the previous image, now in XPL. The yellow colors remain, but blended with other shades of color, very intense and luminous. The yellow is reminiscent of the yellow of staurolite in PPL, but it is more intense.
Globular aggregates, probably of jarosite, in PPL, with the typical intense yellow color.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying jarosite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with jarosite, such as sulfides and goethite.

Sample preparation: Despite its generally powdery nature, jarosite is easy to polish, as is the case with other clay minerals.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Bluish gray.       

Pleochroism: No.      

Reflectivity:  ~15% (?)       

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Without anisotropy.        

Internal reflections: Widespread, strong, orange to red-brown.      

May be confused with: Goethite, especially in sections with low-quality polishing. 

Caption.