KRÖHNKITE

Kröhnkite – Na2Cu(SO4)2·2H2O – is a fairly rare sulfate, typically occurring as a secondary copper mineral. Due to its rarity, it is not an important ore.

It is classified in the Roselite Group and has an anhydrous phase, saranchinaite, which can be formed by heating kröhnkite to 200ºC. It is readily soluble in water.

1. Characteristics

Crystal system: Monoclinic prismatic.         

Color: Blue, blue-green, light blue.
Once exposed, it turns green and “opaque”.

Habit: Short prismatic or pseudo-octahedral. Prismatic or fibrous aggregates, crusts, massive, granular.

Cleavage: {010} perfect, {-101} poor.

Tenacity: Brittle.    

Twinning: {101common. It forms heart-shaped twins.}

Fracture:  Conchoidal.

Mohs Hardness: 2.5 – 3

Parting: No.  

Streak: White.

Lustre:  Vitreous.

Diaphaneity: Transparent.     

Density (g/cm³): 2.9

 

2. Geology and Deposits

It occurs in the oxidized portions of copper deposits, but only in very arid regions, since it is highly soluble in water.

 

3. Mineral Associations

In the type locality, which is the Chuquicamata mine (Chile), it is associated with natrocalcite, chalcantite, blödite, atacamite, and antlerite.

Also with caracolite, brochantite, changoite, malachite, and chalcedony.

 

4. Transmitted Light Microscopy

Refraction indices:  nα:  1.544       nβ: 1.578       nγ: 1.601

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless to pale blue.  

Relief: Low.     

Cleavage: {010} perfect.

Habits: Short or elongated prismatic. May form octahedrons. Prismatic or fibrous aggregates, crusts, massive, granular.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.057, corresponding to intense and colorful hues up to the end of the 3rd order.

Extinction: Oblique with an angle of 48º  

Elongation sign: No information available.  

Twins:  On {101}, common,   

Zoning: No information available.       

CONVERGENT LIGHT

Character: B(-)     

2V angle: 78º

Alterations: Kröhnkite alters to saranchinaite, which is the anhydrous phase.     

May be confused with: At least a dozen secondary blue Cu ores.

Caption.

5. Reflected Light Microscopy

Sample preparation: xx

PLANE POLARIZED LIGHT – PPL

Reflection color: xx

Pleochroism: xx

Reflectivity: xx

Bireflectance:  xx

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: xx

Internal reflections:  xx

May be confused with: xx

General Characteristics: xx

Caption.