DESCLOIZITE

Descloizite – PbZn(VO4)(OH) – is a fairly rare vanadate, characteristic of oxidation zones of Pb and Zn sulfide deposits. It is part of base metal ores, but is never the main ore mineral of the deposit. On the rare occasions when it occurs in larger quantities, it is used as a V ore. A famous occurrence of descloizite, now exhausted, is located in the Otavi Mountains, in Namibia. There were several deposits, considered “the largest vanadium deposits in the world”.

It is classified in the Adelite-Descloizite Group (it is the most common of the Group), in the Descloizite-Mottramite Series. It is isomorphic with olivenite. It may contain Ga, Cu and Ge. There are two varieties (with Cu, with As).

1. Characteristics

Crystal system: Orthorhombic bipyramidal.          

Color: Reddish-brown, reddish-orange, reddish-brown, blackish-brown or almost black. 

Habit: It forms crystals up to 5 cm, equidimensional, pyramidal {111}, prismatic {001}, rarely tabular {100} or short prismatic {100}. Crystalline faces are often irregular or rough due to subparallel growth. Zoned crystals are common. It can be crusty, granular, stalactitic, massive with a fibrous structure and a mammillated or botryoidal surface.

Cleavage: No.       

Tenacity: Brittle.        

Twinning: No.       

Fracture: Irregular, subconchoidal.       

Mohs Hardness: 3 – 3.5

Parting: No.         

Streak: Orange to reddish-brown.         

Lustre:  Subvitreous, resinous, greasy, waxy.         

Diaphaneity: Transparent.           

Density (g/cm³):  6.1 – 6.2

 

2. Geology and Deposits

Desclozite is a very rare secondary mineral that occurs in the oxidation zone of base metal deposits, of the Pb-Cu-Zn-U-V association.

 

3. Mineral Associations

It is associated with common minerals from oxidation zones, such as quartz, limonite (goethite), hematite, and carbonates (calcite, dolomite).

It occurs with secondary Pb minerals (pyromorphite, vanadinite, wulfenite, mimetite, cerussite, mottramite, brackebuschite, minium) and secondary Zn minerals (smithsonite, willemite, hydrozincite).

Also with molybdenite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 2,185      nβ: 2,265      nγ: 2,350

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Pleochroic, from weak to strong:

X = Y = canary yellow and greenish yellow;

Z = brownish yellow.

Relief: Very high.           

Cleavage: No.           

Habits: Granular, prismatic, and pyramidal crystals, more rarely tabular or short prismatic. Also in crusts, stalactitic aggregates, massive with a fibrous structure and a mammillated or botryoidal surface.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.165, very high, with colors up to the eighth order, masked by the mineral’s own color.

Extinction: Paralell, as expected for a orthorhombic mineral.           

Elongation sign: No information available, probably very difficult to recognize due to the high interference colors of descloizite.

Twins: No.         

Zoning: Frequent, with zones of varying colors.

CONVERGENT LIGHT

Character: B(-), rarely B(+).     

2V angle: 85-90º.         

Alterations: No information available.         

May be confused with: No information available.       

Caption.

5. Reflected Light Microscopy

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

Sample preparation: Desclozite has a higher polishing hardness than wulfenite.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Medium gray with a brownish tint.       

Pleochroism: Strong, in shades of brownish gray or between gray and grayish blue.      

Reflectivity: ~17%        

Bireflectance:  Discreet.      

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Conflicting information: some literature reports that it has no anisotropy, while others report that the anisotropy is very strong.

Internal reflections: Conflicting information: some literature reports that they are widespread, ranging from yellow to brown. Others report that they are rare, ranging from colorless to brownish.

May be confused with: No information available.

General Characteristics: 

Grain shape: occurs in idiomorphic crystals and granular aggregates.

Zoning is frequent.

Caption.