BISMUTITE

Bismuthite – (BiO)2CO3 – is a relatively common carbonate, typical of oxidation zones of Bi ores. It is not an important Bi ore.

Its habits have no diagnostic features. Because of this, the literature on bismutite is extremely scarce, and there is little information about it. One variety, hydrobismuthite, has proven to be identical to bismutite. Bismuthite is soluble in acids with effervescence.

When it forms tabular crystals, macroscopically it is indistinguishable from beyerite and kettnerite, which are Ca+Bi carbonates.

1. Characteristics

Crystal system: Orthorhombic pyramidal.

Color: Yellow to brown, greenish, greenish-gray, gray, black, colorless. 

Habit: It generally occurs in earthy, fibrous, or powdery masses, whose only diagnostic feature is its high density. It can form opaline crusts, as well as radial to fibrous crusts. Very rarely it forms crystals, up to 0.5 mm, which can be prismatic, scaly, or lamellar.

Cleavage: {001} good, only visible under a microscope.

Tenacity: Brittle.        

Twinning: Pseudo-tetragonal twins.       

Fracture: No information available. 

Mohs Hardness: 2.5 – 3.5

Parting: No.       

Streak: Gray.         

Lustre: Vitreous, pearly, dull, earthy.          

Diaphaneity: Transparent.           

Density (g/cm³): 6.7 – 7.4

           

2. Geology and Deposits

Bismuthite forms in oxidation zones of ore deposits containing Bi, such as hydrothermal veins or granitic pegmatites containing bismuthinite and native bismuth.

 

3. Mineral Associations

It occurs associated with other Bi minerals, both primary (native bismuth, bismuthinite, tetradymite) and secondary (bismuth, beyerite, kettnerite, mixite, sillenite and pucherite).

It is also associated with other carbonates (cerussite, malachite), oxides common in oxidation zones (limonite, goethite, wulfenite, mimetite) and quartz.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 2,120 – 2,150     nβ:  2,120 – 2,150

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:Colorless, it may have faint yellowish or greenish tones.

Relief: Very high.

Cleavage: {001} good, only visible in larger grains.

Habits: Massive, earthy, powdery, with rare prismatic, scaly and lamellar crystals, up to 0.5 mm.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.13 to 0.16.  Very high, difficult to classify, pearly colors like many carbonates.        

Extinction: Paralell.           

Elongation sign: ES(+)            

Twins: Pseudo-tetragonal twins. The literature does not specify whether they are visible under a microscope, but this would be difficult due to the generally very small size of the grains.         

Zoning:  No information available.            

CONVERGENT LIGHT

Character: B(-), simulates being uniaxial due to twinning.          

2V angle: 45º         

Alterations: No information available.           

May be confused with: No information available.          

Caption

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for identifying the mineral, nor is there any information available on the behavior of bismutite under this technique.

In the presence of primary Bi minerals, such as native bismuth, bismuthinite, and tetradymite, the presence of bismutite is possible.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption.