WITHERITE

Witherite – BaCO3 – is a rare carbonate, but it is the second most common Ba mineral after barite, and therefore an important ore of Ba.

It is classified in the Aragonite Group, forms a series with strontianite (of Sr) and develops epitaxial growths with barite and baritocalcite. May contain Sr and Ca.

Witherite crystals, up to 12 cm, always form cyclic twins of three individuals (“trillings”) by {110}, pseudohexagonal, similar to a short bipyramid hexagonal prism, with six segments. The faces are rough and have horizontal striations. In addition, witherite can show a granular, botryoidal, spherical, columnar and fibrous habit. It can occur massively.

Witherite fluoresces light blue under short and long wave UV light. It is also phosphorescent in white (in shortwave UV light) and bluish white (longwave UV light). It is fluorescent and phosphorescent under X-rays and electron beams. It is soluble with light effervescence with cold dilute HCl. Witherite has a much higher density than other carbonates, which turn its recognition in hand sample easier.

Watch out! Ingestion of witherite is lethal, the mineral was used until the 18th century as rat poison. Care must be taken to avoid inhaling witherite dust when grinding, sawing or shaping the mineral. After handling witherite samples, it is advisable to wash your hands.

1. Characteristics

Crystal system: Orthorhombic bipiramidal.          

Color: Colorless, white, pale gray. It may show shades of pale yellow, pale brown, or pale green.

Habit: See above.       

Cleavage: {010} distinct, {110} poor, {012} poor. Horizontal striations.      

Tenacity: Brittle.        

Twinning: See above.       

Fracture: Irregular.       

Mohs Hardness: 3 – 3.5

Parting: No.         

Streak: White.         

Lustre: Vitreous, resinous.          

Diaphaneity: Transparent.          

Density (g/cm³): 4.3

 

2. Geology and Deposits

Witherite forms in small amounts in low-temperature hydrothermal veins associated with limestone or other types of limestone rock or sediment.

Typically it is an alteration product of barite; it may be a product of alteration of celestine, even forming pseudomorphs.

It may be of anoxic sedimentary origin, with the Ba coming from hot springs of volcanic origin.

It can occur in association with coal seams, but is uncommon.

 

3. Mineral Associations

It occurs associated with other carbonates such as calcite, aragonite, barytocalcite, siderite, alstonite, paralstonite and strontianite.

Also with sulfates (celestine, barite), sulfides (galena, sphalerite, chalcopyrite) and fluorite.

The association with barite is very typical.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1,529      nβ: 1,676     nγ: 1,677

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.

Relief: The relief varies between low and moderate to high every 90º to the platinum rotation in crystals with well defined cleavage. This phenomenon has been dubbed “relief pleochroism” or “chagrin change” and is typical of carbonates (calcite, dolomite, aragonite, siderite, rhodochrosite, magnesite, etc.).

When microcrystalline, these carbonates do not show “relief pleochroism” or it is very difficult to perceive.           

Cleavage: {010} distinct, clearly visible in both basal and longitudinal sections. There are two other bad cleavages, usually not visible.           

Habits: Massive, short prismatic, may be fibrous. Short prismatic crystals of pseudohexagonal habit, always twinned.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.148: high order colors (up to 8th order), pearly, difficult to determine, as other carbonates too (calcite, dolomite, magnesite, etc.).           

Extinction: Parallel to the cleavage in the longitudinal sections. 

Elongation sign: ES(-) in basal and longitudinal sections in relation to cleavage.            

Twins: Always occurs, pseudohexagonal, cyclic, forming sections with six zones.         

Zoning: No.             

CONVERGENT LIGHT

Character: B(-)          

2V angle: 16º         

Alterations: witherite can alter to barite. And barite can alter to witherite.          

May be confused with: other orthorhombic carbonates such as aragonite and strontianite.

Trigonal (rhombohedral) carbonates such as calcite, dolomite, siderite, rhodochrosite and magnesite are uniaxial and show rhombohedral cleavage.

Aragonite has almost the same indices and cleavage, but its density is much lower.

Strontianite has lower refractive indices and two good cleavages.

Witherite 01: In PPL, witherite is colorless, without any typical form and without cleavage in this case.
Witherite 01: The same section as of the previous image, now in XPL. The interference color is typical for carbonate and the absence of twins and cleavage in these crystals indicate that the mineral is not any of the trigonal carbonates (calcite, dolomite, rhodochrosite, magnesite). However, other analytical methods are necessary to confirm the witherite; petrography is not enough.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of witherite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with witherite.

Sample preparation: witherite, like other carbonates (calcite, dolomite, siderite, magnesite) acquires a good polish with some ease.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.       

Pleochroism: Strong, between light gray and dark gray, like calcite.

Reflectivity:  Low (<10%)       

Bireflectance:  Possible, but the strong pleochroism makes it difficult to identify.      

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy, which can be seen clearly, despite internal reflections.

Internal reflections: Widespread, colorless to milky.      

May be confused with: other carbonates, as common carbonates have virtually identical optical characteristics.       

General Characteristics: 

Polishing scratches, due to the low hardness, will always be present.

Polishing pits, due to cleavage, are possible but usually do not occur.

Twins are not visible, at least in the analyzed samples. Neither twins as in calcite, much less pseudohexagonal twins.

Witherite 01: In PPL, witherite with some chalcopyrite grains (golden). The pleochroism of witherite is easy to observe.
Witherite 01: The same section as of the previous image, now in XPL. Chalcopyrite is very dark, almost black. Witherite exhibits white internal reflections.
Cavity in the polished section, observed in XPL, into which some witherite crystals grew. The internal reflections correspond to the color of the mineral in hand specimen.
In XPL, aggregate of witherite crystals forming an approximately radial coating around another grain.