BARYTE

Baryte – BaSO4 – is a relatively common sulfate, being the main ore mineral of Ba.

Baryte crystals can exhibit a characteristic chisel-like shape, so one of the German names is “Meisselspat” (“Meissel” = chisel, “spat” = cleaved crystal). The crystals reach lengths of up to 85 cm. More than 70 different combined forms have been recorded.

Baryte forms a solid solution with celestine (SrSO4) and can be confused with it. The distinction can be made by the flame test: baryte powder thrown into the flame produces a faint green color, celestine powder produces a red color.

Baryte is diamagnetic and may contain Sr, Ca and Pb. It can be fluorescent in yellow, orange and pink. The color of the baryte may change to white or colorless if exposed to the sun.

It has 14 varieties, generally based on different habits or higher levels of a certain element (Pb, Ce, Ra, etc.). The “bologna stone” variety is an impure baryte that, once exposed to the sun, becomes phosphorescent in green-white (thermoluminescence).

1. Characteristics

Crystal system:  Orthorhombic bipiramidal.         

Color:  White, colorless, light shades of blue, yellow, green, gray, brown. May be zoned or multicolored.    

Habit: Tabular, massive, nodular, fibrous, banded, fibrous, etc.       

Cleavage:  {001} perfect, {210} less perfect, {010} imperfect. 

Tenacity: Brittle.        

Twinning: No.       

Fracture: Irregular.       

Mohs Hardness: 3

Parting: No.         

Streak: White.         

Lustre:  Vitreous to resinous, may be pearly.         

Diaphaneity: Transparent.           

Density (g/cm³):  4.5

          

2. Geology and Deposits

Baryte occurs in a wide variety of geological environments.

It is a late gangue mineral from low to medium temperature (epi– and mesothermal) hydrothermal veins of Pb and Zn ores hosted in limestone. It also occurs as tufa in hot springs, in iron ores, and in sedimentary deposits derived from weathered baryte limestones.

Baryte is an accessory mineral of igneous rocks such as carbonatites. It occurs as a primary component of massive volcanogenic submarine sulphide deposits and ocean floor smokestacks (fumarolas – “black smokers”). It can be deposited by biogenic, hydrothermal, evaporation and other processes. It replaces wood, shells and fossils, forms concretions and “desert roses”. It is found in well-formed crystals in geodes (from basaltic rocks, for example) and in miarolitic cavities (from other igneous rocks). It is one of many minerals that can form speleothems in caves.

 

3. Mineral Associations

It occurs with quartz, carbonates (calcite, aragonite, dolomite, rhodochrosite, siderite, cerussite), sulfides (pyrite, chalcopyrite, stibnite, sphalerite, galena), sulfates (anglesite, celestine, gypsum), fluorite, apatite, native sulphur, vanadinite and manganite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.636     nβ: 1.637     nγ: 1.648

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Usually colorless, without pleochroism.

It can be pleochroic in soft tones in the following colors (X,Y,Z): brown: straw yellow, wine yellow, violet; yellow: pale yellow brown, yellow brown, brown; green: almost colorless, pale green, amethyst; blue-green: blue-violet, blue-green, violet. 

Relief:  Moderate.          

Cleavage:  {001} perfect, {210} less perfect, {010} imperfect. Cleavage is visible only in the larger crystals, parallel to the elongation. It can be pseudo-cubic, a finding that needs to be made very carefully.          

Habits: Usually thin to thick tabular crystals that may form rosette aggregates with radial habit. Baryte can have many different habits: elongated, prismatic, equidimensional, concretionary, fibrous, nodular, stalagtitic, banded, earthy or massive. Granular Barytes are common and difficult to identify.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Low birefringence, from 0.011 to 0.012: gray to 1st order yellow-orange colors.          

Extinction:  Parallel in the direction of the cleavage (001), symmetrical in {001} (basal sections), oblique in sections with another orientation.          

Elongation sign: ES(+)            

Twins:  Rare, lamellar, polysynthetic by {110}.        

Zoning: No.             

CONVERGENT LIGHT

Character: B(+)          

2V angle: 36 – 40º        

Alterations: Can be replaced by SiO2 and carbonates. Chalcedony forms pseudomorphs on Baryte.          

May be confused with: baryte is a mineral that easily goes unnoticed due to the multitude of different habits it can present.

Celestine forms a series with baryte and is very similar, but has lower refractive indices.

Gypsum is very similar, but has low relief.         

Baryte 01: In PPL, baryte crystals. Colorless, with some cleavage and moderate relief, but impossible to identify in PPL.
Baryte 01: The same section as of the previous image, now in XPL. The shape is very typical and interference colors usually do not reach 1st order straw-yellow.
Larger crystals may exhibit the characteristic pseudo-cubic cleavage. Image taken in PPL with the diaphragm closed by more than 60%.
Large baryte crystals in XPL. The crystal in the middle of the image show its extinction parallel to shape and best cleavage.
In XPL, baryte as a pore filler in a sandstone.
Idiomorphic baryte crystals, typically ending with an acute angle, similar to a chisel. XPL.
Diagonally in the image, in XPL, baryte crystal showing its typical cleavage parallel to its elongation.
In XPL, granular baryte, whose recognition is more difficult. Diagnostic are moderate relief and cleavage.

5. Reflected Light Microscopy

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

Sample preparation:  polishing baryte is relatively simple and becomes of very good quality. Its polishing hardness is between that of calcite and the hardness of most rock-forming silicates.      

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray.       

Pleochroism: No.      

Reflectivity: Low (~8%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy was not observed.       

Internal reflections:  Widespread clear to milky, sometimes multicolored.     

May be confused with: difficult to recognize in a polished section, because its characteristics are similar to many other gangue minerals. Absence of bireflectance excludes carbonates, medium hardness excludes most silicates, which are harder. Phyllosilicates of similar hardness have much better cleavage, are darker in PPL, have much more polishing scratches, and are often bent. When baryte has its typical shape – tabular crystals with one side ending in a point – it is easier to recognize. It is usually necessary to make a thin section for confirmation.       

General Characteristics: 

Cleavage parallel to (001) is clearly visible, with associated polishing pits.

Baryte 01: Tabular baryte crystals in PPL. The reflectivity is somewhat higher than the gangue minerals around them (the crystals are lighter) and they are very similar to gypsum.
Baryte 01: The same section as of the previous image, now in XPL. The widespread clear to yellowish internal reflections make baryte recognition very difficult, if not impossible.