BERTHIERITE

Berthierite – FeSb2S4 – is a relatively rare sulfide, characteristic of hydrothermal veins containing Sb minerals. It is a secondary Sb ore. Do not confuse it with “berthierin,” which is a greenish phyllosilicate (a mica) that occurs in marine sediments, lateritic soils, and polar soils.

It is a member of the Berthierite Group, which also includes clerite and garavellite. Impurities may include Pb, Ag, Mn, and Cu. Its relative rarity can be explained by the fact that it is stable only in very narrow ranges of fO2 and fS2.

Its physical characteristics and habit in radial aggregates of acicular crystals cause it to be easily confused with stibnite, even in hand specimens.

1. Characteristics

Crystal system: Orthorhombic holohedral.          

Color: Dark steel gray, usually with tarnished iridescent or bronze brown.

Habit:  Secondary acicular (001), typically without well-developed terminations. Fibrous, plumose or radial, granular or massive.

Cleavage: Indistinct prismatic. Striations // a (001).       

Tenacity: Brittle. 

Twinning: No. 

Fracture: Irregular. 

Mohs Hardness: 2 – 3

Parting: No. 

Streak: Dark brown gray. 

Lustre: Metallic.          

Diaphaneity: Opaque.           

Density (g/cm³): 4.64 

 

2. Geology and Deposits

It occurs in low-temperature hydrothermal veins of Sb, in the sulfur-deficient portions.

 

3. Mineral Associations

Berthierite occurs associated with common gangue minerals such as quartz, calcite, siderite, and barite.

In the specific paragenesis of antimony, it is associated with Sb oxides such as valentinite and a series of Sb-containing sulfides, such as stibnite, boulangerite(+Pb), pyrargyrite(+Ag), jamesonite (+Pb,Fe), breithauptite(+Ni), and andorite(+Ag, Pb).

In addition, it is associated with Fe-containing sulfides (pyrite, pyrrhotite, and arsenopyrite), Pb sulfides (galena), Zn sulfides (sphalerite), Hg sulfides (cinnabar), and Ni sulfides (nickeline).

It is also associated with native elements such as native gold and native silver, silver sulfosalts, and Ni-Co-Fe arsenides.

 

4. Transmitted Light Microscopy

This does not apply, as berthierite is completely opaque.

5. Reflected Light Microscopy

Sample preparation: Despite its low hardness, berthierite polishes well. Its polished hardness is slightly greater than that of stibnite, but less than that of sphalerite. It is very similar to the hardness of cinnabar.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Grayish-pink. According to other sources, grayish-white with pinkish or brownish hues.  

Pleochroism: Strong and highly diagnostic in shades of gray. The literature insists on this strong pleochroism, easy to observe:
// “a”: brownish pink, similar to the color of pyrrhotite.
// “b”: gray-white
// “c”: white, the lightest of the three colors.      

Reflectivity: 31.59 – 39.63%        

Bireflectance: Distinct.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy, similar to that of stibnite, but in more vivid colors, in gray, blue (“slate blue”), white, brown, and pink. Complete extinction.  

Internal reflections: No.      

May be confused with: When only sections “b” and “c” are present in the polished section, it is easily confused with stibnite, which usually occurs in association, but the brownish-pink color of the sections parallel to “a” is very conspicuous.

Stibnite shows almost identical anisotropy, but exhibits much weaker pleochroism.

Jamesonite also exhibits an acicular habit, but with more yellow colors.

Compared to other sulfosalts, reflection pleochroism is very diagnostic.

General Characteristics: 

Grain shape: It can basically occur in three different habits. It generally forms acicular crystals arranged in subparallel or radial aggregates, always in small sizes. It can also occur granular, without defined shapes. In some cases, it occurs fibrous.

Cleavage is generally not visible in polished section.

Twinning does not occur.

Myrmekites, well-developed, of berthierite and chalcopyrite are common.

Oriented intergrowths with stibnite sometimes occur, and only berthierite residues may be present in the stibnite.

Oxidation, incipient, especially forms marcasite, as does marcasite with weathering of pyrrhotite and sternbergite (AgFe2S3).

Alteration to stibnite and pyrite is quite common.

Substitution by berthierite can occur with pyrite, pyrrhotite, arsenopyrite, sphalerite, and gudmundite.

Inclusions of berthierite occur in pyrite, marcasite, pyrrhotite, and arsenopyrite.

Berthierite 01: In PPL, gnague minerals (dark gray), sphalerite (light gray) and berthierite, pleochroic between white and rose/cream. This is very diagnostic and important for differentiating this mineral from stibnite, which exhibits much less pronounced pleochroism.
Berthierite 01: The same section as of the previous image, now in XPL. Gangue with clear internal reflections, sphalerite with honey-colored internal reflections, and berthierite without internal reflections but with strong anisotropy between gray and greenish.
Berthierite 02: In PPL, an aggregate of several berthierite crystals. It is necessary to pay attention to the subtle color differences between the grains to detect the diagnostic pleochroism of berthierite. The image has been processed to better show the pleochroism colors. Gangue is in dark gray.
Berthierite 02: The same section as of the previous image, now in XPL+2º. .In its extinction position, berthierite is very dark, with no internal reflections. When the stage is rotated, a bluish-gray color and a slate-green color appear, depending on the section of the mineral. Surrounding it are gangue minerals with yellowish internal reflections.
Aggregate of berthierite crystals in XPL. Gangue with yellowish internal reflections. The typical anisotropy aids in the diagnosis of berthierite. Twinning, so common in stibnite, is absent in berthierite.