CHLORARGYRITE

Chlorargyrite – AgCl – is a relatively rare halide that occurs associated with Ag ores. It constitutes an Ag ore.

Chlorargyrite is dimorphic with bromargyrite, with which it forms a solid solution. It may contain small amounts of Hg and Na. Like all silver minerals, it changes color once exposed to light, usually darkening. In polished sections, this darkening can occur in a few hours.

There are three varieties of chlorargyrite (with Br, with Br+I, with Br+I+Cl). It melts at 455ºC, separating into Ag and Cl. Chlorine is a toxic gas with a pepper+pineapple smell and a metallic taste.
Due to the great historical interest in silver mining, there are more than 30 names in various languages ​​attributed to chlorargyrite.

1. Characteristics

Crystal system: Cubic hexaoctahedral. 

Color: Colorless, pale yellow, light green. When exposed to light, it turns gray, brown, brownish-violet, or purple.     

Habit: It forms cubic crystals up to 1 cm, modified with octahedrons and rhombododecahedrons, in parallel or subparallel groupings. It rarely forms stalactites, columnar aggregates, or coralloids. Also rarely, it is fibrous or forms masses resembling wax or horns (“horn silver”) when altered in arid regions. It can be massive (very common) or occurs as encrustations, shells, and thin films filling fractures. It can occur as cement in carbonate and sandy rocks.

Cleavage: No. 

Tenacity: Sectile and ductile, very plastic. 

Twinning: On {111}.

Fracture: Irregular to subconchoidal. 

Mohs Hardness: 1.5 – 2.5

Parting: No.

Streak: White. 

Lustre: Resinous to adamantine. 

Diaphaneity: Transparent. 

Density (g/cm³): 5.55

           

2. Geology and Deposits

Chlorargyrite is a secondary mineral that forms in the oxidation zone of primary silver ores when chlorine is present. The origin of this chlorine has always been much debated. In arid climates, the mineral is preserved on the surface.

In outcrop, chlorargyrite is a discreet mineral that easily goes unnoticed. It constitutes small, very rich silver deposits that have given rise to many short-lived mines and towns, since the primary ore at depth does not have an economically viable silver content.

 

3. Mineral Associations

Chlorargyrite occurs associated with primary silver ore minerals, such as native silver, acanthite, and stephanite.

As a secondary mineral, formed in the oxidation zone of polymetallic ores, it is associated with:

– secondary Ag minerals (iodargyrite, bromargyrite),

– secondary Pb minerals (cerussite, pyromorphite, wulfenite, anglesite),

– secondary copper minerals (atacamite, malachite, chrysocolla, covellite),

– secondary Zn minerals (smithsonite),

and other minerals typical of oxidation zones such as quartz, calcite, barite, pyrite, native gold, augelite, Fe hydroxides and sulfates (goethite, jarosite), and Mn minerals (“wad”).

 

4. Transmitted Light Microscopy

Refraction indices: n: 2,071

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:Colorless, yellow or green.

Relief: Very high.           

Cleavage: No.          

Habits: Massive, crusty, cavity-filling formations. Cubic or modified cubic crystals are possible, but rarer.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Isotropic.          

Extinction: Isotropic. 

Elongation sign: Isotropic. 

Twins: Isotropic. 

Zoning: Isotropic. 

CONVERGENT LIGHT

Character: Isotropic. 

2V angle: Isotropic. 

Alterations: Chlorargyrite is an alteration mineral. It can be dissolved.          

May be confused with:  Other secondary silver minerals, but these are anisotropic.        

Caption.

5. Reflected Light Microscopy

Sample preparation:  Due to its low hardness, chlorargyrite is difficult to polish and will always leave many polishing grooves, as is the case with acanthite. Good quality polishing will only be achieved if the sample contains only low-hardness minerals.      

PLANE POLARIZED LIGHT – PPL

Reflection color: Medium gray with a subtle greenish tint.

Pleochroism: No.      

Reflectivity: 20% if the polishing is of good quality. With poor polishing, the reflectivity is lower.  

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Isotropic.        

Internal reflections: Abundant, widespread, light green.

May be confused with:  Low hardness and poor polish quality are very diagnostic. Paragenesis greatly aids in its recognition, as secondary minerals from oxidation zones of lead and zinc ores (which are partly similar to chlorargyrite) are generally distinctly anisotropic and polish much better.      

General Characteristics: 

Light attack is only observed in polished sections after a long time; it is not a process that can be used to visualize structures, zoning, and other features.

Substitutions of native silver by chlorargyrite are common.

Chlorargyrite 01: In PPL, chlorargyrite with the typical very dark reflective color and low-quality polish. Surrounding it are iron hydroxides. Holes in black.
Chlorargyrite 01: The same section as of the previous image, now in XPL. Chlorargyrite shows characteristic internal reflections in green. Iron hydroxides (goethite?) with red reflections.
Chlorargyrite 02: In PPL, xenomorphic grain of chlorargyrite. Around it, aggregates of very small crystals, probably of Mn-minerals (light gray) and Fe-minerals (dark gray), with coloform textures typical of oxidation zones.
Chlorargyrite 02: The same section as of the previous image, now in XPL. Chlorargyrite shows green internal reflections. Mn-minerals in gray/black colors and Fe-minerals with reddish internal reflections.
Chlorargyrite is typical of oxidation zones, which are ores with many cavities. Well-formed crystals of dozens of different minerals grow inside these cavities. In this image, in XPL, obtained from the same polished section as the previous images, shows a small cavity into which an idiomorphic crystal of chlorargyrite has grown. The focus is on the crystal, so the surface of the section is out of focus.