IODARGYRITE

Iodargyrite – AgI – is a rare halide, a typical secondary mineral of Ag deposits in arid climates. It contains 46% Ag. Sometimes the volumes of the mineral in certain occurrences make it a silver ore. Large volumes of iodargyrite have been found only in Broken Hill (Australia) and Chanarcillo (Chile). Both occurrences are depleted.

Iodargyrite may contain Cl and Br. It forms paramorphs on miersite, with mimetic repeating twins, with [001] of iodargyrite parallel to {111} of the original miersite. It forms twins by {30-34} and may constitute twins with 4 individuals (“fourlings”.

1. Characteristics

Crystal system: Hexagona, pyramidal, dihexagonal.    

Color: Colorless when fresh. After exposure to light, it turns pale yellow, greenish yellow, yellow, brownish, pearly gray.

Habit: Prismatic to tabular crystals, generally hemimorphic, up to 1.5 cm. Lamellar, barrel and sphere shapes, rosettes, massive.

Cleavage: {0001} perfect.

Tenacity: Sectile, flexible.       

Twinning: It forms twins by {30-34} and may constitute twins with 4 individuals (“fourlings”.

Fracture:  Conchoidal.

Mohs Hardness: 1.5 – 2

Parting: No. 

Streak: Bright yellow.

Lustre:  Adamantine, resinous.

Diaphaneity: Transparent.   

Density (g/cm³): 5.69

 

2. Geology and Deposits

Iodargyrite is a secondary mineral that forms in the oxidized zone (gossan) of silver-bearing ore deposits in arid climates.

 

3. Mineral Associations

It is associated with common minerals in oxidized zones, such as quartz, calcite, “limonite” (goethite), wad, and alunite.

It also occurs with other Ag minerals, such as native silver, acanthite, chlorargyrite, chlorargyrite-(Br), jalpaite, bromargyrite, and bromargyrite-(I).

Also with secondary Cu (+-Pb) minerals (malachite, azurite, beaverite-(Cu), chrysocolla, clinoclase, osarizawaite), secondary Pb minerals (anglesite, cerussite, desclozite, mimetite, pyromorphite, vanadinite, coronadite), and secondary Zn minerals (smithsonite).

 

4. Transmitted Light Microscopy

Refraction indices:  nω: 2,210     nε: 2,220                    

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless.  

Relief: Very high.    

Cleavage: {0001} perfect.

Habits: Prismatic to tabular crystals, generally hemimorphic, up to 1.5 cm. Lamellar, barrel and sphere shapes, rosettes, massive.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.010 (gray to straw yellow of the 1st order). Exhibits anomalous green interference colors.

Occasionally exhibits anomalous wavy birefringence due to deformations. 

Extinction: No information available, probably paralell.  

Elongation sign: No information available.   

Twins:  Possible.     

Zoning: No information available.     

CONVERGENT LIGHT

Character: U(+), can be anomalously biaxial due to deformations.

2V angle: No.

Alterations: No information available.     

May be confused with: No information available.

Caption.

5. Reflected Light Microscopy

Sample preparation: Due to its low hardness, iodargyrite is very difficult to polish (as is cerargyrite), and polished sections, in addition to not acquiring a good polish, will always show many polishing scratches, which are somewhat diagnostic when combined with its other properties.

PLANE POLARIZED LIGHT – PPL

Reflection color: Medium gray.

Pleochroism: No.

Reflectivity: ~15%

Bireflectance:  No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: It does not exhibit anisotropy.

Internal reflections: Widespread, bright, in intense yellow.

May be confused with: Other transparent secondary minerals with yellowish colors.

Iodargyrite 01: In PPL, iodargyrite with typical low reflectivity and polishing scratches.
Iodargyrite 01: The same section as of the previous image, now in XPL. Internal reflections are widespread in the characteristic color, a yellow with a defined greenish hue.