CORONADITE

Coronadite – Pb(Mn4+6Mn3+2)O16 – is a common manganese oxide, but rarer than other Mn oxides. It is a minor component of Mn ores.

It is classified in the Coronadite Group, in the Hollandite Supergroup. It may contain Al, Fe, and H2O.

Conclusive identification of many Mn oxides is obtained only or preferably with X-ray Diffraction or Infrared Spectroscopy.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Dark gray to black.  

Habit: Massive, granular, botryoidal crusts with a fibrous structure. Microscopic curved lamellae. Laminated. 

Cleavage:  No.  

Tenacity: No information available. 

Twinning: No. 

Fracture: No information available. 

Mohs Hardness: 4.5 – 5

Parting: No.

Streak: Blackish brown.

Lustre: Metallic, sub-metallic, dull, earthy. 

Diaphaneity:  Opaque.  

Density (g/cm³): 5.25 – 5.5

 

2. Geology and Deposits

Coronadite forms in hydrothermal veins.

It also forms as a product of the alteration of Mn-containing rocks, integrating into the assemblage of secondary minerals in oxidation zones.

It occurs in layered sedimentary deposits.

Coronadite also occurs in hot springs.

 

3. Mineral Associations

It is associated with common gangue minerals such as quartz.

Of course, it also occurs with other Mn minerals such as hollandite, psilomelane, pyrolusite, and wad.

As an alteration product, it is part of the typical minerals of oxidation zones in polymetallic deposits, such as goethite, smithsonite, wulfenite, cerussite, chalcophanite, vanadinite, kaolinite, mottramite, malachite, chlorargyrite-Cr, plumbogummite, and pyromorphite.

 

4. Transmitted Light Microscopy

The literature indicates that coronadite is biaxial.

5. Reflected Light Microscopy

Sample preparation: Its hardness upon polishing is greater than that of manganite.

PLANE POLARIZED LIGHT – PPL

Reflection color: White to grayish-white, sometimes identical to galena.      

Pleochroism: Strong, somewhere between dark brown and gray.

Reflectivity: ~35%        

Bireflectance: Distinct.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy between white, dark gray, and dark brown. Parallel extinction, with isotropic basal sections. 

Internal reflections: No.      

May be confused with: Other Mn oxides.       

General Characteristics: 

Grain shape: Coronadite forms botryoidal, irregular, cellular, lamellar, or fibrous masses, and can be granular. It can form concentric zones, alternating between fibrous zones and gel-like zones, which are isotropic. It probably occurs as isolated crystals with an extremely acicular habit.

Twins: spearhead-shaped twins may occur.

Substitutions: coronadite replaces manganite.

Coronadite 01: In PPL, banded structures of coronadite. The pleochroism is not so easy to perceive. Hole in black.
Coronadite 01: The same section as of the previous image, now in XPL. Anisotropy in white and brown is very clear, easy to observe.
Coronadite 02: Another view of coronadite in PPL. The banding is typical of Mn oxides and similar to the one exhibited often by chalcedony.
Coronadite 02: The same section as of the previous image, now in XPL. Often it is useful to uncross the polarizers a bit to observe the anisotropy colors.
Coronadite 03: In PPL, detail of one of the coronadite bands. Even in PPL it is possible to see the individual crystals.
Coronadite 03: The same section as of the previous image, now in XPL. White/brown anisotropy is very evident and the individual crystals are easy to see.
In XPL, coronadite band with decreasing grain sizes from top to bottom.
In XPL, coronadite bands formed of almost fibrous paralell crystals.
In XPL, coronadite crystals seen with high magnification (10x ocular, 50x objective) with typical anisotropy colors.