CHALCOPHANITE
Chalcophanite – ZnMn4+3O7 · 3H2O – is a fairly rare oxide associated with Zn and Mn ores.
It is classified in the Chalcophanite Group and may contain Fe and/or Mn replacing Zn.
When it occurs with a pseudo-octahedral habit, it can be confused with heterolite, with which it is frequently associated.
It is almost indistinguishable from aurorite (its Zn analogue) and jianshuiite. It does not fluoresce and is soluble in HCl.
1. Characteristics
Crystal system: Trigonal rhombohedral.
Color: Iron black, purplish black, bluish black. In thin sheets it is dark red.
Habit: Chalcophanite generally occurs in drusic, botryoidal, reniform habits or in stalagtitic crusts, with tabular crystals arranged along the rim. It can occur massively, densely, granularly, or tabularly and fibrously. It forms cryptocrystalline, colloform bands that border and fill cavities between previously formed Mn minerals. Rarely, it forms isolated tabular crystals up to 8 mm in size, which can cluster to form open rosettes. Idiomorphic crystals are pseudo-octahedral. It can fill fractures in psilomelane, pyrolusite, and other Mn oxides.
Cleavage: {0001} perfect.
Tenacity: Brittle, flexible.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 2.5
Parting: No.
Streak: Dark brown, reddish-brown.
Lustre: Metallic to sub-metallic.
Diaphaneity: Transparent.
Density (g/cm³): 3.9 – 4.1
2. Geology and Deposits
Chalcophanite is a common secondary mineral that occurs in the oxidation zone of polymetallic Zn and Mn deposits.
3. Mineral Associations
In the type locality (Franklin Mining District, New Jersey, USA), it is associated with quartz, calcite, zincite, willemite, and franklinite.
It occurs with minerals characteristic of oxidation zones of polymetallic deposits, such as:
– secondary Cu minerals (native copper, malachite),
– secondary Zn minerals (smithsonite, hemimorphite, adamite, woodruffite, hetaerolite),
– secondary Fe minerals (limonite, goethite, hematite), and
– secondary Mn minerals (cryptomelane, manganite, birnessite, todorokite).
Also with kaolinite.
4. Transmitted Light Microscopy
Information regarding the characteristics of chalcophanite under transmitted light is fragmentary and can be summarized as follows:
Refractive indices: nω: 2.720 nε: 2.720
Color: reddish-brown to opaque.
Relief: very high.
Birefringence: maximum birefringence of 0.000, corresponding to black colors.
5. Reflected Light Microscopy
Sample preparation: Chalcophanite has a very low polishing hardness, making it difficult to produce a polished section without grooves. In XPL, some polishing scratches will always be visible, but less conspicuous than those in hausmannite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Pure white with a gray tint, without any other color tones.
Pleochroism: Strong, ranging from white to a much darker gray, similar to molybdenite and graphite.
It resembles the pleochroism of hausmannite, but without its yellowish hue.
Reflectivity: 9.30 – 26.02%
Bireflectance: Very strong, ranging from light white to dark gray, very characteristic and diagnostic, stronger than that of molybdenite.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very strong, ranging from light gray to white and grayish-white to bluish-gray.
Basal sections are isotropic.
Internal reflections: Abundant, between carmine red and dark red, when the chalcophanite is rich in Zn. Zn-deficient chalcophanite does not show internal reflections.
May be confused with: Lithiophorite and aurorite, which are very similar.
General Characteristics:
Grain shape: see habits described in the introductory text.
Cleavage: perfect basal cleavage is always visible in crystals.
Intergrowths with tabular, twinning-like crystals are not uncommon. They resemble the structures formed by the triple twinning of cristobalite.
Alternating banding with psilomelane and cryptomelane may occur.
Substitutions: chalcophanite substitutes franklinite, manganite, and pyrolusite.
