MINIUM
Minium – Pb3O4 – is a very rare oxide, a secondary mineral of lead. It participates in the secondary mineral assembly of polymetallic ores, but is never the main ore mineral.
Macroscopically, it can be confused with litharge (tetragonal PbO, also very red) and with powdery cinnabar (HgS).
As it contains lead, it is potentially toxic and dust inhalation, licking or ingestion should be avoided. It is advisable to wash your hands after handling.
1. Characteristics
Crystal system: Tetragonal, ditetragonal dipyramidal.
Color: Bright red, reddish-brown, sometimes with yellowish tones.
Habit: Massive, earthy, powdery. It forms microscopic crystalline scales.
Cleavage: {110} perfect, {010} perfect.
Tenacity: No information available.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 2.5
Parting: No.
Streak: Yellow orange.
Lustre: Resinous, dull.
Diaphaneity: Transparent.
Density (g/cm³): 8.9 – 9.2
2. Geology and Deposits
Minium is a rare alteration mineral of extremely oxidized lead ores. It occurs in many localities, but always in very small quantities.
It is also formed by fires in mines.
3. Mineral Associations
It ocurrs with several gangue minerals such as barite, gypsum and calcite.
In addition, with primary lead minerals such as galena and a number of other secondary lead minerals such as cerussite, mimetite, wulfenite, plumbojarosite, massicot (orthorhombic PbO), litharge (tetragonal PbO) and native lead.
It also occurs with cuprite and is sometimes associated with Zn minerals.
4. Transmitted Light Microscopy
Refraction indices: n: 2.42
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Red, with strong pleochroism between X = deep reddish brown, Z = almost colorless.
Relief: Very high.
Cleavage: The crystals are too small to show cleavage.
Habits: Very small flakes, earthy, powdery.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.000: black colors, but may show anomalous birefringence of green colors.
Extinction: Paralell.
Elongation sign: ES(-)
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: Uniaxial.
2V angle: No.
Alterations: does not alter, it is an alteration mineral.
May be confused with: other red minerals that even occur in the same paragenesis, such as cinnabar, litharge and others.
5. Reflected Light Microscopy
Sample preparation: polishing is not difficult; the polishing hardness is lower than the hardness of galena, litharge and massicot.
PLANE POLARIZED LIGHT – PPL
Reflection color: Due to strong internal reflections, it shows a pinkish-yellow-gray color. The normal color is bluish.
Pleochroism: Weak, yellowish gray to orange gray and bluish gray.
Reflectivity: 17%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy is masked by strong internal reflections.
Internal reflections: Abundant, widespread, intense and luminous red orange.
May be confused with: other minerals with orange internal reflections, such as orpiment, wulfenite and others.
General Characteristics:
Grain Shape: Generally extremely fine grained.
Replaces and forms pseudomorphs on galena and cerussite.