PHOSGENITE
Phosgenite – PbCO3Cl2 – is a fairly rare carbonate, typical of alteration (oxidation) zones of Pb sulfide deposits. Due to its rarity, it is not considered an ore.
Phosgenite has some special characteristics:
– the hardness in the mineral is anisotropic, as in kyanite; in this case the hardness in {110} is lower parallel to [001] and higher perpendicular to [001].
– it is possible to bend the crystals easily in directions perpendicular to [001].
– it can form epitaxies with cerussite (PbCO3); it alters or is easily replaced by cerussite.
– it may exhibit yellow fluorescence under short-wave and long-wave ultraviolet light, X-rays and cathode rays (see images at the end of the sheet).
– it decomposes slowly in cold water, which extracts Pb chloride.
Caution! As it contains Pb, wash your hands after handling. Never inhale dust. Never lick. Never ingest. Never saw, grind, or polish dry, as this generates dust.
1. Characteristics
Crystal system: Tetragonal, ditetragonal bipyramidal.
Color: Light colors in various shades: colorless, smoky, yellow, brown, pink, violet, greenish.
Habit: Short or long prisms, very rarely tabular. Massive, granular. Usually pinacoid {001}.
Cleavage: {001} distinct, {110} distinct, {010} bad
Tenacity: Sectil
Twinning: Translation twins.
Fracture: Conchoidal
Mohs Hardness: 2 – 3
Parting: No.
Streak: White.
Lustre: Adamantine
Diaphaneity: Transparent.
Density (g/cm³): 6.12 – 6.15
2. Geology and Deposits
Phosgenite forms in the oxidation zones of Pb deposits, usually from galena. The supply of Cl is important for its formation, so it occurs especially when seawater or other chlorine sources react with Pb minerals.
3. Mineral Associations
It is associated with the primary lead mineral, which is galena.
It occurs with other secondary lead minerals, such as cerussite, anglesite, diaboleite, paralaurionite, fiedierite, pseudoboleite, matlockite, laurionite, and nealite.
In oxidation zones, it occurs alongside many other secondary minerals, including gypsum.
4. Transmitted Light Microscopy
Refraction indices: nω: 2,118 nε: 2,145
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless. In sections with thicknesses above 30 microns, there is a slight pleochroism between reddish and greenish.
Relief: Very high.
Cleavage: The literature doesn’t specify, but the cleavage, although of poor quality, is likely visible.
Habits: Short to long prisms, massive, granular.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.027, corresponding to colors up to the middle of the 2nd order: white, yellow, orange, red, purple, blue up to greenish tones.
Extinction: The literature doesn’t provide this information, but it’s probably paralell.
Elongation sign: No information available.
Twins: Translation twins, may be visible.
Zoning: No information available.
CONVERGENT LIGHT
Character: U(+), can be B(+) if the phosgenite has been subjected to stress.
2V angle: No information available. Probably small, when it occurs anomalously.
Alterations: Phosgenite can alter to cerussite.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying phosgenite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with phosgenite, such as galena.
Sample preparation: No information available yet.
PLANE POLARIZED LIGHT – PPL
Reflection color: No information available yet.
Pleochroism: No information available yet.
Reflectivity: No information available yet.
Bireflectance: No information available yet.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: No information available yet.
Internal reflections: No information available yet.
May be confused with: No information available yet.