CROCOITE
Crocoite – PbCrO4 – is a very rare chromate, formed under very special conditions. It can be a Cr ore and has been used for paints, but in the collector’s mineral market it fetches very high prices. Exceptional collector’s pieces come from Dundas Mine and Adelaide Mine (Dundas, Tasmania).
It may contain Zn and S. Exposure to ultraviolet light causes the crystals to lose some of their transparency and luster.
CAUTION! Crocoite, in addition to lead (which is toxic), contains the hexavalent ion Cr, which is highly toxic, carcinogenic, and mutagenic. Always wash your hands after handling crocoite samples. Avoid inhaling dust when breaking the mineral. Never lick or ingest it. Never dispose of Cr solutions down the drain.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Intense orange-red, called “hyacinth-red”.
Habit: Crocoite is usually found in the form of long prismatic crystals with poorly terminated tips and an almost square basal section, which cluster in radial aggregates or form randomly arranged aggregates. More rarely, crocoite forms equidimensional crystals or pseudo-octahedrons. When in very small crystals it can be light yellow to orange, some crystals are dark red. Crystals are often cavernous or hollow. It can be massive, imperfectly columnar or granular.
Cleavage: {110} distinct, {001} indistinct, {100} indistinct. Striations // a [001].
Tenacity: Brittle and sectile.
Twinning: No.
Fracture: Conchoidal to irregular.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: Orange-yellow.
Lustre: Subadamantine, subvitreous, resinous.
Diaphaneity: Transparent.
Density (g/cm³): 5.9 – 6.1
2. Geology and Deposits
Crocoite is a rare secondary Pb-Cr mineral because its formation depends on very specific conditions: hydrothermal vein environments of oxidized Pb, with galena and other lead minerals, in the presence of ultramafic rocks that provide the Cr-bearing fluids necessary for crocoite formation.
3. Mineral Associations
It is associated with some common gangue minerals such as quartz, “limonite” (goethite), barite and gibbsite.
It evidently occurs with other secondary Pb minerals such as pyromorphite, cerussite, cerussite-Cr, anglesite, mimetite-Cr (“bellite”), vauquelinite, vanadinite, descloizite, wulfenite, dundasite, coronadite, philipsbornite, phoenicochroite, hidalgoite and embreyite.
Also with gold and asbolane.
4. Transmitted Light Microscopy
Refraction indices: nα: 2.290 nβ: 2.360 nγ: 2.660
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Orange-red, with weak pleochroism between orange-red and blood-red.
Relief: Very high.
Cleavage: {110} distinct, {001} indistinct and {100} indistinct.
Habits: Long prismatic with incomplete terminations. May be granular or pseudo-octahedral.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.370: very high-order colors, reminiscent of calcite colors.
Extinction: Probably oblique.
Elongation sign: No information available. Probably difficult to determine due to the strong colors of the mineral itself, which mask interference colors.
Twins: No.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+)
2V angle: 57º
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying the mineral. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with the mineral, like galena.
Sample preparation: No information available.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray.
Pleochroism: Pleochroism ranging from greenish to grayish-violet. May exhibit subtle internal reflections in orange.
Reflectivity: ~32%
Bireflectance: Weak.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy between greenish and violet.
Internal reflections: Widespread, orange-red, very bright, clear.
May be confused with: No information available.