VANADINITE
Vanadinite – Pb5(VO4)3Cl – is a rarer vanadate, characteristic as a secondary mineral in polymetallic ores containing Pb. It is one of the most important V ores and a very secondary source of Pb.
It is classified in the Apatite Group and is isostructural with pyromorphite and mimetite, with which it forms solid solutions. It can contain As, Ca, and P. There are two varieties (with As, with Cu).
Since vanadinite is neither a transparent rock-forming mineral nor an opaque ore mineral, its microscopic characteristics, both in transmitted and reflected light, are not addressed in either of the two groups into which mineralogy textbooks and other sources are divided. Furthermore, its occurrence in oxidized ores, typically cavernous/porous, makes the preparation of thin sections and polished sections difficult, requiring impregnation. That’s why there’s almost no information, and even fewer images, of vanadinite.
1. Characteristics
Crystal system: Hexagonal bipyramidal.
Color: Can be intense, in orange-red, reddish-brown, deep red, or brown. It can also be whitish or light straw-yellow. Zoning occurs, as well as concentric zones of distinct compositions.
Habit: Well-developed hexagonal prisms, which can be very short (tabular) to very elongated (almost acicular) prismatic. They reach 15 cm. It can also be acicular, capillary, fibrous, cavernous, or skeletal. Rarely rounded to globular.
Cleavage: No.
Tenacity: Very brittle.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: White to pale yellow and light brownish-yellow.
Lustre: Sub-adamantine to resinous
Diaphaneity: Transparent.
Density (g/cm³): 6.88
2. Geology and Deposits
Vanadinite is typically a secondary mineral, formed in the oxidized zones of lead ores from the oxidation of galena. It occurs only in arid climates. Vanadium originates from the leaching of silicates from the surrounding rocks.
3. Mineral Associations
Its occurrence in oxidation zones implies its association with many dozens of other minerals that can develop there. Oxidation zones can contain up to more than 300 different mineral species. Any list of associated minerals here will always be incomplete.
Vanadinite is evidently associated with other secondary Pb minerals such as anglesite, cerussite, descloizite, mimetite, mottramite, pyromorphite, and wulfenite.
It occurs with other minerals typical of oxidation zones, such as limonite (goethite), carbonates (calcite, siderite), quartz, barite, and manganese oxides.
4. Transmitted Light Microscopy
Refraction indices: nω: 2,416 nε: 2,350
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless to subtly colored with faint pleochroism.
Relief: Very high.
Cleavage: No.
Habits: Hexagonal prisms. Short prismatic, almost tabular, can be long prismatic. Also fibrous, acicular, cavernous, skeletal. Rarely globular.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.066, corresponding to interference colors of up to the 3rd order higher, very colorful, intense, dominated by green and pink.
Extinction: Probably parallel in longitudinal sections.
Elongation sign: No information available.
Twins: No.
Zoning: Possible, with variations in color and chemical composition.
CONVERGENT LIGHT
Character: U(-)
2V angle: No.
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 vanadinite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with vanadinite.
Sample preparation: No information available.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray. Orange reflections may occur internally.
Pleochroism: No.
Reflectivity: ~20%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy in shades of gray, masked by strong internal reflections.
Internal reflections: Widespread in intense colors, in various shades of yellow, orange, red and red-brown.
May be confused with: No information available.