WULFENITE
Wulfenite – PbMoO4 – is a relatively common molybdate and a typical secondary mineral in lead deposits. It occurs in polymetallic ores, primarily those containing lead and zinc. There is an industrial process (patented by Union Carbide and Carbon Corporation) for extracting molybdenum from wulfenite. Well-formed wulfenite crystals command high prices in the collector mineral market.
Wulfenite is classified within the Scheelite Group and forms a complete solid solution series with stolzite (PbWO4). Specific names have been assigned to some members of this solid solution (e.g., chillagite for 64% wulfenite and 36% stolzite), but the International Mineralogical Association has ruled that these names are unnecessary. There are four additional varieties (containing Ca, Cr, W, or V).
Common impurities in wulfenite include Cr, As, W, Ti, V, and Ca. When occurring as pyramidal or prismatic crystals, it exhibits distinct hemihedry. The crystals, which can reach up to 11 cm in size, may be piezoelectric. It sometimes fluoresces dark yellow to orange under short-wave UV light and dark orange to red under long-wave UV light.
1. Characteristics
Crystal system: Tetragonal dipyramidal.
Color: Yellow-orange, yellow, honey-yellow, reddish-orange, rarely colorless, gray, brown, olive green, and even black.
Habit: Thin tabular (square) to pyramidal, can be elongated, cubic, prismatic, etc. Granular, massive.
Cleavage: {011} distinct, {001} indistinct, {013} indistinct.
Tenacity: Brittle.
Twinning: Contact twins on {001}, common.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: Yellowish white.
Lustre: Adamantine, resinous.
Diaphaneity: Transparent.
Density (g/cm³): 6.5 – 7.5
2. Geology and Deposits
Wulfenite is one of the many secondary minerals that form in the oxidized zones of hydrothermal Pb ores, typically where galena is associated with sphalerite. The Mo in its formula was generally introduced into the deposit from external sources.
Exceptionally, wulfenite can precipitate from volcanic gases in fumaroles at temperatures between 400 and 550°C.
3. Mineral Associations
It occurs associated with common gangue minerals such as quartz, barite, fluorite, and carbonates (calcite).
It naturally occurs with galena (from which it formed) and with other secondary Pb minerals, such as cerussite, pyromorphite, descloizite, mimetite, plattnerite, vanadinite, and anglesite.
Sphalerite is generally present in the paragenesis; consequently, secondary Zn minerals such as smithsonite and hemimorphite are found. Cu sulfides also occur, leading to the presence of secondary Cu minerals like malachite and dioptase.
It is found with molybdenite, goethite, and psilomelane.
4. Transmitted Light Microscopy
Refraction indices: nω: 2.405 nε: 2.283
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Exhibits weak pleochroism between orange and yellow.
Relief: Very high.
Cleavage: {011} distinct, {001} e {013} indistinct.
Habits: Granular, massive; may be earthy. Crystals are generally thin and tabular; they may resemble cubes.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.122, very high, corresponding to pearly interference colors of up to the 6th order, which are difficult to determine.
Extinction: Paralell.
Elongation sign: No information available.
Twins: Contact twins on {001}, very common.
Zoning: No information available.
CONVERGENT LIGHT
Character: U(-); it could be an anomalous B(-).
2V angle: Up to 8º.
Alterations: Alters to descloizite, ilsemannite and vanadinite.
May be confused with: Other yellow to orange transparent minerals such as orpiment and chlorargyrite, but the paragenesis is different.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for identifying wulfenite. However, preparing a polished section is important for identifying opaque minerals associated with wulfenite, such as galena.
Sample preparation: Wulfenite takes a reasonable polish with some effort. Cleavage planes tend to create numerous pits on the mineral’s surface. Its polishing hardness is lower than that of descloizite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray.
Pleochroism: No.
Reflectivity: ~15%
Bireflectance: Weak.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy masked by internal reflections.
Internal reflections: Widespread, strong, ranging from colorless to various shades of yellow and orange in intense hues.
May be confused with: Other transparent minerals that exhibit yellow to orange colors in hand samples, such as orpiment.
General Characteristics:
Grain shape: occurs as idiomorphic crystals and aggregates of xenomorphic grains.
Polishing scratches are generally visible due to the mineral’s low hardness.
