POWELLITE
Powellite – Ca(MoO4) – is a rare molybdate that forms as a secondary mineral. It rarely forms an ore, despite molybdenum being a strategic element. Powellite crystals, on the other hand, fetch high prices in the collector mineral market.
Powellite is classified in the Scheelite Group and forms a solid solution series (it is isostructural) with it. It frequently contains some tungsten.
It is very easy to confuse powellite with scheelite, but powellite fluoresces from creamy white to yellow and golden yellow under shortwave ultraviolet light, and scheelite fluoresces in white-blue and light blue colors under the same light. Calcite, which can also be very similar, when fluorescent shows reddish, blue, or discreetly greenish colors.
1. Characteristics
Crystal system: Tetragonal bipyramidal.
Color: Hay yellow, greenish yellow, brownish yellow, brown, colorless, blue, almost black, may be zoned.
Habit: Powellite crystals, when idiomorphic, can be pseudo-octahedral, reach 8 cm, and can exhibit combinations of many different shapes. Bipyramidal crystals, more rarely thin tabular crystals, can be as thin as paper. In crusts, powdery or massive.
Cleavage: {011} indistinct, {112} indistinct, {001} indistinct. Striations on the faces may occur.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular, conchoidal.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: Light yellow.
Lustre: Subadamantine, resinous, pearly.
Diaphaneity: Transparent.
Density (g/cm³): 4.25
2. Geology and Deposits
Powellite typically occurs as a secondary mineral in the oxidized zones of hydrothermal molybdenum ores. It frequently forms as an alteration product of molybdenite, assuming the original hexagonal or foliated form of molybdenite. Sometimes it only partially replaces molybdenite.
It sometimes occurs in granitic pegmatites, in skarns (tactites), and in cavities (vesicles) of basalts, such as in the Deccan Traps (India). It forms in tailings from Mo mines.
3. Mineral Associations
When it occurs as a secondary mineral in hydrothermal deposits, it is associated with molybdenite, szenicsite, brochantite, cuprite, pseudomalachite, malachite, azurite, atacamite, scorodite, and many others.
In basalt cavities, it is associated with quartz, calcite, prehnite, apophyllite, celadonite, and zeolites (stilbite, laumontite, scolecite, mordenite).
In skarns, it is associated with scheelite. Powellite alters to ferromolybdite. Some other associated minerals are dolomite, magnetite, and piemontite.
In general, powellite can occur associated with hundreds of different minerals. It has been found, for example, in Tsumeb (360 different minerals), Chuquicamata (148 minerals), Currais Novos (42 minerals), and Franklin Hill (272 minerals), all four locations with enormous mineral diversity. Therefore, the list of associated minerals will never be complete.
4. Transmitted Light Microscopy
Refraction indices: nε: 1.984 nω: 1.974
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: When macroscopically black, it appears to be a deep color. When it shows strong colors, it is pleochroic between blue and green.
Relief: Very high.
Cleavage: No.
Habits: Crusts, massive, powdery, pseudo-octahedral bipyramidal crystals.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Very low birefringence, maximum of 0.010: colors from gray to white, does not reach first-order straw yellow.
Extinction: Probably parallel because it is a tetragonal mineral.
Elongation sign: No information available.
Twins: No.
Zoning: Sometimes powellite is zoned.
CONVERGENT LIGHT
Character: U(+)
2V angle: No.
Alterations: Powellite alters to ferromolibdite.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying powellite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with powellite, such as molybdenite and magnetite.
Sample preparation: Polishing powellite is relatively simple.
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray, slightly lighter than gangue minerals such as silicates.
Pleochroism: No.
Reflectivity: Low (<10%)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: No anisotropy is perceived due to its internal reflections.
Internal reflections: Widespread, strong, white.
May be confused with: Other white transparent minerals.
