THE MINERAL POWELLITE

Chemistry: CaMoO₄, Calcium Molybdenate
Class: Sulfates
Subclass: Molybdenates
Uses: As a minor ore of molybdenum (an important industrial metal) and as mineral specimens.
Specimens

Powellite is one of only a handful of relatively common molybdenum minerals. Other molybdenum minerals include wulfenite, molybdenite, ferrimolybdite, molybdite and sidwellite. Powellite is named for the American geologist, Major John Wesley Powell, a former director of the U. S. Geological Survey. Most of powellite's occurrences are the result of hydrothermal reactions with the primary sulfide mineral molybdenite, with a formula of MoS₂. Powellite in fact, forms pseudomorphs after molybdenite. A pseudomorph is an atom by atom replacement of one mineral's chemistry for another; all the while the crystal retains the outward shape of the original mineral (pseudomorph means "false shape"). These pseudomorphs will have the shape of molybdenite crystals, but are actually made of powellite. Powellite also is known to form as a primary mineral in quartz veins.

Powellite forms an incomplete series with the mineral scheelite, CaWO₄. Scheelite differs from powellite by the substitution of the molybdenum in powellite by the tungsten (W) in scheelite. Some tungsten is usually found in powellite and thus sometimes the formula of powellite is written as Ca(Mo, W)O₄ to reflect this substitution. Scheelite is a popular fluorescent mineral as it typically glows a bright bluish white. Powellite is less well known for its fluorescence, but some specimens can display a delightful golden yellow under ultraviolet light. Crystals of powellite resemble the much more common but heavier crystals of scheelite, fortunately the difference in fluorescence is a key to distinguishing them. Powellite, scheelite and the silicate mineral scapolite all belong to an exclusive symmetry class called the Tetragonal Dipyramidal Class with a symmetry of 4/m.

PHYSICAL CHARACTERISTICS:

Color is yellow, brown, gray, blue, white or black.
Luster is adamantine to greasy.
Transparency: Crystals are translucent to transparent.
Crystal System is tetragonal; 4/m
Crystal Habits include small four sided pyramidal crystals (pseudo-octahedral) and thin plates. Commonly as crusts or films around altered molybdenite and as pseudomorphs after molybdenite.
Cleavage is distinct in four directions (bipyramidal).
Fracture is uneven.
Hardness is 3.5 - 4.
Specific Gravity is approximately 4.2 - 4.3 (heavy for nonmetallic minerals).
Streak is white.
Other Characteristics: Fluorescent golden yellow.
Associated Minerals are quartz, zeolites, molybdenite and lindgrenite.
Notable Occurrences include the Peacock Lode, Seven Devils district, Idaho (the type locality); Keewenaw Peninsula, Michigan; Tungsten, Nevada; Superior, Arizona and Randsberg, California, USA; Nasik, India; Turkey; Russia; Scotland; Clayton Quarry, Panama Canal Zone, Panama and Morocco.
Best Field Indicators are crystal habit, color, fluorescence, association with molybdenite and cleavage.