Lets Talk Gemstones

By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962

Phenakite Group

[A NESOSILICATE]

Willemite

As a gemstone, faceted willemite is rare. It is so rare that Dr. Joel Arem tells us in the Color Encyclopedia of Gemstones that “stones larger than one to two carats are worthy of museums”, though the Franklin, New Jersey deposit has produced faceted gems up to ten carats. Little space is devoted to the material in readily available reference materials, and the information is often contradictory. One of the goals the author has set is to provide information, especially about little-known gems, to those interested in and involved with gemstones. The format used to do this has been greatly influenced by Cornelis Klein and Cornelius S. Hurlbut, Jr.’s Manual of Mineralogy after J. D. Dana. This volume lists willemite, as well as phenakite, as a member of the Phenakite Group. It is the only reference work that does so. The 1995 edition of the Michael Fleisher. Joseph A. Mandarino Glossary of Mineral Species does not list a Phenakite Group; nor do other references.

The following quote from the Manual of Mineralogy is of particular interest. “Willemite is isostructural with phenakite, with SiO4 and ZnO4 tetrahedra. Because Zn2+ (radius = 0.74 A) is much larger than Be2+ (radius = 0.35 A) the structure of willemite is much expanded over that of phenakite.] Manganese frequently replaces a substantial portion of the zinc, and small amounts of iron may be incorporated in the chemical composition of willemite. Troostite is the name given to this manganese-iron bearing variety of willemite. Willemite was named to honor King William I of the Netherlands, and the troostite variety derives its name from the American mineralogist, Gerard Troost.

The Color Encyclopedia of Gemstones [photo #239] and Walter Schumann’s Gemstones of the World [page 204] present photos of willemite gems. A picture of an excellent crystal specimen from a New Jersey source can be found on page 69 of An Illustrated Guide to Rocks and Minerals by Michael O’Donoghue.

Willemite is usually found in massive or granular forms. Crystals are rare. Its structure is listed as hexagonal [Manual of Mineralogy- pg.373 and Color Encyclopedia- pg.202]; – trigonal [Gemstones of the World- pg18]; and tetragonal [Gemstones of the World- page 204]. Zinc bearing metamorphic deposits are the most common source. The Manual of Mineralogy indicates that crystals may have developed by the metamorphism of smithsonite or hemimorphite in crystalline limestone. The absence of water in willemite distinguishes it from hemimorphite. There are rare occurrences of willemite crystals as a secondary mineral in oxidized zones of zinc deposits. Deposits at Franklin and Sterling Hill, New Jersey yield crystals, masses, and the manganiferous troostite associated with franklinite, zincite, and calcite. Willemite has been found in Utah, in Inyo County in California, in the Merritt mine in New Mexico, and at Tiger, Arizona. Other locales for willemite include Algeria, Belgium, Greenland, Zambia, Zaire, and at Tsumeb, Namibia. Gem quality blue crystals can be found at Mt. Saint Hilaire in Quebec, Canada.

Rhombohedral terminations of the hexagonal prisms [short and stubby or acicular] are the norm. Pure willemite is white and infusible, but heating with cobalt nitrate on charcoal results in a blue assay. A violet-red color appears on a borax bead when troostite is subjected to an oxidizing flame. Material from Franklin, New Jersey exhibits a strong green to yellow-green fluorescence, frequently followed by an intense green phosphorescence in both long wave and short wave ultraviolet light.

Willemite exhibits a conchoidal fracture, a vitreous to resinous luster, and a hardness of 5.5. Most faceted willemite gemstones are colorless or varying shades of green, orange, reddish brown, yellow, gray or white. Blue gems are exceptional. Attractive cabochons are cut from massive troostite and from white calcite bearing willemite with red zincite and black franklinite. It is neither practical nor commercially feasible to use willemite in jewelry, except for the collector of unusual gemstones.

Silicate Properties

Composition    Zn2SiO2 zinc silicates
Class    Silicates
Group    phenakite, per the Manual of Mineralogy
Species    willemite
Variety    by color and Troostite
Crystal System    (see above article)
Habit    prismatic, acicular, granular, and massive
Cleavage    good, per the Manual of Mineralogy; poor, per Arem
Streak    white or gray
Fracture    conchoidal
Fracture Lustre    no information
Lustre    vitreous to resinous
Diaphaneity    transparent, translucent, and opaque
Colors    green, orange, yellow, reddish-brown, gray, white, & blue
Phenomena    none known
Specific Gravity    3.89 to 4.20; usually 4.10
Hardness    5.5
Toughness    fragile and brittle
Refractive Indices    o=1.691; e=1.719
Birefringence    0.028
Optic Character    uniaxial (+)
Dispersion    no information
Pleochroism    varies by color
Luminescence    (see above article)
Spectrum    strong band at 4210; weak bands at 4320, 4420, 4900, 5400, and 5830
Chelsea Filter    no information
Aqua Filter    no information
Solubility    no information
Thermal Traits    avoid thermal shock; infusible (see above article)
Treatments    no information
Inclusions    no information