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