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The Mineral willemite

Willemite Var. Troostite Crystal

Willemite is one of the most well-known fluorescent minerals, and its fame is attributed to the Franklin District of New Jersey where it forms in abundance with many different colors and habits. Willemite was named in honor of King William I of the Netherlands (1772 - 1843), who was known locally as King Willem. The type locality for Willemite is in present-day Belgium, which was part of the greater Kingdom of the Netherlands when this mineral was named in 1830.

Chemical Formula

Zn₂SiO₄

Color

Brown, reddish-brown, green, white, and gray. Less commonly yellow, orange, blue, colorless, or multicolored green and brown.

Crystal System

Hexagonal

Properties

Streak
White

Hardness
5.5

Transparency
Transparent to opaque

Specific Gravity
3.9 - 4.2

Luster
Vitreous, resinous, pearly

Cleavage
2,3

Fracture
Conchoidal to uneven

Tenacity
Brittle

Other ID Marks
Strongly fluorescent green, and often strongly phosphorescent. Also triboluminescent.

Crystal Habits

In wide prismatic or short stubby crystals. Most often massive, grainy, botryoidal, drusy, and in small rounded grains. Less commonly in rhombohedral crystals, columnar, fibrous, acicular, radiating, and in rounded balls.

3D Crystal Atlas

Additional Information

Composition
Zinc silicate

In Group
Silicates; Nesosilicates

Striking Features
Strong fluorescence and mode of occurrence.

Environment
In zinc-rich ore bodies of metamorphosed marble, and in hydrothermal replacement deposits.

Rock Type
Metamorphic

Varieties

Troostite

- A manganese-rich variety of Willemite from the Franklin District zinc mines of New Jersey. Troostite often describes the blocky reddish-brown crystallized form of Willemite.

Uses

Willemite is an important mineral among collectors, and is especially popular among collectors specializing in fluorescent minerals. It has been used as an important ore or zinc.

Noteworthy Localities

The Franklin Mine in Franklin, New Jersey, and the Sterling Hill Mine in Ogdensburg, New Jersey, are the most important localities for this otherwise uncommon mineral. These two mines in the same zinc ore body have produced interesting specimens of Willemite of all shapes, habits, and colors in abundance. The only other U.S. locality of importance is the Mammoth-St. Anthony Mine, Tiger, Pinal Co., Arizona, which has produced small crystals of Willemite together with Wulfenite.

Botryoidal Willemite groups are found in the Potosí Mine, Santa Eulalia District, Chihuahua, Mexico. Brown, drusy crystals were found in the type locality at Kelmis, Moresnet, Belgium. In Namibia, Berg Aukas, in the Grootfontein District, produces acicular Willemite crystals. The most significant locality after the Franklin District is Tsumeb, Namibia, which has produced beautifully colored botryoidal and ball shaped aggregates of white, green, yellow, and blue Willemite. The most prized Tsumeb colors are the deep blue and cadmium-rich deep yellow.