By Christa Lynn Van Eerde
From television screens and lighting treatments to make teeth whiter-than-white, fluorescence is known to all thanks to its presence in everyday life.
The Sterling Hill Mine in the small town of Ogdensburg, New Jersey, was once an internationally renowned zinc mine with one of the world’s richest zinc ore deposits. New Jersey was a key player in copper, iron and zinc mining, the latter of which was unparalleled in its richness and purity. Zinc is essential in myriad industries. To name but a few: automobiles (carburetors, door handles, and fuel pumps, all die-cast parts made of zinc, and tires), ceramics, and footwear.
New Jersey's Fluorescence Finds
There are over 370 minerals in the zinc mines in Franklin, New Jersey, 90 of which fluoresce. Franklin was named “The Fluorescent Mineral Capital of the World” in 1968, and has remained the ultimate locality for fluorescent minerals. It is most well-known for its combinations of intense red calcite (calcium carbonate), deep yellow esperite (calcium lead zinc silicate), profound violet-blue hardystonite (calcium zinc silicate), and brilliant green willemite (zinc silicate), which is phosphorescent and continues to emit light after the energy source has been removed.
There are elements, such as nickel, iron and cobalt that quench fluorescence. Franklinite, a member of the spinel group, is made of zinc, manganese and approximately one third iron, and shows no fluorescence. However, it is often found alongside calcite, willemite and zincite. These minerals are all found together thanks to billions of years of geological events.
As William Kroth, current president and executive director of the Sterling Hill Mine explained, it all began in the Pre-Cambrian era (geologic time prior to 600 million years ago), with sediments of metal-rich fluids that were deposited in a shallow sea floor with white calcium carbonate mud that became limestone. Around 1.25 to 1.3 million years ago, the sediments were buried, heated and distorted, which caused chemical reactions and metamorphism.
At 1400 degrees Fahrenheit and a depth of 10 to 12 miles, the pure limestone then morphed into Franklin Marble, a textural recrystallization of calcite into large, interlocking calcite grains. Intruded by igneous dikes, penetrated by hydrothermal waters and exposed to weathering, the ore oxidized and uplifted.
Millions of years of glaciation eroded the soft marble and bared ore minerals, and this alteration of chemistry resulted in the creation of new minerals. Over the 136-year period the Sterling Mine was in operation, it mined 11 million tons of ore, twenty percent of which was zinc.
The Sterling Hill Mine became a museum run by brothers Richard and Robert Hauck after it ceased operations in 1986 due to adverse market conditions, and it was in the early 1990s that Kroth became involved. The Sterling Hill Mine is the fourth oldest mine in the United States, and New Jersey’s only underground mine tour, with a quarter mile of tunnels to explore.
Overall, the mine has 35 miles of tunnels, but public access is very limited. Although visitors cannot venture down to the 2,550-foot level, it is possible to get a glimpse into the mine and view historic mining machinery and equipment, a demonstration of drilling and blasting, and “The Rainbow Room,” where one can see fluorescent minerals in situ underground.
The museum is open seven days a week and currently welcomes between 55,000 and 60,000 visitors per year. It is an essential pilgrimage for curious individuals and rock and mineral enthusiasts of all ages.
For more information, visit sterlinghillminingmuseum.org.