Minerals in New Jersey

June 26, 2014


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Franklinite

Willemite, Franklinite
and Calcite

under UV light


Willemite, Calcite
and Franklinite


Calcite, Franklinite
and Willemite


Hardystonite


Wollastonite

The mines of Franklin and the Sterling Hill Mine at Ogdensburg, Sussex County in northwestern New Jersey are world famous and deservingly so. No other site can boast the same assortment of rare and interesting minerals. Over three hundred different minerals were found at these mines and most are listed in . Over 60 new minerals to science were also described from samples taken from these mines, thus claiming these mines as their type locality (these minerals are shown in bold in the table). While other great localities can have similarities with other sites, there simply are no real good parallels with the mineral assortments of Franklin and Sterling Hill, New Jersey.

The geological reasons for this diversity of minerals is somewhat complex. It involves zinc, manganese and iron rich sediments on a pre-Cambrian sea floor being swept up into a regional orogenic event that created a mountain chain in the approximate position of the current Appalachian Mountains. This event however occurred more than a billion years ago. The origin of the zinc, manganese and iron sediments is theorized to have been manganese nodules and/or sulfide producing "black smokers" that we see along mid-oceanic ridges today. Whatever the case, the manganese and zinc is what drives almost all of the unique and exotic mineral species that are found here. Later contact and regional metamorphism, hydrothermal alterations and weathering produced unusual results and thus a whole "mess" of rare zinc and manganese minerals.

The minerals that were found here are unlike those found anywhere else. Unusual manganese and zinc oxides and silicates as well as a few arsenates, are the hallmark of this locality. The primary ore minerals are the franklinite (an iron, zinc and manganese oxide) and willemite (a zinc silicate) and to a lesser extent zincite (a red colored zinc oxide) and hemimorphite (a zinc silicate). Iron is the primary product, in terms of weight, while zinc and manganese are rather significant.

When first exploited, it was thought that the ore minerals were magnetite (an iron oxide) and cuprite (a red colored copper oxide), but the ore behaved differently than other magnetite ores in the smelting process and the "cuprite" yielded no copper. Of course most of the "magnetite" turned out to be a new mineral to science, franklinite and the "cuprite" turned out to be zincite, one of the first new minerals identified in the New World and one of many to come from this locality. As the mining continued, more uses and better techniques for the exploitation of zinc and manganese were found and the mine became a boom for the area. The steel, paint and coal industries (needed to smelt the ore) were all positively affected by these mines. The iron/manganese alloys strengthened steel; while the zinc was used in a variety of paints and in certain alloys.

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