FRANKLIN  MARBLE

 

Significant zinc ore bodies in the Franklin Marble of Mesoproterozoic age have been mined in northern New Jersey, USA for many decades.  The sample shown below is a zinc ore from Sterling Hill, New Jersey.  The two zinc minerals here are black franklinite ((Zn,Fe)Fe2O4 - zinc iron oxide) & red zincite (ZnO - zinc oxide).

 

Zinc ore (= zincite-franklinite-calcite rock) (above & below; above: ~6.2 cm across; below: ~2.1 cm across) 

 


 

The New Jersey zincite-franklinite bodies are traditionally considered as skarn deposits - the result of contact metamorphism of Cambro-Ordovician limestones by igneous intrusions.  Elsewhere, igneous intrusion of limestones does result in the formation of odd mineral suites by contact metamorphism.

 

However, these zinc ores do not appear to be skarn deposits.  Their exact origin is still debated in the literature, but published research suggests that the zinc ore bodies were originally Zn-rich metalliferous sediments deposited in the margin of a marine basin.  The marine basin was subsequently metamorphosed by subduction during the Grenville Orogeny (1.03-1.08 billion years ago) and became enclosed in marble host rocks by inverse diapirism.

 


 

In addition to their economic geologic significance, the rocks and minerals from the Franklin and Sterling Hill zinc orebodies of New Jersey are famous for their gorgeous fluorescent colors under ultraviolet (UV) light.

 

The rocks shown below are from the Franklin zinc orebody in the Mesoproterozoic-aged Franklin Marble at Franklin, New Jersey.  They contain three minerals.  Under white light, the dominant mineral is whitish-grayish in color - that’s calcite (CaCO3 - calcium carbonate).  Calcite is what makes marble marble.  Under UV light, manganiferous calcite will fluoresce an intense orangish color (see below).

 

The black mineral is franklinite ((Zn,Fe,Mn)(Fe,Mn)2O4) - zinc iron manganese oxide), a dominant zinc ore mineral in New Jersey.  It does not fluoresce under black light.

 

The light brown to peachy-colored mineral is willemite (Zn2SiO4 - zinc silicate).  Willemite varies considerably in color under white light, but will always have an intense greenish fluorescence under UV light (see below).

 


 

Zinciferous marble (above & below) (5.5 cm across) from the Mesoproterozoic-aged Franklin Marble of Franklin, New Jersey, USA under white light (above) and ultraviolet black light (below).

Above: white = calcite; light brown = willemite; black = franklinite.

Below: orangish-red fluorescence = calcite; green fluorescence = willemite; black = franklinite.

 


 

Zinciferous marble (above & below) (4.4 cm across) from the Franklin Marble of New Jersey under white light (above) and ultraviolet black light (below).

Above: white = calcite; light brown = willemite; black = franklinite.

Below: orangish-red = calcite; green = willemite; black = franklinite.

 


 

Zinciferous marble (above & below) (field of view 7.4 cm across) from the Franklin Marble of New Jersey under white light (above) and UV black light (below).

Above: whitish-grayish = calcite; peachy colored = willemite; black = franklinite.

Below: orangish-red = calcite; green = willemite; black = franklinite. 

 

Zinciferous marble (7.4 cm across) from the Franklin Marble of New Jersey under ultraviolet/black light.

Orangish-red = calcite; green = willemite; black = franklinite.  

 


 

Zinc ore under black light (UV light).  Green = willemite.  Black = zincite & franklinite (not fluorescent).  (OSU public display, Orton Geology Museum, Ohio State University, Columbus, Ohio, USA)

 


 

Why do some minerals fluoresce under UV light?  When short-wavelength UV radiation, long-wavelength UV radiation, or x-rays bombard atoms, electron excitation occurs.  But the electrons do not remain in an energetically excited state.  They quickly give off energy and resume their normal energy levels.  If the electron energy release is in the visible spectrum of light, a mineral glows, or fluoresces.

 


 

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