Franklin and Sterling Hill
Franklin and Sterling Hill, Sussex County, New Jersey: zinc orebodies in Franklin Marble, over 360 species, about 90 fluorescent; Sterling Hill worked to 1986.
Franklin and Sterling Hill are zinc orebodies in the Franklin Marble of Sussex County, New Jersey, worked for franklinite, willemite and zincite; Sterling Hill until 1986. The Sterling Hill Mining Museum counts over 360 species there, about 90 fluorescent; 15 of this site's 176 Handbook sheets name Franklin or Sterling Hill.
In short
- Two orebodies, one marble. The Sterling Hill Mining Museum puts the Franklin orebody 2.5 miles north of Sterling Hill. Both are wholly encased in the Franklin Marble, about 1.3 billion years old. The USGS MRDS record calls the district a first-rank zinc district of unique geology with no known counterpart anywhere in the world.
- The species count is the museum’s, not ours. It gives more than 360 species, about 90 fluorescent and more than two dozen found nowhere else. Its list is current as of September, 2008 and revised by the Franklin-Ogdensburg Mineralogical Society. The museum’s ore-body page says more than 365.
- 15 of 176. Fifteen Handbook of Mineralogy sheets in this site’s reference set name Franklin or Sterling Hill, Sussex Co., in their Distribution field. Six of those record fluorescence (ligature-aware search, controls hemimorphite and willemite): aragonite, corundum, hemimorphite, marialite, uvite and willemite.
- Worked zinc from the ore mode. USGS MRDS quotes Palache (1935): 43 wt% franklinite, 26% willemite, 1% zincite, 25% carbonates, 5% silicates. Ideal formulas give 27.7% Zn. With the Handbook’s New Jersey franklinite analysis (ZnO 20.77%) it falls to 23.2%. The MRDS decade grades are 19–21 wt%.
- Closure dates. The museum and MRDS agree that Sterling Hill operated until 1986. For Franklin, the same MRDS record says the mine closed in 1957, while its production table ends with the period 1951–1954. We print both and do not choose.
| Species | Formula (Handbook) | Hardness | Density (meas.) | UV and other response (sheet wording) | Zn by mass | In the 176? |
|---|---|---|---|---|---|---|
| Franklinite | ZnFe3+2O4 | about 6 | 5.05–5.22 | None recorded; “Strongly to weakly magnetic” | 27.12% | No |
| Willemite | Zn2SiO4 | 5.5 | 3.89–4.19 | “Typically strongly fluorescent in yellow-green to yellow-orange under SW and LW UV; may be phosphorescent” | 58.68% | Yes |
| Zincite | (Zn,Mn2+)O; IMA: ZnO | 4 | 5.66 | “Rare pale yellow fluorescence under LW UV” | 80.34% | No |
| Hemimorphite | Zn4Si2O7(OH)2·H2O | 4.5–5 | 3.475 | “may fluoresce bluish under SW UV” | 54.29% | Yes |
| Aragonite | CaCO3 | 3.5–4 | 2.95 | “fluorescent red or yellow under LW and SW UV, phosphorescent” | 0% | Yes |
| Marialite | 3NaAlSi3O8·NaCl | 5.5–6 | 2.50–2.62 | “Commonly fluoresces orange to bright yellow or red under LW or SW UV or both” | 0% | Yes |
| Tephroite | Mn2+2SiO4 | 6 | 3.87–4.12 | None recorded | 0% | Yes |
Two orebodies in one marble
The setting. The Sterling Hill Mining Museum’s ore-body page describes both deposits as wholly encased in the Franklin Marble. That unit underlies much of the Wallkill River valley between Sparta, New Jersey, and Warwick, New York, and is about 1.3 billion years old. The ores are stratiform and folded with the marble. The museum says they are rich in zinc (more than 20% by weight), iron and manganese, and that no similar deposits have been found anywhere else on Earth.
How they formed, in the museum’s summary. Hot, metal-rich fluids discharged onto a shallow sea floor about 1.3 billion years ago. The resulting layers were later buried to 15–18 km and heated to 750–800 °C. The limestone became marble and the metal-rich layers became franklinite, willemite and zincite. The USGS MRDS record describes the deposits as stratabound, sedimentary exhalative and unique, and notes that their origin has been disputed.
What the ore was. MRDS quotes Palache (1935) for Franklin ore: 43 wt% franklinite, 26% willemite, 1% zincite, 25% carbonates and 5% silicates. Sulfides such as sphalerite and galena are present in the merest traces. The other localities are on the localities index.
How many species: the numbers and who gives them
The museum’s figures. The Sterling Hill Mining Museum’s minerals page gives more than 360 species from Sterling Hill and Franklin together, which it calls a world record for such a small area. It says about 90 are fluorescent and more than two dozen have been found nowhere else on Earth. The list is current as of September 2008 and is revised by the Franklin-Ogdensburg Mineralogical Society’s Mineral List Committee. The museum’s ore-body page says more than 365 and puts the area in the top five most mineralogically complex localities. We repeat these figures and attribute them; we have not counted them.
Our own count is of Handbook sheets, and it is smaller by design. Method: a script reads the Distribution field (from Distribution: to the next Name:) of each of the 176 sheets in this site’s reference set as pdftotext -layout text, strips diacritics, expands ligatures, joins line breaks and matches case-insensitively. The pattern requires Franklin or Sterling Hill followed by Sussex Co., New Jersey in the same sentence. That excludes vermiculite’s Franklin, Macon Co., North Carolina. It matches 15 sheets: actinolite, aragonite (Passaic mine, Sterling Hill), arsenopyrite, austinite (Sterling Hill), biotite, corundum, graphite, hemimorphite, hydroxyapophyllite, marialite, phlogopite, tephroite, tremolite, uvite and willemite. Hematite names Franklin only in its chemistry block, so it is not counted.
The two ore minerals that are not in the set. The franklinite and zincite sheets were fetched as comparators. Each opens its Distribution field with Franklin and Sterling Hill. The franklinite sheet says the name is for Franklin Furnace, now Franklin, and for Benjamin Franklin. The IMA list gives USA as the country for franklinite (1819) and zincite (1845).
Why Franklin willemite glows green
The museum’s explanation. The Sterling Hill Mining Museum’s fluorescence page names divalent manganese (Mn2+) substituting for zinc in willemite as the emission centre for its green glow. It says calcite, willemite and wollastonite do not fluoresce when pure, but with a little Mn2+ they fluoresce red, green and yellow respectively. The Handbook willemite sheet’s two Franklin analyses carry 0.12% and 8.96% MnO.
What the sheets record. Of the 15 sheets naming Franklin or Sterling Hill, six record fluorescence. Those sheet statements describe the species, not Franklin material in particular. Of the 176 sheets in the set, 31 record fluorescence. The search is ligature-aware, because pdftotext renders the willemite sheet’s word as °uorescent. The general background is on fluorescent minerals explained and is calcite UV reactive. For lamps, see the best UV lamp for mineral collecting. This page gives no UV safety advice.
Hemimorphite and zincite are the quieter ones. Hemimorphite may fluoresce bluish under SW UV. Zincite has rare pale yellow fluorescence under LW UV. Franklinite records none and is magnetic instead, strongly to weakly; see which minerals are magnetic.
1986, 1957 or 1954: the closure dates and the ore
Sterling Hill: 1986. The museum’s history page says the zinc minerals were identified and several small companies began mining in 1836. The New Jersey Zinc Company consolidated the properties in 1897. The mine then operated until 1986, first as an open pit and later underground, to 2,675 feet (815 m) with 35 miles of tunnels. The museum’s own site advertises rock collecting on its Mine Run Dump; any terms are the museum’s, and we say nothing beyond that. USGS MRDS also gives 1986 and about 11 million tons of ore from about 1875.
Franklin: two years in one record. USGS MRDS 10073174 says zinc mining was continuous until the Franklin mine closed in 1957, with about 22 million tons of ore. Its production table, however, ends with the period 1951–1954, and the district record’s 22-million-ton line is dated 1850–1954. We could not fetch a primary source that settles it, so no single Franklin closure year is given here.
Worked zinc check. On standard atomic weights, franklinite ZnFe2O4 is 27.12% Zn, willemite Zn2SiO4 58.68% and zincite ZnO 80.34%. Palache’s mode, 0.43 × 27.12 + 0.26 × 58.68 + 0.01 × 80.34, gives 27.7% Zn. Real franklinite carries manganese; the Handbook’s first New Jersey analysis has ZnO 20.77% (16.69% Zn), which brings the mode to 23.2%. MRDS gives 19–21 wt% Zn by decade. The remaining gap is not explained by the sources. The species pages are on the species index. Nothing here is offered for sale; the wanted list is the only commercial route.