Cummingtonite, an iron magnesium silicate hydroxide mineral of the amphibole group, was first reported from Sterling Hill (as grunerite) by Reilly (1983). It occurred as small dark-brown needle-like crystals, as an alteration product of diopside, and was associated with franklinite, titanite, and calcite in drill-hole #124, 111 feet from its origin on the 340 level. Cummingtonite was also found as an alteration product of hedenbergite and iron oxides, associated with andradite, ferroactinolite, franklinite, and calcite in drill-hole #124, 119 feet from its origin. No symmetry determination was given by Reilly (1983); the monoclinicity was apparently inferred optically. The mineral described as zinc-manganese-cummingtonite by Bauer and Berman (1930) is now known as tirodite.
Microprobe analyses by Reilly (1983) of the occurrences noted above yielded SiO2 48.63, 46.42; CaO 0.14, 0.36; Al2O3 6.87, 8.00; FeO 29.92, 29.96; MnO 1.77, 2.30; MgO 2.33, 2.49; K2O 1.61, 1.32; ZnO 5.09, 5.65; TiO2 0.49, 0.73; total = 96.85, 97.23 wt. %. Iron occupies 3.96 to 4.00 of the 7 cations and is insufficient for the designation grunerite according to the nomenclature conventions of Leake (1978). (Dunn, 1995)

 Location Found: Ogdensburg
 Formula: ☐{Mg2}{Mg5}(Si8O22)(OH)2
 Essential Elements: Hydrogen, Magnesium, Oxygen, Silicon
 All Elements in Formula: Hydrogen, Magnesium, Oxygen, Silicon
 IMA Status: Approved
 To find out more about this mineral at minDat's website, follow this link   Cummingtonite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.453

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.52

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 9, No. 1 - February 1968, pg. 17The Exclusive Minerals of Franklin/Ogdensburg, N.J. (as of January 1968) by Frank Z. Edwards - Zinc-Manganese Cummingtonite

Cummingtonite, Sterling Hill Mine, Ogdensburg, NJ
Cummingtonite (dark-brown) from the Sterling Hill Mine, Ogdensburg, NJ. Field of view 1/2". From the collection of, and photo by, DW.

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