Magnetite



Magnetite is a ferrous-iron ferric-iron oxide mineral of the spinel group and the Fe2+ analogue of franklinite and jacobsite. Local material has not been much studied, except as an indirect and often accidental result of studies of franklinite. The magnetite in the
Furnace Magnetite Bed is very pure (Palache, 1935) and contains very little or no Zn, but has appreciable Mn and minor Mg. Magnetite occurring in exsolution intergrowths with franklinite in gahnite (Carvalho, 1978; Valentino, 1983) has 99 mole % FeFe2O4. Sterling Hill magnetite varying from 96-100% of the end-member was studied by Johnson (1990).
Magnetite was reported in the early 1820's and was known long before that simply as "iron ore." Mining in the local area was originally for magnetite ore and not in the Zn-rich orebodies.
Magnetite occurs for the most part as massive material, resembling franklinite in most respects, but not easily distinguished. Intergrowths of these two isostructural minerals give rise to anomalous magnetic effects. Masses vary from all franklinite to all magnetite. It is black and opaque, with varying degrees of metallic to submetallic luster. No physical or optical data exist. Exsolution relations and localized replacement of magnetite by hematite were discussed by Carvalho (1978).
Magnetite occurs in great abundance at Franklin in the Furnace Magnetite Bed. The Furnace Magnetite Bed lies between the west limb of the orebody and the underlying Cork Hill Gneiss. Ore from this bed is graphitic, and this is one of several diagnostic site indicators, compared with the non-graphitic character of the other proximal magnetite deposits. The Furnace Magnetite Bed is 3-8 feet (0.8-2.4 meters) in average thickness and underlies most of the west limb.
In the zinc orebody at Franklin, some material labeled franklinite, which is highly magnetic and commonly associated with andradite, may be wholly or in part magnetite; this might have been abundant and remains uninvestigated. Magnetite also occurs intergrown with the blocky and well-known pseudocubic cleavage-masses of hematite from Franklin. Superb 1-2 cm magnetite crystals have been found associated with chalcocite and silver on the 1050 level at Franklin; they are octahedral in habit, with numerous surface trigons giving a markedly stepped appearance. Magnetite also occurs as lamellar aggregates in calcite and in breccias from the Buckwheat Open Cut.
The reactants formed at both deposits by the interaction of late-stage hot sulfides and common franklinite/willemite ore are fine-grained, dense, black rims of magnetite and sphalerite, up to several cm in thickness; these, however, are minor features. Magnetite is commonly found in minor amounts in some sulfide assemblages and similarly in the nickelarsenide assemblage.
Magnetite is found in substantial amounts, associated with andradite, pyrite, and hedenbergite, in the iron mines on Balls Hill; these deposits supported the local iron industry.
Magnetite is present at Sterling Hill, but is unstudied except for the work of Carvalho (1978) and Valentino (1983). It is likely much more common at both deposits than has been recognized to date. (Dunn, 1995)


 Location Found: Franklin and Ogdensburg
     
 
     
 Formula: Fe2+Fe23+O4
 Essential Elements: Iron, Oxygen
 All Elements in Formula: Iron, Oxygen
     
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
     
     
 To find out more about this mineral at minDat's website, follow this link   Magnetite

     
 References:
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.582

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.65


The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 31, No. 1 - Spring 1990, pg. 3Breithauptite From The Nickel-Arsenide Assemblage at Franklin, New Jersey, Magnetite (small description)
     
Images

     
Magnetite ore from the Balls Hill Iron Mine
Magnetite ore from the Balls Hill Iron Mine, Franklin, New Jersey. Photo by JVF







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