MAGNETITE

FeFe³⁺₂O₄
Cubic

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.

Description

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 on {100} and localized replacement of magnetite by hematite were discussed by Carvalho (1978).

Composition

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. The writer’s studies of chemical compositions are discussed in the section entitled “Regional and local geology of the Franklin-Sterling Hill area;” see also Frondel and Baum (1974), who cite previously unpublished studies. 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). See the discussion under franklinite and the work of Frondel and Baum (1974).

Occurrence and paragenesis  

Magnetite occurs in great abundance at Franklin in the Furnace Magnetite Bed, which is discussed in detail in the section entiled “Regional and local geology of the Franklin-Sterling Hill area” and in the historical sections. 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 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 nickel-arsenide 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. More discussion of these deposits is given in the section entitled “Regional and local geology of the Franklin-Sterling Hill area.”

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.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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This page created: January 11, 2001