FRANKLIN AND STERLING HILL NEW JERSEY: THE WORLD'S MOST MAGNIFICENT MINERAL DEPOSITS
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ZINCITE


The spinel group

FRANKLINITE

GAHNITE

HERCYNITE

JACOBSITE

MAGNETITE

SPINEL


Other oxides

ANATASE

AURORITE

BIRNESSITE

BROOKITE

BRUCITE

CHALCOPHANITE

CIANCIULLIITE

CORUNDUM

CRYPTOMELANE

CUPRITE

FEITKNECHTITE

GOETHITE

GROUTITE

HAUSMANNITE

HEMATITE

HETAEROLITE

HYDROHETAEROLITE

ILMENITE

MANGANITE

MANGANOSITE

PYROCHROITE

PYROPHANITE

ROMEITE

RUTILE

TODOROKITE

URANINITE

WOODRUFFITE

CORUNDUM

Al2O3 
Hexagonal

Corundum, an aluminum oxide mineral, was first reported from the local area by Vanuxem and Keating (1822b) and Fowler (1832). It is found in the Franklin Marble. For a discussion of names using the term “ruby,” see the discussion under zincite.

Description

Corundum occurs, for the most part, as euhedral to subhedral crystals, but deformed crystals, as well as crystals whose shape is inherited in part from contact minerals, are common.

 
 
 
  Figure 22-53. Crystal drawings of corundum. The top drawings are of crystals from the Furnace Quarry in Franklin; the bottom drawings are of crystals from the calamine pits at Sterling Hill. Drawings are from Palache (1935) who provided crystallographic data.  
   

The morphology of local corundum was described and illustrated by Palache (1935), who also noted that crystals occur up to 5 inches (~13 cm) in size; cm-sized crystals are more common. The habits vary from equant, to barrel-shaped, to steeply dipyramidal (Figures 22-53 and 22-54). Most if not all corundum crystals occur rock-locked, not in vugs, and many specimens result from partial dissolution of the enclosing marble matrix, either by weathering or by intentional etching away of the surrounding calcite.

 
 
 
  Figure 22-54. A cluster of ruby corundum crystals from Franklin. The smallest crystal has been attached. Specimen is 5 cm in maximum dimension. Mineralogical Museum, Harvard University, #96071. Photo by Chip Clark.  
   

The color of local corundum is variable; much is blue or bluish gray, and violet and fine red material is also known. Color zoning may be present, but inconsistently. The luster is vitreous, and parting is sometimes present. Red corundum has a moderate red fluorescence in longwave ultraviolet; it is weak in shortwave. No physical or optical measurements have been made on local corundum, nor any chemical studies.

Occurrence and paragenesis

Corundum occurs throughout the local extent of the Franklin Marble and close to but not within the orebodies. Few notes were kept, and the quantity and quality of corundum likely varied from quarry to quarry. Palache (1935) reported it associated with spinel, rutile, graphite, pyroxene, garnet, titanite, and phlogopite. Spinel and phlogopite are very common associated minerals. Early collections have some fine corundum specimens (Figures 22-54 and 22-55).

 
 
 
  Figure 22-55. Crystals of red corundum with diopside (light gray, upper right) and small crystals of phlogopite, spinel, and altered pyrite in the white calcite of the Franklin Marble from Franklin. Specimen is 9 cm in maximum dimension. Mineralogical Museum, Harvard University, #121252. Photo by the author.  
   

Much corundum has been found in the vicinity of Sparta, and some of this has been mislabeled as having come from Franklin. Replacement of spinel by corundum was reported by Frondel (1972), and some corundum alters to mica locally.

Abundant corundum was found in the Franklin Marble immediately adjacent to the Sterling Hill orebody, associated with phlogopite, rutile, arsenopyrite, gahnite, and other minerals (Dunn and Frondel, 1990) and described herein under margarite. This material was locally abundant. Many specimens do not show the whole assemblage. Fine corundum specimens were found in the debris of the Sterling Hill hemimorphite pits, likely derived from the marble by weathering.

 

 

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Copyright © 1995 by Pete J. Dunn
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CHAPTER 22. OXIDES AND HYDROXIDES