SPINEL

MgAl₂O₄
Cubic

	Figure 22-44. Octahedral crystals of spinel in the white calcite of the Franklin Marble from Franklin. Specimen is 5 cm in maximum dimension. Smithsonian Institution, #R1943-1. Photo by the author.

Spinel is moderately common in the Franklin Marble, was noted by Nuttall (1822), and is in other early lists. Much of the material from here is true spinel, with varying solid solution. The “spinel” so-labeled from the zinc orebodies is largely gahnite. The name ceylonite was used by the analyst Vogel (Palache, 1935), and ceylanite was used by Fowler (1825) and by Torrey, who also used the name pleonaste (Thomson, 1828).

Description

Local spinel occurs as predominantly idiomorphic crystals; massive material is uncommon. The crystals are mostly octahedral with dodecahedral modifications; the cube form is rare. Crystals are commonly 2-6 mm in size, but have been found in cm-sized euhedra, and large crystals are known (Figures 22-43 and 22-44).

	Figure 22-43. Octahedral crystals of spinel (dark gray) with meionite (light gray) in the white calcite of the Franklin Marble from Franklin. Specimen is 12 cm in maximum dimension. Mineralogical Museum, Harvard University, #83516. Photo by Chip Clark.

The color of local spinels varies from gray to green, pink, and blue; grayish hues are the most abundant, and blue or pink is the most aesthetic. The luster is vitreous. There is no discernible fluorescence in ultraviolet, and no other physical data have been obtained. It is easily recognized by its octahedral habit, hardness, and associations.

Composition

Spinel is a magnesium aluminum oxide mineral of the spinel group. Local material is largely unstudied chemically. Palache (1935) reported an analysis with 8.91 wt. % FeO, but questioned its value. It is likely that much of the darkly colored spinel from the marble is partially ferroan, in solid solution toward hercynite.

Occurrence and paragenesis

Spinel is found throughout the extent of the Franklin Marble and is locally found associated with calcite, corundum, phlogopite, rutile, chondrodite, meionite, and other minerals. Spinel also occurs as masses of crystals with interstitial calcite, but it is predominantly idiomorphic. Rhodonite is a commonly associated mineral in specimens occurring near the zinc orebody. Frondel (1972) reported spinel replaced by corundum. Local spinel-bearing assemblages are largely unstudied.

Large crystals, up to 17 cm, have been found near Sparta and in the Franklin Marble in New York.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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