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ZrSiO4
Tetragonal
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| Figure 15-95. Crystal drawings of zircon from Balls Hill in Franklin. Drawings are from Palache (1935) who provided crystallographic data. | ||
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Zircon, a zirconium silicate mineral, was first reported by Vanuxem and Keating (1822b), but it is relatively rare in the ore deposits. It is found at both Franklin and Sterling Hill, but nearly all specimen material is from Franklin, especially from Balls Hill. It occurs sporadically as an accessory mineral, and fine crystals have been recovered.
The morphological description was given by Palache (1935), and little can be added here. Some crystals show three tetragonal dipyramids, some from Balls Hill have {001} as a terminal form, and many are sharp and euhedral. Most Franklin zircon is dark brown, with vitreous luster. Most of the observed crystals are not discernibly fluorescent in ultraviolet; those that are have a weak orange fluorescence in shortwave, and some have been reported to have a weak red fluorescence. Little chemical analytical work has been done on local material; some Balls Hill zircon is Hf-free and has near end-member composition.
The best known specimen of Franklin zircon, described by Cook (1889) and illustrated by Palache (1935, Plate 16-a) as being from Balls Hill, is a fake specimen composed of many crystals glued together, as discussed in the section entitled “The specimen base.” Zircon is found on valid Balls Hill specimens as free-growing, prismatic 5 mm crystals associated with garnet, pyroxene, epidote, and ferroactinolite.
Franklin zircon, although generally sparse, was most common in the pegmatite of the Trotter Shaft area. One fine 2 cm crystal was collected from the calcium silicate units in the Franklin Mine by John L. Baum, Resident Geologist at Franklin.
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| Figure 15-96. Zircon crystal on acid-etched anorthite from the corundum-margarite occurrence in the Franklin Marble at Sterling Hill. Field of view is 0.5 mm in maximum dimension. | ||
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Zircon is less common in the orebodies; those from the Trotter Shaft are commonly rock-locked in microcline and are brittle, and thus are commonly found in fragments. Fine 2 cm zircon crystals were found with 5 cm andradite and unstudied pyroxene from Franklin; similar crystals are also associated with fluorapophyllite and epidote from the Palmer Shaft. Fine 5 cm crystals have been found associated with rhodonite. Among the most uncommon specimens is one of light pinkish brown, 1 cm, slightly bulging crystals which have gray color in the cores and near the terminations of the crystals; the final growth is of dark brown lustrous zircon.
Zircon was reported and illustrated from the Sterling Hill corundum/margarite occurrence by Dunn (1979c). Here it occurs as tiny crystals, slightly resorbed, in the Franklin Marble, associated with rutile, anorthite, arsenopyrite, goldmanite, and other species (Dunn and Frondel, 1990).
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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Link
to homepage
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This
page created: January 11, 2001 |
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