FLUORITE

CaF₂
Cubic

	Figure 23-1. Cubic fluorite crystal from Sterling Hill. Field of view is 1.3 mm in maximum dimension.

Fluorite was first reported, purportedly from Franklin, by Bruce (1810b), but the occurrence was actually in Hamburg, north of the ore deposits and the presently defined “Franklin-Sterling Hill Area”. However, fluorite was reported as “fluate of lime” in many of the reports of the 1820’s and as ittro cerite by Gibbs (1823), spelled yttrocerite by Palache (1935).

Description

Local fluorite occurs in masses, as interstitial grains in granular aggregates and as fine, small crystals in vein assemblages. Palache (1941a) noted crystal forms {001}, {113}, and {011}. Fiveling twins, similar to that of hetaerolite illustrated by Palache (1935), and illustrated herein (Figure 22-64), are rare.

Flow textures are occasionally evident; rounded aggregates are typical, especially where associated with calcite (Figure 23-3). It is found in red, pink, gray, brown, violet, and other colors; violet and pink are the most common. Some fluorite is violet at the contacts with nickel arsenides. Cleavage is perfect on {111}, and the density is 3.19 g/cm³. Fluorite is readily distinguished by its cleavage.

Colorless Franklin fluorite has been shown to be thermoluminescent and triboluminescent (Wick, 1937), and brilliant yellow thermoluminescence and white triboluminescence have been reported by Northup and Lee (1940). Colored material is known to fade on exposure to sunlight. Fluorite from the orebodies is largely fluorescent in ultraviolet; that found in the Franklin Marble is commonly non-fluorescent. Where evident, the fluorescence is green to bluish green in longwave and shortwave ultraviolet and rarely vivid violet, stronger in longwave. Some material, when freshly obtained, is phosphorescent in response to incandescent light; see Bostwick (1982). Some orange-brown material which exhibits a bluish-green fluorescence is visible-light-sensitive  and the fluorescence diminishes in quality after the specimen has been exposed to visible light for any appreciable time (R. C. Bostwick, personal communication). Additional observations of responses to ultraviolet were given by Millson and Millson (1950) and Jones (1961).

Composition

Few chemical data have been published since the analysis given by Palache (1935), except for trace element studies of Sterling Hill fluorite by Buis (1983). She reported that all specimens studied were enriched in Mn, Zn, and Fe and that yttrium was a minor component, confirming the earlier finding of Humphreys (1904). Franklin fluorite was reported to host the rare-earths Dy and Sm (Wicks, 1937); Pb, Sn, Mn, Mg, and Fe were reported by Northup and Lee (1940).

Occurrence and paragenesis

	Figure 23-2. Cubic fluorite crystals with coign modifications by the trioctahedron. Field of view is 1.5 mm in maximum dimension.

Fluorite occurs in the two orebodies and in the Franklin Marble. At Franklin, massive material is locally common, especially in the Trotter Shaft area, and as an occasional intergranular constituent of the franklinite-willemite  ore. Fine crystals are found in some uncommon vein assemblages but, in general, are rare here, unlike at many other ore deposits (Figures 23-1 and 23-2). Fluorite is associated with many minerals, most commonly with calcite, pyrite, dolomite, sphalerite, and quartz, and also with willemite, titanite, rhodonite, phlogopite, scapolite, rhodochrosite, and others.

	Figure 23-3. Rounded masses of fluorite in the white calcite of the Franklin Marble from Franklin. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #R1278. Photo by the author.

Koenig (1890) reported fluorite associated with the rare nickel arsenide assemblage at Franklin (Figures 21-30, 21-31, and 21-32). Fluorite is also known as fine crystals in the Buckwheat Dolomite (Peters et al., 1983). Fluorite is also present as druses, late-stage crystals in veins, and breccia cements; an especially notable breccia consists of yellow willemite prisms up to 6 cm in length cemented by colorless fluorite. Additionally, fluorite is an accessory mineral in many associations; see figure 15-90.

At Sterling Hill, fluorite may have been less common, except in the sulfide veins. Fine crystals were found associated with brown manganaxinite, epidote, and actinolite and separately as colorless dodecahedra associated with chlorophoenicite, rhodochrosite, and allactite. It is found sporadically in small amounts. Buis (1983) examined rare-earth elemental ratios in a significant fluorite band surrounding the black-willemite zone at Sterling Hill and suggested it may have formed in part hydrothermally.

Fluorite is common in the Franklin Marble, especially in fractures, but has not been studied in detail. Occurrences are sporadic, and many are in cavities; well-formed 1 cm violet crystals have been found. Fluorite also is found as a marker-bed in the Kittatinny Limestone. 

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