FLUORAPATITE

Ca₅(PO₄,AsO₄)₃F
Hexagonal

	Figure 25-1. Crystal drawings of apatite, likely fluorapatite, from Franklin; the drawing at the right is of a crystal from the Parker Shaft area. Drawings are from Palache (1935) who provided crystallographic data.

Fluorapatite is the most common member of the apatite group at Franklin, but is much less common at Sterling Hill; it is also found in the Franklin Marble. The apatite of Palache (1935) is mostly this species. It was reported by Nuttall (1822) and Penfield (1880); the extant data were summarized by Palache (1935). Much blue and green fluorapatite from the orebodies has been mislabeled as svabite, not known with certainty from here. The aquatite of Koening (1889) is a printer’s misspelling for apatite.

Description

Franklin fluorapatite occurs as prismatic crystals; they were reported by Frondel (1972) to be over a foot long, but few exceed 15 cm in length. They are commonly terminated by a shallow dipyramid, as figured by Palache (1935). Crystals may be crude or well-formed (Figure 25-1); resorbed edges are common on calcite-hosted crystals.

The color of local fluorapatite varies substantially: blue, green, and bluish green are the most common colors, and colorless, white, gray, and brownish hues are also known. The luster is vitreous, and the mineral is brittle. The density varies, in good part due to the pervasive substitution of arsenic discussed below. Fluorapatite fluoresces in ultraviolet with blue, violet-blue, or orange colors in shortwave ultraviolet (Bostwick, 1972), but the fluorescence may be absent or imperceptible. Optically, fluorapatite is uniaxial, but the optic sign and indices of refraction vary significantly with As substitution. It is best verified by both X-ray and chemical means.

Composition

Fluorapatite is a calcium phosphate fluorine mineral of the apatite group. Some microprobe analyses of fluorapatites from the orebodies, with a large estimated error of plus or minus 4% of the amount given, are presented in Table 24; specimens from the Franklin Marble were not studied, but were stated by Frondel (1972) to be fluorapatite. The given analyses indicate much variation in arsenic, which is pervasive in orebody fluorapatite, and show minimal Mn substitution for Ca. Much work remains to be done on the characterization of local apatites.

	Table 24. Chemical analyses of minerals in the apatite group.

The limited given data are slightly misleading in that they do not show completely the variation in Cl, OH, and F in local material. Midmembers between many species occur here; solid solutions of the fluorapatite, svabite, johnbaumite, turneaureite, hedyphane, and hydroxylapatite members of the group are common. Numerous specimens have compositions that plot near the mutual F-OH join for the above-listed minerals, with Cl a minor component.

An analysis of apparent johnbaumite was given as one of svabite by Palache, correctly for the time, inasmuch as johnbaumite was not known then and the compositional variation in the apatite group was not understood yet. Although a number of analyses of arsenian fluorapatite were obtained by the writer, there is none whose composition entered the svabite compositional space defined as (Ca > Pb,Sr,Ba)(As > P)(F > Cl,OH). Svabite is unknown from these deposits.

In general, Cl is not present in significant amounts in the high-temperature primary apatites, except for hedyphane and turneaureite. The six analyses given are all of blue and green apatites, but color is not a useful discriminant. There is minimal substitution of Pb, Sr, and Ba in local fluorapatite.

Occurrence and paragenesis

Fluorapatite occurs sparingly in a variety of textures with the silicate minerals and is stable as a high-temperature, primary mineral. It occurs as euhedral crystals, most commonly in association with calcite, and more commonly as massive to granular material in association with rhodonite, andradite, calcite, feldspars, pyroxenes, micas, willemite, franklinite, and other species. It occurs less commonly with hematite and with hardystonite. Palache (1935) noted many occurrences on Mine Hill, including the Buckwheat Mine, the Trotter Mine, and the Parker Mine, and observed that it occurred near pegmatite-marble contacts. It was illustrated from the Buckwheat Dolomite by Peters et al. (1983).

Fluorapatite is uncommon at Sterling Hill, but was found in 1983 as 2-cm rounded crystals in calcite and sphalerite on the 600 level at the southern end of the orebody. It was also found in 1977 associated with fluorite, calcite, franklinite, willemite, and graphite in the black-willemite zone on the 1400 level. See also Jenkins (1994).                           

Blue prismatic crystals are found within the Franklin Marble, but have not been studied.

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