The luster of freshly broken franklinite is splendent metallic. The luster of crystal faces is splendent metallic to semimetallic to submetallic to dull. In general, franklinite crystals from Franklin are much more lustrous than those from Sterling Hill; there are numerous exceptions to this observation. Secondary crystals in vein assemblages are locally commonly splendent. False-lusters are common, resembling in many cases tarnishes and iridescent films, some of which resemble those of bornite and locally are called “peacock ore;” these surficial phenomena have not been studied on natural material.

	Figure 22-25. Cubic franklinite crystals modified by the octahedron and other forms and associated with rhombic calcite crystals and druses of clinochlore from Franklin. Field of view is 2.5 mm in maximum dimension.		Figure 22-26. Octahedral crystals of franklinite with willemite (gray) and calcite (white) from Franklin. Specimen is 10 cm in maximum dimension. Mineralogical Museum, Harvard University, #96778. Photo by Chip Clark.

Franklinite is megascopically and predominantly black and opaque on casual observation. However, franklinite can vary in intrinsic color from red to brown to black and is known to lend false colors to host minerals in cases where finely-dispersed microscopic material is present as inclusions, most notably in willemite. As observed under reflected-light and transmitted-light microscopy, much franklinite is seen to be red or reddish brown; this is also evident when viewing transparent 0.1 - 0.3 mm microcrystals in secondary vein assemblages with a common binocular microscope or the naked eye. Brown, brownish-red, and intermediate hues are also found.

	Figure 22-27. Modified cubic franklinite crystals with rhombic calcite crystals and platy clinochlore from Franklin. Field of view is 3.0 mm in maximum dimension.

The density varies from 5.05 to 5.22 g/cm³. Cleavage is absent, but partings on {111} are common and have been erroneously considered as cleavages. The hardness is approximately 6, with VHN₁₀₀ = 852-882. In general, franklinite is weakly magnetic, but this magnetism is apparently enhanced by exsolved magnetite. Farrington (1851) found the magnetism of “franklinite” to vary proportionately with distance from the wallrock, being most intense within 4 feet of contacts with non-marble rocks he called syenites (likely pegmatite) and diminishing with distance therefrom; he may have studied magnetite in part. Detailed magnetic studies were given by Barsanov and Kolesnikov (1966).

The most noteworthy textural features of franklinite derive from its numerous exsolutions. The initial study was of “zincian vredenburgite” by Mason (1948), who described hetaerolite exsolved in manganoan franklinite. A broad study of franklinite exsolutions was published by Frondel and Klein (1965), who reported exsolution intergrowths of hetaerolite, gahnite, and hematite in franklinite specimens from Franklin and Sterling Hill.

These exsolutions were also noted by Ramdohr (1980) and are discussed in part under headings for those species. Exsolutions in gahnite/franklinite and magnetite/franklinite  from Sterling Hill were both comprehensively studied by Carvalho (1978) and Carvalho and Sclar (1988). Franklinite/magnetite intergrowths from Franklin were studied by Ramdohr (1980) and Sclar and Leonard (1992). Magnetite/ franklinite/pyrophanite intergrowths from Sterling Hill, a spin-off of Carvalho’s study, were investigated by Valentino (1983).

Additionally, franklinite occurs as inclusions of tiny lath-like crystals, < 0.5 mm in size, in ordered arrangements in hardystonite, willemite, and other minerals. These may represent exsolutions as well or covered epitactic relationships, albeit of uncommon and unstudied kinds.

Franklinite is easily confused with magnetite and jacobsite and less so with hetaerolite. Strong magnetism is indicative of magnetite, at least in part. Franklinite is locally replaced by hematite.

Optically, franklinite is isotropic and moderately white in reflected light. The reflectance (470-700 nm) varies from R = 18.7-17.0 % in air and 6.28-5.33 % in oil (Criddle and Stanley, 1986). Optical observations, descriptions, and other data have been given by Ramdohr (1931, 1980), Picot and Johan (1982), and Criddle and Stanley (1986, 1993). Anomalous anisotropy was described by Klemm (1962) and Libowitzky (1994).

Composition

	Figure 22-28. Chemical composition of 132 selected samples of so-called franklinite calculated on the basis of Zn+Mn+Fe = 100 in atom percent. The data include 56 unpublished analyses from the New Jersey Zinc Company, 40 unpublished analyses made by J. Ito and C. Hepburn for this study, and 36 analyses from Frondel and Klein (1965). The average composition of the Sterling Hill (left) and Franklin (right) franklinite mill concentrates is indicated by crosses. The data is representative of the range of composition, but not of the relative abundance of different compositions. Caption abstracted from that of Frondel and Baum (1974) who provide more detail.

Ideally, franklinite is a zinc ferric-iron oxide mineral of the spinel group. Natural material, however, contains much Mn²⁺, Mn³⁺, and Fe²⁺, together with lesser amounts of other elements, such as Al; it is a complex solid solution. Indeed, end-member material is unknown; some Mn or Fe²⁺ (mostly Mn) substitutes for Zn in all specimens studied to date.

The best bulk analyses of franklinite are those of Dr. Jun Ito, as reported by Frondel and Klein (1965). They reported local specimens to vary in composition from slightly manganoan and zincian magnetite through franklinite to material with Zn as the dominant “A” cation and with total Mn [web note: 'approximately equal to' sign in original] total Fe. 

Frondel and Baum (1974) plotted 132 analyses of franklinite, including those mentioned above (Figure 22-28), and provided compositional data on franklinite mill-concentrates (see section entitled “Mineral assemblages”).

The definitive modern studies of franklinite have been done on Sterling Hill material at Lehigh University by Sclar, Squiller, Carvalho, and Valentino, as noted below and in the references; these are superb contributions.

Squiller (1976) analyzed 394 Sterling Hill franklinites (summary of results is given in Table 19). He found correlations between franklinite compositions and the color of the host willemites. Franklinite from red willemite has the lowest iron content, Al₂O₃ up to 7 wt. %, ZnO 19-28 wt. %, and MnO 5-13 wt. %. Franklinite from black willemite has the highest iron content and the lowest ZnO (17-21 wt. %), with MnO from 3.9-7.5 wt. % and Al₂O₃ quite variable, to a maximum of 3.2 wt. %. Franklinite from brown willemite has intermediate values for Fe₂O₃ and average values for ZnO (21-23 wt. %).

Squiller also performed homogeneity studies, finding no systematic variation for Mn, Fe, or Zn within individual franklinite crystals, and assigned a homogeneity index of less than one for approximately ¾ of the 394 crystals studied. He found homogeneity among individual grains, however, to be rare; chemical equilibrium was attained only over a distance of centimeters, not tens of centimeters. Squiller also noted that, in general, on a large scale, compositional variation along the strike of an ore layer is less than across it. Carvalho (1978) reported Fe values for Sterling Hill franklinite in excess of those given by Frondel and Baum (1974), but still with Zn > Fe in the tetrahedral site. Squiller’s study is the standing definitive one on franklinite.

Leavens and Nelen (1990) reported that, at Franklin, the most Fe-rich franklinites are associated with hematite and andradite; the most Mn-rich with tephroite; and the most Al-rich with bustamite. They also reported that Sterling Hill franklinites have more Fe than those at Franklin, supporting the observations of Frondel and Baum (1974). Johnson (1990) reported 12 franklinite analyses from ore specimens and 2 from calcium silicates, all from Sterling Hill.

	Table 19. Chemical analyses of Sterling Hill franklinite from Squiller (1976) in weight percent.		Table 20. Chemical analyses of secondary franklinite by the writer.

The writer has investigated the composition of secondary franklinite, which occurs as sharp euhedral crystals in vein assemblages at both Franklin and Sterling Hill. The results are given in Table 20, and show that the compositions of these crystals from varying assemblages in different parts of the mines are remarkably constant. They are all very close to (Zn_0.75Mn_0.25)Fe³⁺₂O₄, containing approximately 2 Mn, 6 Zn, and 16 Fe atoms per unit-cell. Inasmuch as the constancy of compositions suggested the possibility of cation ordering, which would require a decrease in symmetry, analyzed crystals were investigated by single-crystal methods in two laboratories. They did not show any deviation from cubic symmetry (Drs. F. Pertlik and R. Rouse, personal communications). Their compositional similarity remains enigmatic; it may be a “comfortable balance” composition for presumably low-temperature material.

Minor-element concentrations in franklinite have also been studied. Carvalho (1978) reported a strong partitioning of titanium into franklinite and a strong correlation between Al and Ti in franklinite/ gahnite intergrowths, suggesting they were deposited together in the original sediment. Titanium is present in Sterling Hill franklinite which hosts exsolved magnetite; it contains up to 3.4 wt. % TiO₂. The most titanium reported in Sterling Hill franklinite, 5.75-9.93 wt. % TiO₂, was in samples co-existing with gahnite (Davis, 1993), but the sums of the four given analyses (87.37, 106.68, 92.49, 91.55 wt. %) suggest very poor precision; these high titanium values should be considered with much caution. 

For aluminum, Carvalho found a maximum of 5 mole % gahnite in solid solution in natural franklinite. Franklinite was reported to be the major host for trace scandium at Franklin at 3 ppm (Frondel, 1970), but that was before the 1981 discovery of local thortveitite.

Occurrence and paragenesis

	Figure 22-29. Franklinite crystals. Drawing by Dave Woods, used with permission.

Franklinite is the most common ore mineral at Franklin and Sterling Hill, comprising approximately 42 and 32%, respectively, of the crude ore milled at Franklin in 1930-1934 and at Sterling Hill in 1935 (Frondel and Baum, 1974). It also is the only one of the principal ore minerals to occur as individual, generally monomineralic ore units. See additional discussion in the section entitled “Mineral assemblages.”

	Figure 22-30. Drawing of octahedral franklinite crystals and prismatic willemite crystals from Sterling Hill. Drawing by Kenneth Sproson, used with permission. See Plate 13, drawing B, in Palache (1935).

At Franklin, franklinite occurs as massive material and, most commonly, as granular aggregates (average grain-size = 2-3 mm) with and without willemite and more or less calcite and zincite, in large-scale lenticular masses with gneissic texture (Figures 12-1, 12-2, 12-3, 12-4, and 12-5). Franklinite grains, commonly interlocked with willemite in calcite-free ore, are more euhedral and increase in grain-size concomitantly with an increase in the calcite component of the ore (Figure 15-87).

At Franklin, franklinite accompanies most of the vast number of minerals known from here. In addition to the primary occurrence in the ores, it is a common constituent of the calcium-silicate units as well, occurring in markedly lesser amounts, but nearly ubiquitous in its distribution. In highly recrystallized silicate assemblages, franklinite is commonly rimmed by andradite and/or willemite.

	Figure 22-31. Superb black franklinite crystals from Sterling Hill, associated with calcite (white) on granular ore consisting of franklinite, calcite, and abundant willemite. Specimen is 8 cm in maximum dimension. Smithsonian Institution, #127555. Photo by the author.		Figure 22-32. Distorted black franklinite crystals composed of octahedral and dodecahedral forms, associated with calcite (white) and willemite (gray) on massive franklinite-willemite ore from Franklin. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #R16067. Photo by the author.

Superb hand-sized and cabinet-sized specimens containing fine euhedral crystals are found in almost all major collections (Figures 22-18, 22-20, 22-26, 22-32, 22-33, 22-35, 22-36, and 22-38). Franklinite is also known as a secondary mineral, forming superb highly lustrous euhedral crystals in vein assemblages (Figures 15-88, 22-21 through 22-25, and 22-27), and associated with most of the minerals found in such veins and fissures. It occurs as durable slickensides with high luster.

	Figure 22-33. Sharp octahedral crystal of franklinite in white calcite from Franklin. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #B12361. Photo by the author.		Figure 22-34. Octahedral crystal of franklinite in calcite on common Sterling Hill ore. Note apparent resorption of crystal faces. Specimen is 8 cm in maximum dimension. Smithsonian Institution, #C1603. Photo by the author.

At Sterling Hill franklinite occurs sporadically as lenticular masses which are in general on the order of a few meters in thickness. There is little mention of large monomineralic franklinite units, although Cook (1879) hinted at one near the keel. This might be part of the “franklinite-zone,” comprised of highly magnetic, low-zinc franklinite-magnetite, which was reported to occur at Sterling Hill by Metsger et al. (1958); it was found in the east limb near the keel of the orebody and wraps around the keel of the deposit.

Sterling Hill franklinite occurs predominantly as granules in granular ore commonly, but not always, associated with calcite; as material intergrown with willemite; and as inclusions in willemite. Gneissic franklinite-willemite  ore is known here, particularly in the central zincite zone, in the central part of the cross-member, in the outer parts of the outer zincite zone, and as isolated units within disseminated ore. An important complication is that the coarse texture of much Sterling Hill ore is such that a clearly gneissic texture on a large scale in situ might not be at all evident in hand-specimens once removed from that site; grain-size problems commonly obscure the recognition of textural features.

	Figure 22-35. Multi-faceted crystals of black franklinite showing a generally rounded aspect on Franklin calcite. Specimen is 3 cm in maximum dimension. Smithsonian Institution, #R1988. Photo by the author.		Figure 22-36. Dodecahedral crystals of black franklinite sparsely distributed in white calcite from Franklin. Specimen is 14 cm in maximum dimension. Franklin Mineral Museum, #SG-266. Photo by the author.

Much if not most Sterling Hill franklinite is in disseminated ore with willemite, more or less zincite, and much calcite. In such ore, the calcite content varies greatly. Franklinite grains tend to be somewhat evenly distributed in both the hand-specimen and in the larger geologic occurrence. Fine crystal specimens are shown in figures 15-87, 22-20, 22-30, 22-34, and 22-37.

	Figure 22-37. Superb multi-faceted, slightly rounded franklinite crystals associated with willemite (gray) and calcite (white) from Sterling Hill. Specimen is 8 cm in maximum dimension. Privately owned. Photo by the author.		Figure 22-38. Abundant octahedral crystals of franklinite on calcite (white) with zincite (dark gray massive at right) from Franklin. Specimen is 9 cm in maximum dimension. Franklin Mineral Museum, #SG-265. Photo by the author.

Metsger et al. (1958) described franklinite inclusions from Sterling Hill, finding them to be limited to nesosilicate minerals and not found in pyroxenes. They proposed that these franklinite inclusions formed by the serpentinization of willemite, that their color was dependent on Fe/Mn ratios in the primary willemite, and that they colored the willemite host. This theory was confirmed in part by Squiller (1976), as noted above, for inclusions which differed in composition from primary franklinite grains in the same host assemblages.

The magnofranklinite (Canfield, 1889) from the Noble Mine at Sterling Hill has not been studied in detail. The crystal described by Palache (1935; p. 48, Plate 7a) as being this material is presently in the U. S. National Museum. Analysis by the writer of a chip from the outside of this specimen finds it to have the composition (Zn_0.49Mn_0.34Fe_0.17)Fe³⁺₂O₄, and therefore to be manganoan franklinite. Large modified octahedra of franklinite were found in the weathered pits at Sterling Hill, which provided some of the largest intact crystals known.

The alteration of franklinite was described in part by Moore (1875) and Palache (1935) and is discussed herein under chalcophanite. Some interesting syntheses were noted by Worner et al. (1990), who did not cite the earlier work by Amstutz (1957).

Name

Franklinite was named by Berthier (1819) who said it was “derived from Franklin, in order to remind us that it was found, for the first time, in a place to which the Americans have given the name of a great man, whose memory is venerated equally in Europe as in the new world by all the friends of science and humanity.” Clearly, it was named for the then-village of Franklin Furnace, with an intent to honor Benjamin Franklin.

The specific origin of the village’s name has been questioned by Frondel (1972), who discusses the possibility that it may have been named for William Franklin (1729-1813) rather than his father, Benjamin Franklin (1706-1790). This writer does not accept Frondel’s suggestion. The naming of the local town for Benjamin Franklin would have paralleled similar actions in Massachusetts (1778) and Pennsylvania (1795) and the naming of the short-lived state of Franklin in the late 18^th century.

Quite aside from the origin of the borough’s name, Berthier’s reference to Franklin’s venerableness in Europe and his reference to science surely implies that Benjamin Franklin was on his mind when he named the mineral. Benjamin Franklin had decided early in his life to devote much effort to scientific investigation, a most admirable decision.

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