Franklinite



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 ≈ total Fe.
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. 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 3/4 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.
[Dunn] has investigated the composition of secondary franklinite, which occurs as sharp euhedral crystals in vein assemblages at both Franklin and Sterling Hill. The results 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 (Zn0.75Mn0.25)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. % Ti02. 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.
Franklinite is king of Franklin and Sterling Hill. It is the most abundant ore mineral at these deposits, occurring in a great variety of crystal forms, textures, habits and in a substantial range of chemical compositions. It is, together with willemite, among the most ubiquitous and varied minerals found locally.
It was first described as francklinite from Franklin, by Berthier (1819), but it had been known earlier and had been considered to be a poor ore of iron. It was also known as zincoferrit (Hintze, 1933), and magnetic material was known as magnofranklinite (Canfield, 1889). Synthetic studies use the term zinc ferrite. The literature of franklinite is very extensive; no full recitation is given here. Palache (1935) summarized many studies prior to his work; others are discussed in this text. Some early uses of the ore were described by Bornet (1858). X-ray powder diffraction data were given by Berry and Thompson (1962), and the system for Fe2O3 - Mn3O4 - ZnMn2O4 - ZnFe2O4 was discussed by Mason (1947) and that for MnFe2O4 - Mn3O4 by Osawa et al. (1992). Additional modern references are given by Marshall and Dollase (1984).
Although locally abundant, franklinite is exceedingly rare elsewhere. Several occurrences were summarized by Hintze (1933). In recent times, the best documented of such occurrences are those at Langban, Sweden, described by Burke and Kieft (1972) and also by Nysten (1984), who described Langban franklinite associated with willemite. Other confirmed trivial occurrences include ones at the Niazatas Fe-Mn deposit, Kazakhstan, U.S.S.R (Smol'yaninova et al., 1981) and at the Hraniend magnetite deposit, Silesia, Czechoslovakia (Litochleb, 1974). A less wellconfirmed, perhaps doubtful occurrence is at the Koduru Manganese pit, Andhra Pradesh, India (Krishna Rao, 1968).
The name franklinite has been applied locally to almost all the opaque ore material from the orebodies which has not been shown to be other species. This great generalization is assuredly partly in error; much magnetite is present locally, both as isolated masses and in exsolution intergrowths with franklinite, and jacobsite may have been abundant locally. Although a zinc ore, franklinite is an ore of iron, containing 65-75 wt. % Fe2O3.
Franklinite has the spinel structure and is a "normal" spinel with zinc in tetrahedral coordination (Verwey and Heilmann, 1947; Konig and Chol, 1968; and Marshall and Dollase, 1984). Cation distributions were studied by Shirakashi and Kubo (1979) using NMR; they found Mn2+ primarily in tetrahedral coordination with Zn, and they found Fe3+ and Mn3+ in octahedral coordination in one specimen from primary ore at Franklin. The degree of inversion from normal spinel is temperature-dependent (O'Neill, 1992; see also Fitzner, 1979), and the magnetic structure was discussed by Vogel and Evans (1975).
Franklinite occurs in euhedral to rounded anhedral crystals and in all intermediate degrees of crystal perfection. It also occurs as formless masses, forming thick layers in the orebodies, and most commonly as equidimensional grains with willemite and more or less calcite and zincite.
Crystals of franklinite are commonly equant, but may be distorted or rounded; the dominant habit is octahedral, and dodecahedral, cubic, and trapezohedral modifications are common. Some highly modified crystals were found at the Hamburg Mine (Palache, 1935).
Secondary franklinite crystals in vein assemblages are commonly highly lustrous and complexly modified; cubic habits are known. Such secondary occurrences have provided the very best crystals.
The size of franklinite crystals varies in all gradations from at least 17 cm to the submicroscopic. Much larger ones ( > 30 cm) have been found, but have broken upon removal from the rock. Crystals are commonly twinned, but spinel-law twins with evident re-entry angles are uncommon. Crystals with pseudo-crystal-faces (inherited from contact minerals) are not uncommon, and such faces may be found in combination with true crystal faces of rational forms. Rounded crystals, some showing hexoctahedral faces, have been found at Franklin, and deformed crystals are not uncommon; such deformed crystals may be highly distorted or unsymmetrical. Much material is granular in habit.
Many specimens of franklinite have been repaired or faked (Dunn et al., 1981a). Tradition tells us that miners spent long winters and inactive periods filling broken areas or coigns of crystals with plaster and subsequently covering such "repairs" with lampblack or other blackening agents to disguise their handiwork. There are many such specimens, especially in the Canfield collection in the Smithsonian Institution.
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.
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.
The density varies from 5.05 to 5.22 g/cm3. Cleavage is absent, but partings are common and have been erroneously considered as cleavages. The hardness is approximately 6, with VHN1 = 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.
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."
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. 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.
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. Superb hand-sized and cabinet-sized specimens containing fine euhedral crystals are found in almost all major collections. Franklinite is also known as a secondary mineral, forming superb highly lustrous euhedral crystals in vein assemblages, and associated with most of the minerals found in such veins and fissures. It occurs as durable slickensides with high luster.
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.
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.
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 [Dunn] of a chip from the outside of this specimen finds it to have the composition (Zn0.49Mn0.34Fe0.17)Fe3+2O4, and therefore to be manganoan franklinite. Large modified octahedral 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 Womer et al. (1990), who did not cite the earlier work by Amstutz (1957). (Dunn, 1995)


 Location Found: Franklin (Type Locality) and Ogdensburg
     
 
 Year Discovered: 1819
     
 Formula: Zn2+Fe23+O4
 Essential Elements: Iron, Oxygen, Zinc
 All Elements in Formula: Iron, Oxygen, Zinc
     
 IMA Status: Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"
     
     
 To find out more about this mineral at minDat's website, follow this link   Franklinite

     
 References:
Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.566

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.56


The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 34, No. 2 - Fall 1993, pg. 13Recent Mineral Finds From The Sterling Mine Ogdensburg, New Jersey - Franklinite
View IssueV. 30, No, 2 - Fall 1989, pg. 19Research Reports, Franklinite-Gahnite Exsolution Intergrowths
View IssueV. 11, No. 2 - August 1970, pg. 10Additional Mineral Notes - Franklinite (small article)
View IssueV. 10, No. 1 - February 1969, pg. 6Mineral Notes - Franklinite
View IssueV. 9, No. 1 - February 1968, pg. 12The Exclusive Minerals of Franklin/Ogdensburg, N.J. (as of January 1968) by Frank Z. Edwards - Franklinite
View IssueV. 7, No. 2 - August 1966, pg. 8The Minerals of Sterling Hill 1962-65 by Frank Z. Edwards - Franklinite
View IssueV. 7, No. 1 - February 1966, pg. 7Franklinite (small article)
     
Images

     
Franklinite crystal in calcite matrix, Sterling Hill Mine
Franklinite crystal (about 1 1/2 inches across) in calcite matrix, Sterling Hill Mine, Ogdensburg, NJ. From the collection of, and photo by JVF.


Franklinite crystal 1 7/8 inch with minor willemite and calcite from Sterling Hill, NJ.
Franklinite crystal 1 7/8 inch with minor willemite (red) and calcite (white) from Sterling Hill, NJ. From the collection of, and photo by Robert A. Boymistruk.


Franklinite crystals in calcite from Sterling Hill Mine
Franklinite crystals (black) in calcite (white) from Sterling Hill Mine, NJ. 4" x 2 1/2" x 2". From the collection of, and photo by Robert A. Boymistruk.


Gneissic banded ore, willemite, calcite, zincite, and iridescent franklinite from Franklin, NJ
Gneissic banded ore, willemite (light green), calcite (white), zincite (red), and franklinite showing a "peacock ore" patina (green, violet, blue) from Franklin, NJ. 4" x 2". From the collection of JVF, and photo by WP.







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