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Franklinite is an opaque, black, sub metallic octahedral crystal. It is a member of the spinel group of minerals, and an end member of the magnetite series. It is an oxide of zinc, Manganese and iron. the composition dominantly ZnFe3+2O4,  but always with some substitution of  Mn2+ and Fe2+ for Zn,and Mn3+ for Fe3+[1][6]. Franklinite is usually associated with brilliantly fluorescent Calcite and Willemite, and may also occur with orange Zincite, providing specimens of nice color contrasting. Franklinite is sometimes classified as a variety of the mineral Magnetite, but it is in fact a distinct mineral species[2] (Dunn 1995). Franklinite is a mineral first described at Sterling Hill Mine. It  is a slice of Lower Cambrian to Middle Devonian rocks that are collected in a half graben and are detached from the Valley and Ridge sequence. It is not found in massive or granular form. The true character of the mineral was first determined by the French chemist Berthier, who named it Franklinite[3].

Composition

The chemical formula of franklinite is ZnFe2O4 [1]. 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 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[1]. Atomic structure

Franklinite is a mineral with formula ZnFe2O4. It has a normal spinel-type structure and contains zinc, manganese and iron. Franklinite has the spinel structure and is a “normal” spinel with zinc in tetrahedral coordination[6]. Cation distributions by 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[4]. The degree of inversion from normal spinel is temperature-dependent[5] and the magnetic structure  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 The principal forms are {111}, {100}, {110}, {211}, and {311}; the less common forms {310}, {510}, {531}, {331}, and {221} are also known. Some highly modified crystals were found at the Hamburg Mine[3][5].

Physical properties

Franklinite surrounded by white calcite and/or greenish willemite with a sprinkling of red zincite. Specimens of this exotic and interesting mineral are truly valued by mineral collectors. it is dark black, Luster is metallic, and Crystals are opaque. Crystal System is isometric 4/m bar 3 2/m crystal Habits include octahedrons often with dodecahedral faces modifying the edges of the octahedron. The modifying can lead to an overall rounding of the crystal. Also as massive and granular. Cleavage is absent. Fracture is conchoidal.Hardness is 6 Specific Gravity is 5.0 - 5.2 (slightly above average for metallic minerals)Streak is reddish brown. Other characteristics: Slightly magnetic. Associated Minerals include willemite, zincite, calcite, rhodonite and other minerals found at Franklin, New Jersey. Notable Occurrences are limited to the world famous mines at Franklin, New Jersey, USA from where franklinite gets its name. Best Field Indicators are crystal habit, streak, associations with other zinc minerals and locality[1].

Geologic occurrence

Franklinite, with only minor exception, is confined to the zinc deposits at Franklin, New Jersey, where, enclosed in granular lime stone, it is associated with zincite and willmite[1]. Franklinite is a major component of the Zn-Mn-Fe ore bodies at Franklin and Sterling hill, and was mined in great quantity as an ore of zinc. Franklinite is black in color, and transparent reddish-brown in small fragments. Some samples, particularly when found with tephroite, have an iridescence not unlike "peacock ore" (bornite).Unlike the other two zinc ore minerals found in the district, willemite and zincite, the composition of local franklinite varies considerably iron. Franklinite is found in massive, granular and, less often, in crystallized forms. Grains range from sub-millimeter to several centimeters in size, and occur as rounded through subhedral to finely formed crystals[5].

Origin of the name

Franklinite was named by Berthier in 1819[3]. 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 intent to honor Benjamin Franklin. The specific origin of the village’s name has been questioned[1], who discusses the possibility that it may have been named for William Franklin (1729-1813) rather than his father, Benjamin Franklin (1706-1790).

References

1.Frondel, C. and Klein, Cornelis Jr. (1965) Exsolution in Franklinite. The American mineralogist. Vol. 50, PP. 1670-1680

2.Dunn, P.J. (1995) Franklin and Sterling Hill. No publisher, pp. 566-578

3.Wolff, J.E. (1903). Zinc and manganese Deposits of Franklin furnace, N.J. united geology Survey, issue 213. Pp.214-215

4.Shirakashi, T. and Kubo, T. (1979) cation distribution in Franklinite by nuclear magnetic resonance. American mineralogy. Vol.64, pp. 599-603.

5.Palache, C. (1928) Mineralogical notes on Franklin and Sterling Hill, N J. American mineralogy. Vol.13, pp.308.

6.Burke, E.A.J. and Kieft, C. (1972) Franklinite from Laangban, Sweden; a new occurrence. Lithos, Vol. 5, Issue 1, pp. 69-72.

7.Dana, James D.(1868). A System of Mineralogy. John Wiley and son. Vol. 2, pp. 664-667.

8.Tarr, A. W. (1929). The origin of zinc Deposits at Franklin and sterling Hill, New Jersey. American mineralogist. Volume 14, pp. 207-221.

9.Allaby, A. and Allaby, Michael. (1999) "franklinite." A Dictionary of Earth Sciences. Encyclopedia.com. (October 27, 2011). http://www.encyclopedia.com/doc/1O13- franklinite.html

10.http://www.mindat.org/gallery.