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Black Franklinite octahedron on calcite (white) from the Sterling Mine, Sterling Hill, Ogdensburg , Sussex County (New Jersey) (dimensions of the largest crystal: 10 cm × 9 cm × 7 cm)
General and classification
other names

Zinc ferrite (after Hintze, 1921)

chemical formula
  • ZnFe 3+ 2 O 4
  • (Zn, Mn 2+ , Fe 2+ ) (Fe 3+ , Mn 3+ ) 2 O 4
Mineral class
(and possibly department)
Oxides and hydroxides
System no. to Strunz
and to Dana
4.BB.05 ( 8th edition : IV / B.01b)
Crystallographic Data
Crystal system cubic
Crystal class ; symbol cubic hexakisoctahedral; 4 / m  3  2 / m
Space group Fd 3 m (No. 227)Template: room group / 227
Lattice parameters a  = 8.47  Å
Formula units Z  = 8
Frequent crystal faces {111}, {100}
Twinning on {111}
Physical Properties
Mohs hardness 6 to 6.5, VHN 100 = 852-882
Density (g / cm 3 ) measured: 5.05 to 5.22; calculated; 5.163
Cleavage is missing; Isolation after {111} possible
Break ; Tenacity uneven to slightly scalloped
colour iron black, brown, red
Line color reddish brown to black
transparency opaque, translucent in thin layers
shine Metallic luster
magnetism strong to weak
Crystal optics
Refractive index n  = 2.36 (2)
Birefringence none, as it is optically isotropic
Other properties
Chemical behavior soluble in HCl

Franklinite , also outdated as Zinkoferrit known, is a rarely occurring minerals from the mineral class of "oxides and hydroxides" with the idealized chemical composition ZnFe 3+ 2 O 4 and is thus chemically seen a zinc - iron - oxide . Structurally, franklinite is a spinel .

Franklinite crystallizes in the cubic crystal system in the structure of spinel and typically develops octahedral crystals of up to 22 cm in size, the corners of which are usually rounded. Coarse or fine-grained inclusions in other minerals are also found. The mineral is generally opaque and only thinly translucent in deep red. The crystals can be iron black, brown or red in color. Fresh samples show a metallic sheen on the surfaces .

Etymology and history

New Jersey Zinc Company mine, Franklin

Franklinite was first discovered in several pits in the vicinity of the city of Franklin in the US state of New Jersey . The first description was in 1819 by Pierre Berthier , who named the mineral Franklin after its type locality and Benjamin Franklin after its namesake .

Type material for this mineral is not defined.


The current classification of the International Mineralogical Association (IMA) is one of the Franklinite to spinel supergroup , where he together with chromite , Cochromit , Coulsonit , Cuprospinell , Dellagiustait , Deltalumit , Gahnit , Galaxit , Guit , Hausmannite , Hercynit , Hetaerolith , Jakobsit , maghemite , Magnesiochromite , magnesiocoulsonite , magnesioferrite , magnetite , manganochromite , spinel , thermoerogenite , titanomaghemite , trevorite , vuorelainenite and zincochromite form the spinel subgroup within the oxispinelle.

Already in the outdated, but still in use 8th edition of the mineral classification according to Strunz , franklinite belonged to the mineral class of "oxides and hydroxides" and there to the department of "compounds with M 3 4 4 - and related compounds", where it, together with Jakobsite, Magnesia ferrite, magnetite and trevorite the group of "iron (III) spinels" with the system no. IV / B.01b .

In the last revised and updated Lapis mineral directory by Stefan Weiß in 2018 , which, out of consideration for private collectors and institutional collections, is still based on this classic system of Karl Hugo Strunz , the mineral was given the system and mineral number. IV / B.02-60 . In the "Lapis system" this corresponds to the department "Oxides with a metal: oxygen ratio = 3: 4 (spinel type M 3 O 4 and related compounds)", where franklinite together with cuprospinel, jacobsite, magnesioferrite, magnetite and trevorite form the group of "Ferrite spinels" forms.

The 9th edition of Strunz's mineral systematics , which has been valid since 2001 and was updated by the IMA until 2009, also classifies franklinite in the department of "Oxides with the molar ratio of metal: oxygen = 3: 4 and comparable". This, however, is further divided according to the relative size of the participating cations , so that the mineral according to its composition in the subdivision to find "With only medium-sized cations" where it along with Brunogeierit , chromite, Cochromit, Coulsonit, Cuprospinell, Filipstadit , Gahnit , Galaxite, hercynite, jacobsite, magnesiochromite, magnesiocoulsonite, magnesioferrite, magnetite, manganochromite, nichromite (N), qandilite , spinel, trevorite, ulvöspinell , vuorelainenite and zinc chromite the "spinel group" with the system no. 4.BB.05 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Franklinite to the class of "oxides and hydroxides" and there in the department of "multiple oxides". Here it is together with magnesioferrite, jacobsite, magnetite, trevorite, cuprospinel and brunogeierite in the " iron subgroup " with the system no. 07.02.02 to be found in the subsection “Multiple Oxides (A + B 2+ ) 2 X 4 , Spinel Group ”.


In chemically pure form, which, however, has not yet been observed in natural Franklinites and has therefore only been realized in syntheses so far, the composition ZnFe 3+ 2 O 4 consists of 27.12% by weight zinc (Zn), 46.33% by weight % Iron (Fe) and 26.55% by weight oxygen (O).

At high temperature are franklinite, jacobsite (Mn 2+ Fe 3+ 2 O 4 ), trevorite (NiFe 3+ 2 O 4 ), magnesia ferrite (MgFe 3+ 2 O 4 ), magnetite (Fe 2+ (Fe 3+ ) 2 O 4 ) and Ulvöspinell Fe 2+ 2 TiO 4 are able to form unrestricted mixed crystals .

Due to the mixed crystal formation, part of the zinc is therefore mostly replaced (substituted) by divalent manganese and / or iron and part of the trivalent iron by equivalent manganese . The mixed formula is given in various sources as (Zn, Mn 2+ , Fe 2+ ) (Fe 3+ , Mn 3+ ) 2 O 4 . Especially between franklinite and magnetite and between franklinite and jacobsite, all intermediate links occur at high temperatures. However, the miscibility decreases as it cools, which leads to the individual phases segregating .

These segregation processes are, among other things, the cause of what appear to be "magnetic Franklinites", which have lamellar magnetite segregation.

Crystal structure

Franklinite crystallizes cubically in the structure of spinel with the space group Fd 3 m (space group no. 227) and the lattice parameter a  = 8.47  Å as well as eight formula units per unit cell . Template: room group / 227


Franklinite (light) and zincite (dark) in the reflected light microscope under normal light

Franklinite is infusible in front of the soldering tube, but it becomes magnetic . It is not very resistant to acids and can be dissolved in hot hydrochloric acid (HCl), with the development of chlorine gas .

Under the incident light microscope , Franklinit reflects the light almost completely, which means that it appears quite pure white. In the air it appears somewhat lighter than sphalerite and much lighter than zincite . The reflectivity of franklinite is, however, greatly reduced in oil and it also shows a color change to gray-green, which allows it to be distinguished from the more reddish magnetite.

With a Mohs hardness of 6 to 6.5, Franklinite is one of the medium-hard to hard minerals that, like the reference mineral orthoclase (hardness 6), can just be scratched with a steel file . Franklinite shows no tendency to split , but secretions are possible after the octahedral surfaces {111}. It breaks with uneven to slightly mussel-looking fracture surfaces.

Education and Locations

Franklinite (black) with zincite (red) from Franklin, New Jersey - exhibited in the Mineralogical Museum of the University of Bonn
Large Franklinite octahedron (9 cm × 9 cm × 8 cm) with willemite (yellowish), zincite (reddish) and calcite (white) from the Sterling Mine, Sterling Hill, Ogdensburg ( New Jersey )

Franklinite forms in veins rich in iron, zinc and manganese , which have been changed in marine carbonate sediments by metamorphosis at high temperatures. As Begleitminerale occur among other Andradite , Berzeliit , Braunit , calcite , Gahnit, hematite , Hausmannite , Hetaerolith , Jakobsit , magnetite, manganosite , rhodochrosite , rhodonite , Sarkinit , Willemit and Zincite on.

As a rare mineral formation, Franklinite could only be detected at a few sites, with around 60 sites (as of 2018) being known to date. In addition to its type locality Franklin , where the mineral was found in several pits and shafts in this largest mining area in New Jersey , Franklinite entered the United States at Moffet Point in the Aleutians East Borough in Alaska, in the Desert View Mine California's San Bernardino Mountains and the Rio Dolores Mine near Central City in Gilpin County of Colorado. Two other sites, the Webber shaft in the Fairview silver mine in Churchill County of Nevada and the "Devine zinc property" in Hidalgo County of New Mexico are so far questionable as the finds have not yet been confirmed.

The only known site in Germany so far is a slag dump from the Genna zinc smelter in the Letmathe district in the Sauerland (North Rhine-Westphalia). In Austria, too, the Stradner Kogel near Wilhelmsdorf (municipality of Bad Gleichenberg) in Styria is the only known location for Franklinite.

Other locations include Iron Knob and Zeehan, as well as in Mukinbudin Shire in Australia, near Vazante in the Brazilian state of Minas Gerais , in the prefectures of Meizhou (Guangdong) and Yichun (Jiangxi) in China, on Ross Island in Ireland, near Nežilovo (Opština Veles) and in the Kratovo - Zletovo district in Macedonia, near East Camp ( Santa Eulalia , Chihuahua) and La Blanca ( Municipio Ojocaliente , Zacatecas) in Mexico, near Trzebinia in Poland, near Ocna de Fier , Răzoare (Maramureș) and Iacobeni ( Suceava) in Romania, in the Pereval marble quarry near Sljudjanka (Eastern Siberia) and at three sites in the Chibinen and Lowosero Tundra in Russia, the Lusaka Province in Zambia, near Garpenberg (Dalarna), Långban and Hasselhojden (Västmanland) in Sweden , at Barberton (Mpumalanga) and in the Edendale lead mine near Pretoria (Gauteng) in South Africa and at the Kerimasi volcano in the Arusha region of Tanzania.


  • P. Berthier : Analysis de deux minéraux zincifères des États-Unis d´Amérique . In: Annales des Mines . tape 4 , 1819, p. 483–494 (French, [PDF; 625 kB ; accessed on June 28, 2019]).
  • Paul Ramdohr : The ore minerals and their adhesions . 4th, revised and expanded edition. Akademie-Verlag, Berlin 1975, p. 969, 995-997 .
  • Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p. 388 .
  • Richard V. Gaines, H. Catherine W. Skinner, Eugene E. Foord, Brian Mason , Abraham Rosenzweig: Dana's New Mineralogy . 8th edition. John Wiley & Sons, New York (et al.) 1997, ISBN 0-471-19310-0 , pp. 300 .

Web links

Commons : Franklinite  - collection of images, videos and audio files

Individual evidence

  1. Carl Hintze : Handbook of Mineralogy . tape  1 . Association of Scientific Publishers Walter de Gruyter, Berlin and Leipzig 1921, p. 66 ( table of contents available from De Gruyter [PDF; 241 kB ; accessed on June 28, 2019]).
  2. zinc ferrite. In: Hudson Institute of Mineralogy, accessed June 28, 2019 .
  3. a b c d Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  188 (English).
  4. Malcolm Back, William D. Birch, Michel Blondieau and others: The New IMA List of Minerals - A Work in Progress - Updated: March 2019. (PDF 1703 kB) In: IMA / CNMNC, Marco Pasero, March 2019, accessed June 28, 2019 .
  5. a b c d Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
  6. a b c d e f g h i j k Franklinite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( [PDF; 71  kB ; accessed on August 20, 2018]).
  7. ^ A b Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  374 .
  8. a b Franklinite. In: Hudson Institute of Mineralogy, accessed June 28, 2019 .
  9. Ferdinando Bosi, Cristian Biagioni, Marco Pasero: Nomenclature and classification of the spinel supergroup . In: European Journal of Mineralogy . tape 31 , no. 1 , September 12, 2018, p. 183–192 , doi : 10.1127 / ejm / 2019 / 0031-2788 (English).
  10. Ernest H. Nickel, Monte C. Nichols: IMA / CNMNC List of Minerals 2009. (PDF 1703 kB) In: IMA / CNMNC, January 2009, accessed June 28, 2019 .
  11. Paul Ramdohr : The ore minerals and their adhesions . 4th, revised and expanded edition. Akademie-Verlag, Berlin 1975, p.  969 .
  12. Paul Ramdohr : The ore minerals and their adhesions . 4th, revised and expanded edition. Akademie-Verlag, Berlin 1975, p.  997 .
  13. a b Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp.  506 (first edition: 1891).
  14. Paul Ramdohr : The ore minerals and their adhesions . 4th, revised and expanded edition. Akademie-Verlag, Berlin 1975, p.  995 .
  15. Localities for franklinite. In: Hudson Institute of Mineralogy, accessed June 28, 2019 .
  16. a b List of localities for Franklinite in the Mineralienatlas and Mindat