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Franklinite
Current inventory:  0 gems
 

Chemistry:  (Zn,Mn²⁺, Fe²⁺)(Fe³⁺,Mn³⁺)₂O₄
[Zinc Iron Manganese Oxide]

Discovered in 1819;   IMA status: Valid (pre-IMA; Grandfathered).
Franklinite is named after its type locality of Franklin, New Jersey, USA which was named after Benjamin Franklin (1706-1790), American statesman, philosopher and inventor.

Classification

Mineral Classification:

Oxides

Strunz 8th Ed. ID:

4/B.02-60

Nickel-Strunz 10th Ed. ID:

4.BB.05

4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
B : Metal: Oxygen = 3:4 and similar
B : With only medium-sized cations

Related to:

Spinel Group. Magnesioferrite - Franklinite Series.

Crystal Data

Crystallography:

Isometric - Hexoctahedral

Crystal Habit:

Typically as octahedral crystals, dodecahedral less common, cubic rare; commonly with rounded edges, to 22 cm; may be modified. Also exsolved within other minerals; coarse to fine granular, massive.

Twinning:

Twinning on {111} as both twin and composition plane, the spinel law, common.

Physical Properties

Cleavage:

None observed; Parting on [111], fair

Fracture:

Irregular/Uneven, Conchoidal

Tenacity:

Very Brittle

Hardness (Mohs):

5.5 - 6.0

Density:

5.07 - 5.22 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Other:

Strongly to Weakly Magnetic; the variation in composition ratio of Zn:Fe results in a variable degree of magnetism.

Optical Properties

Color:

Black, Iron-Black, Brownish Black, Reddish Black, may be tarnished; in reflected light, White to Gray, with dark Red internal reflections.

Transparency:

Opaque, Translucent in fine fragments

Luster:

Metallic, Sub-Metallic

Refractive Index:

2.36(2)  Isotropic
R: (400) 19.2, (420) 19.2, (440) 19.2, (460) 19.2, (480) 19.2, (500) 19.1, (520) 19.0, (540) 18.6, (560) 18.2, (580) 17.8, (600) 17.4, (620) 17.1, (640) 16.8, (660) 16.6, (680) 16.4, (700) 16.3

Birefringence:

0.000  Isotropic

Pleochroism:

None

Occurances

Geological Setting:

In beds and veins formed by high-temperature metamorphism of Fe, Zn, Mn-rich marine carbonate sediments. As a minor mineral in some manganese and iron deposits.

Common Associations:

Andradite, Berzeliite, Braunite, Calcite, Gahnite, Hausmannite, Hematite, Hetaerolite, Jacobsite, Magnetite, Manganosite, Rhodochrosite, Rhodonite, Sarkinite, Willemite, Zincite

Common Impurities:

Mn, Ti, Al, M, Ca

Co-Type Localities:

Franklin Mine, Franklin, Franklin Mining District, Sussex County, New Jersey, USA
Trotter Mine (Lehigh Mine), Franklin, Franklin Mining District, Sussex County, New Jersey, USA
Sterling Mine, Sterling Hill, Ogdensburg, Franklin Mining District, Sussex County, New Jersey, USA

Year Discovered:

1819

View mineral photos:

Franklinite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Franklinite is an ore of zinc and manganese, two important industrial metals. It is a common collector's mineral because of its typical associations with fluorescent minerals such as Calcite and Willemite. Franklinite is rarely available as a faceted gem because it is opaque, black and very brittle.

Franklinite is mostly known from its type locality of Franklin, New Jersey, USA but also found in a few other locations around the world such as Långban, Värmland, Sweden; Hranicná, Czech Republic; Nayzatas, Dzhumart, and in the Ushkatyn deposits, Atasui area, Kazakhstan; the Pereval marble quarry, near Slyudyanka, Sayan Mountains, south of Lake Baikal, Siberia, Russia.
 

  
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