Franklinite

What is franklinite? A zinc iron spinel

Franklinite is ZnFe3+2O4 on the September 2026 IMA list, status G, 1819, USA; the first reference is Annales des Mines 4 (1819), 483 and the second European Journal of Mineralogy 11 (1999), 511. The Handbook of Mineralogy franklinite sheet prints the same formula and files it in the “Spinel group”. Worked on standard atomic weights, ZnFe2O4 is 27.12% zinc, 46.33% iron and 26.55% oxygen. Bosi, Biagioni and Pasero (2019), abstract sort the spinel supergroup by the dominant cations in AB2X4 and describe a spinel subgroup of A2+B3+2O4 oxides; franklinite’s formula has that pattern, though the abstract lists no species by name. A script over the IMA list finds five rows written as zinc, two of a second cation and O4: franklinite, gahnite, hetaerolite, leiteite and zincochromite. The supergroup is set out on spinel.

What the sheet records. Cubic, point group 4/m -3 2/m, space group Fd3m, a = 8.43 to 8.474 Å, Z = 8. Typically octahedra, commonly with rounded edges, to 22 cm; also exsolved within other minerals, coarse to fine granular and massive. Twinning on {111}, the spinel law, common. Parting on {111}, fair; fracture uneven to subconchoidal; brittle; hardness about 6 (VHN 852 to 882). Density 5.05 to 5.22 measured, 5.163 calculated. Opaque, translucent in fine fragments; iron-black, brown or red, may be tarnished; streak reddish brown to black; lustre metallic to semimetallic, may be dull; isotropic, n = 2.36.

Where it forms. “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.” Its associates are zincite, willemite, calcite, andradite, manganosite, rhodochrosite, gahnite, magnetite, rhodonite, hausmannite, hetaerolite, jacobsite, braunite, sarkinite, berzeliite and hematite.

Is franklinite magnetic?

Yes, but not uniformly. The sheet’s physical-properties line ends “Strongly to weakly magnetic.” USGS Open-File Report 99-529, Table 1, by Rosenblum and Brownfield, measured the current at which a laboratory separator pulls each mineral: franklinite 0.01 to 0.40 A in total, 0.10 to 0.20 A at best, over 7 samples. Magnetite, whose sheet says “Strongly magnetic”, is below 0.01 to 0.05 A, best 0.01. The report calls a mineral not drawn even at 1.70 A diamagnetic, and names magnetite first among its ferromagnetic minerals.

The Franklin trio, in the hand. Of the three ore minerals the sheet names together, only franklinite’s sheet prints a magnetic property; Table 1 puts willemite’s best range at 0.30 to 1.10 A and zincite’s at 0.50 to above 1.70 A, both marked “depends on chemistry”. Streak agrees: reddish brown to black for franklinite, white for willemite, yellow-orange for zincite. The site-wide question is on which minerals are magnetic, and a black oxide without a printed magnetic property is worked on is hematite magnetic.

Documented failure: the ranges overlap. Franklinite’s total range, 0.01 to 0.40 A, overlaps willemite’s and zincite’s, both 0.10 to above 1.70 A. A weak pull on a single grain does not identify franklinite, and the sheet’s own “strongly to weakly” says the response varies between specimens. Magnetite also responds; the in-water weights (16.04 to 16.17 g against 16.14 g for 20.00 g) do not separate those two, and hardness overlaps.

What is franklinite made of? The New Jersey recast

Method. Each oxide in the sheet’s three analyses was divided by its formula weight and the cations scaled to 4 oxygens, iron taken as Fe3+ in the two New Jersey analyses as the sheet states. Analysis (1), New Jersey, total 99.51 with 0.71 H2O: Zn 0.605, Mn 0.333, Fe3+ 1.978, Si 0.028, Mg 0.020, Ca 0.018. Analysis (2), New Jersey, microprobe average of five: Zn 0.765, Mn 0.221, Fe3+ 1.984. Analysis (3), Långban, Sweden: Zn 0.882, Fe2+ 0.119, Fe3+ 1.900, Al 0.058, Mn 0.020, Ti 0.021, which reproduces the sheet’s printed formula.

Result. In the New Jersey analyses zinc fills 61.5% and 77.6% of the ideal formula’s ZnO (33.76 wt%), and manganese takes much of the rest: Zn/(Zn + Mn) is 0.645 and 0.776, against 0.978 at Långban. The franklinite on New Jersey labels is a manganese-bearing franklinite, on its own sheet. The zinc it adds to the ore is set out on Franklin and Sterling Hill and is not repeated here.

Documented failure: the cell density. On the ideal ZnFe2O4, the printed cell gives 5.263 to 5.346, above the printed calculated 5.163 and the measured 5.05 to 5.22. The recast shows why the ideal formula is the wrong input for natural material; the sheet is not corrected here. Weighing in water is on how to measure specific gravity at home.

Where is franklinite found? New Jersey and elsewhere

Distribution, as printed. “From Franklin and at Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA. Found at Långban, Värmland, Sweden. Occurs at Hraničná, Czech Republic. In Kazakhstan, from Nayzatas, Dzhumart, and in the Ushkatyn deposits, Atasui area. At the Pereval marble quarry, near Slyudyanka, Sayan Mountains, Siberia, Russia. From the Koduru manganese mine, Srikakulam district, Andhra Pradesh, India. At the Bonnie prospect, near Mukinbudin, Western Australia.” The X-ray powder pattern on the sheet is from Franklin.

The name. “Named for Franklin Furnace (now Franklin), New Jersey, USA; and in honor of the American statesman, philosopher, and inventor Benjamin Franklin (1706-1790).” The sheet marks no type locality and the IMA list gives only the country, USA; see what is a type locality.

Under a UV lamp. The sheet records no luminescence, and the FMS Fluorescent Mineral Database franklinite page read “Sorry! Nothing found” when fetched on 14 November 2026, while the same database lists 163 willemite records. On a Franklin specimen under a lamp, franklinite has nothing recorded to show; the glow is the subject of is willemite fluorescent. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route. More oxides are in the species index.

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