FRANKLINITE
Franklinite – Zn2+Fe3+2O4 – is a very rare oxide. It is, or was, abundant in only a few locations. In the Franklin Mining District (New Jersey, USA), it was an important ore of Zn and Mn. The deposit has been depleted for decades, and today the mines are open to visitors; famous for the spectacular fluorescent minerals they contain. The calcites and willemites associated with franklinite are usually strongly fluorescent.
Franklinite is classified in the Spinel Supergroup, Oxyspinel Group, Spinel Subgroup. It is isostructural with magnetite and heterolite. It may contain Mn, Al, Ti, and Ca.
Franklinite is paramagnetic, ranging from very magnetic to very weakly magnetic; variations in Zn and Fe content result in varying degrees of magnetism.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Iron black, brown, red, may be tarnished.
Habit: Crystals are typically octahedrons, always with rounded edges; they can reach 22 cm. Dodecahedrons occur, but are less common; cubes are rare. Combined forms occur frequently. It may be exsolved in other minerals, and may occur in fine to coarse granular form or massive.
Cleavage: No.
Tenacity: Very brittle.
Twinning: On {111}, Spinel Law.
Fracture: Irregular to subconchoidal.
Mohs Hardness: 5.5 – 6.0
Parting: Good, on {111},
Streak: Black to reddish-brown.
Lustre: Metallic to submetallic, it may be dull.
Diaphaneity: Opaque, translucent in fine fragments.
Density (g/cm³): 5.05 – 5.22
2. Geology and Deposits
Franklinite occurs in layers and veins formed through complex processes of high-temperature metamorphism and metasomatism on marine carbonate sediments rich in Fe, Zn, and Mn.
The Franklin deposit (New Jersey, USA) is unique, and its genesis is still not fully understood. In some Fe and Mn deposits, franklinite occurs as a very secondary mineral.
3. Mineral Associations
In the type locality (Franklin Mining District, New Jersey, USA), franklinite occurs with 273 other minerals. The most important/well-known are calcite, zincite, willemite, and garnet (andradite).
Among the many other minerals in the District are hardystonite, manganosite, rhodochrosite, gahnite, magnetite, rhodonite, sphalerite, hausmannite, hetaerolite, jacobsite, sarkinite, tephroite, and hematite.
It is also associated with braunite and berzelite.
4. Transmitted Light Microscopy
This does not apply, as franklinite is almost completely opaque. Its refractive index is 2.36.
5. Reflected Light Microscopy
Sample preparation: Franklinite has a high hardness, making it difficult to polish, as is magnetite. Its hardness upon polishing is greater than that of zincite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light gray with very subtle greenish tones.
Compared to the color of magnetite, the color of franklinite is very similar, but less pink and more gray-green.
Compared to the color of zincite, the color of franklinite is much lighter.
Compared to the color of sphalerite, the color of franklinite is slightly lighter.
Compared to the color of hematite, the color of franklinite is much darker and grayer.
Pleochroism: No.
Reflectivity: 17.62% Reflectivity can vary considerably between different franklinites.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, but grains that have undergone stress may show very weak anisotropy between pinkish gray and blackish gray.
Internal reflections: No. Some literature reports abundant deep red reflections, but this is not the case in air observations.
With immersion in oil(!), it is often possible to observe, through careful observation, the presence of internal deep red reflections, mainly at the intergranular boundaries.
May be confused with: Other oxides that occur in paragenesis.
Magnetite is extremely similar, it is very easy to confuse it with franklinite, but it does not have internal reflections like franklinite.
Jacobsite has a distinctly yellow reflection color.
General Characteristics:
Grain shape: Idiomorphic and hypidiomorphic crystals are common. Octahedrons are common, typically with rounded edges. Other cubic forms (dodecahedrons, cubes, combined forms) occur more rarely. It can be massive or granular, fine to coarse.
Partition parallel to (111) may be very sharp, spectacularly developed, or completely absent.
Twins parallel to (111) may be visible. Also parallel to (100).
Zoning is indicated by varying amounts of demixing bodies.
Unmixings 1: Unmixing may be absent, mainly in the younger, pegmatitic portions. In other grains, the observed unmixing is of various types.
Unmixings 2: Very small grains or discs arranged parallel to (100) may occur, formed by a transparent mineral of the Spinel Group, perhaps gahnite.
Unmixings 3: another unmixing is formed by thick plates, arranged parallel to (111), composed of a mineral similar to ilmenite, but with much more internal reflections and lower reflectivity, probably pyrophanite.
Unmixings 4: a very fine network oriented parallel to (100) may occur, composed of hausmannite or a Mg-spinel.
Unmixings 5: Unmixings with magnetite are common. At high temperatures, probably all transitions (continuous solid solution) between magnetite, franklinite, and jacobsite occur. Above a certain quantity of one of the members, unmixing occurs, such that all “magnetic franklinites” are almost always franklinite with magnetite unmixings or magnetite with franklinite unmixings. In both cases, lamellae arranged parallel to (100) of the host mineral occur. In limiting situations, unmixings does not occur: the members remain homogeneous, but are less translucent and have much higher reflectivity than normal. In them, a material similar to normal franklinite develops in fractures and cleavage or parting planes, but with many internal reflections and still magnetic.
