JACOBSITE
Jacobsite – Mn2+Fe3+2O4 – is a fairly rare oxide, of no importance as an ore.
It is classified in the Spinel Supergroup, Magnetite Series. It is magnetic like magnetite, forms a series with magnetite, and is dimorphic with a polymorph of it, jacobsite-Q (Mn2+(Fe3+,Mn3+)2O4), and also with iwakiite.
It may exhibit epitaxies with hausmannite (called “vredenburgite”), as well as exsolutions of hausmannite or galaxite. As an impurity, it may contain Zn.
1. Characteristics
Crystal system: Cubic hexoctahedral.
Color: Black to brownish black.
Habit: Massive to fine or coarse granular. Octahedral crystals up to 4 mm are rare.
Cleavage: In {111}
Tenacity: Brittle.
Twinning: On {111} or lamellar.
Fracture: Irregular, subconchoidal
Mohs Hardness: 5.5 – 6.5
Parting: {111} could be a partition.
Streak: Reddish black
Lustre: Resinous, submetallic
Diaphaneity: Opaque.
Density (g/cm³): 4.76
2. Geology and Deposits
Jacobsite occurs in metamorphic and/or metasomatic Mn deposits, where it can be primary or form as an alteration product of other Mn minerals.
3. Mineral Associations
It is associated with calcite, garnet (andradite), vesuvianite-Mn, hematite, magnetite, and various other Mn minerals such as pyrolusite, psilomelane, coronadite, manganosite, tephroite, bixbyite, franklinite, wiserite, galaxite, braunite, plumboferrite, and hausmannite.
4. Transmitted Light Microscopy
This does not apply, as jacobsite is opaque, but the literature indicates that it can be translucent at thin edges.
5. Reflected Light Microscopy
Sample preparation: Jacobsite exhibits a fairly high polishing hardness, similar to that of magnetite and slightly lower than that of braunite. It polishes well.
PLANE POLARIZED LIGHT – PPL
Reflection color: The color varies with the Mn content:
– Low content: pinkish-brown color, very similar to the color of magnetite.
– Medium content: gray color with an olive tint.
– High content: brownish-gray color.
Compared to the color of braunite, the color of jacobsite has an olive-green or yellowish tint; braunite does not have olive tones.
Compared to the color of bixbyite, the color of jacobsite is olive-gray.
Compared to the color of magnetite, the color of jacobsite is distinctly olive.
Pleochroism: No.
Reflectivity: 17.31%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, occasionally very weakly anisotropic, ranging from dark grayish-brown to light gray with a bluish tint.
Internal reflections: Very rare, brown to deep red, especially when rich in Mn.
May be confused with: other Mn minerals.
Braunite is anisotropic and very rarely shows internal reflections. When it does, they are brown.
Hausmannite and manganite have more developed anisotropy.
Magnetite does not show an olive hue on ND nor internal reflections.
Franklinite (from Franklin, New Jersey, USA) shows a complete transition to jacobsite.
General Characteristics:
Grain shape is generally anhedral; rounded subhedral to euhedral crystals up to one millimeter are possible, as well as finely granular aggregates and polygonal grains.
Cleavage is not visible.
Twinning is not visible.
Alteration of jacobsite is very common, almost generalized, and can be partial or total, forming goethite, hematite, psilomelane, and pyrolusite.
Inclusions of jacobsite occur in braunite, pyrolusite, psilomelane, and bixbyite.
Substitutions: Jacobsite is replaced by pyrolusite and psilomelane.
Regular intergrowths (epitaxies) of jacobsite with hausmannite are called β-vredenburgite; in this association, jacobsite forms the matrix and hausmannite the lamellar network. The term “vredenburgite” was originally used for a material from India consisting of hausmannite lamellae in a jacobsite matrix. In Franklin (New Jersey, USA), the term was used for similar features, both heterolite lamellae in franklinite and manganosite lamellae in zincite. Essentially, the term applies to oriented exsolution lamellae of one mineral in another, both being Mn- and/or Zn-rich oxides.
