ULVÖSPINEL
Ulvöspinel – TiFeO4 – (formerly known as “ulvite”) is a relatively rare (or rarely recorded!) oxide that occurs in basic rocks, usually forming exsolutions in magnetite.
It is present in Fe-Ti ores, but is not the main ore mineral. High contents of ulvöspinel, which is not magnetic, create major problems in the beneficiation of magnetite-based iron ore, as the magnetic concentrate ends up having a lower Fe content than the original Fe content of the raw ore, since a significant volume of Fe, contained in the ulvöspinel, was discarded as “gangue”.
It is part of the Spinel Supergroup and forms a complete solid solution with magnetite.
1. Characteristics
Crystal system: Cubic hexaeoctahedral.
Color: Iron black.
Habit: Exsolution in thin lamellae in magnetite or ilmenite. Rare skeletal crystals, up to 2 cm.
Cleavage: No.
Tenacity: No information available.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 5.5 – 6
Parting: No.
Streak: No information available.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.78 (calculated).
2. Geology and Deposits
Ulvöspinel generally occurs as an exsolution in magnetite, forming extremely fine networks along {100}, generating a texture resembling fabric. It also occurs as an exsolution in ilmenite, also along {100}. It can constitute exsolutions in pseudobrookite.
It is a common component of magnetite-based Fe ores.
Ulvöspinel also occurs in kimberlites.
Pure ulvöspinel occurs in strongly reduced basalts containing native iron and/or graphite, both terrestrial and lunar. In lunar basalts it occurs as ulvöspinel-Cr.
It occurs in eucrite-type meteorites.
3. Mineral Associations
It is associated with the typical minerals of the paragenesis of basic and ultrabasic rocks.
Among the rock-forming minerals are plagioclase (anorthite), olivine (fayalite), clinopyroxene (augite), biotite, and apatite. Also volcanic glass.
The associated opaque minerals are native elements (native iron, graphite, cohenite), other oxides (magnetite, magnetite-Ti, ilmenite, chromite, pleonaste, zirconolite, pseudobrookite), and sulfides (troilite, pyrrhotite, chalcopyrite).
In altered ores, it occurs with maghemite.
4. Transmitted Light Microscopy
This does not apply, as ulvöspinel is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Ulvöspinel takes on a very good polish, but requires more vigorous and time-consuming polishing than common silicates (plagioclase, olivine, clinopyroxene). Its hardness upon polishing is equivalent to that of magnetite, perhaps slightly higher.
PLANE POLARIZED LIGHT – PPL
Reflection color: Brown to reddish-brown. It is a color very similar to one of the pleochroism colors of ilmenite.
Compared to the color of magnetite, ulvöspinel is a darker brown.
Pleochroism: No.
Reflectivity: 18.38%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: Ilmenite, which also occurs in the same habit as magnetite and whose anisotropy is very difficult to observe in such fine lattices. Furthermore, Mg-ilmenites show reflectivity as low as ulvospinel.
The additional presence of spinel in this paragenesis requires great care in observations. The development of maghemite, by alteration, can make observations difficult (cloudy). As magnetite with ulvöspinel often contains graphite in the form of spheres or fine-grained aggregates, the presence of this mineral also needs to be considered.
The use of other analytical techniques is indicated to differentiate between ulvospinel and associated minerals.
General Characteristics:
Grain shape: very rarely occurs as idiomorphic octahedral crystals. It generally forms very fine exsolutions in rocks with Ti-rich magnetites. These exsolutions are arranged parallel to (100) and (110) of the magnetite and resemble the texture of a fine fabric. Care must be taken to recognize these exsolutions, as magnetites with ulvöspinel often also contain spinel exsolution lamellae, as well as ilmenite lamellae arranged parallel to (111).
Inclusions of exsolved magnetite cubes may occur in ulvospinel if the original melt was especially rich in Ti.
Twinning does not occur.
Cleavage along (100) may be visible.
Oxidation of magnetites with ulvöspinel generates a network of ilmenite lamellae in the magnetite, which are not exsolutions because there is no miscibility between magnetite and ilmenite.
Alteration of ulvöspinel, even by very subtle oxidation, forms ilmenite, with or without magnetite; sometimes native iron is also formed. A texture may develop that preserves the shape of the original ulvöspinel exsolution lamellae, but which now consist of a zigzag aggregate of ilmenite lamellae. A myrmekitic pattern of magnetite and ilmenite is frequently formed.
