PSEUDOBROOKITE
Pseudobrookite – Fe2TiO5 – is a rare oxide, typical of Ti-rich igneous rocks. It has no importance as an ore.
It is classified in the Armalcolite-Pseudobrookite Series, and there is a polymorph of it, still unnamed. There are many synthetic analogues.
It can form epitaxies with hematite and magnetite. It may contain rutile inclusions. There are two varieties (with Cr, with Mg).
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Blackish brown, reddish brown, black. May exhibit iridescent tarnishing.
Habit: Tabular to elongated, they can be prismatic to acicular. Crystals up to 7 cm.
Cleavage: {010} good. Deep striations parallel to [010].
Tenacity: Brittle.
Twinning: Possible.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 6
Parting: No.
Streak: Brown.
Lustre: Adamantine, resinous, metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.33 – 4.39
2. Geology and Deposits
Pseudobrookite is almost exclusively a product of exhalative alteration formed by pneumatolytic processes in Ti-rich magma cavities, occurring intergrown with hematite. In this case it was formed directly or indirectly from titaniferous magnetite through oxidation at hot fumarole temperatures. It is possible that the hematite formed in this equation was frequently removed.
Therefore, it is typical for magmatic rocks, young volcanic rocks or as a post-volcanic formation in andesites, rhyolites, basalts and others. It is also formed by the reaction of xenoliths contained in these rocks, generally in lithophysae.
It can be generated by ilmenites that have suffered very high temperatures, of ~1,100ºC, a situation that is possible in ejected volcanic material, for example in the ilmenites of some kimberlitic “pipes”.
It rarely occurs in foyaite pegmatites.
3. Mineral Associations
It occurs associated with common silicates of magmatic rocks, such as pyroxenes (enstatite-ferrosilite), sanidine, olivine, quartz, tridymite, and topaz.
With oxides such as hematite, magnetite, ilmenite, rutile, spinel, corundum, and bixbyite.
In pegmatites with garnets (spessartine), beryl, apatite, cassiterite, and mica.
More rarely (in buchites) with mullite, volcanic glass, fluoredenite, and mordenite.
4. Transmitted Light Microscopy
In fine fragments, pseudobrookite is transparent and exhibits the following characteristics:
Refraction indices: nα: 2.350 nβ: 2.390 nγ: 2.420
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Brown, with pleochroism in brown tones.
Relief: Very high.
Cleavage: {010} good.
Habits: Long, prismatic, acicular, very frequently intergrown with hematite.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.070, corresponding to interference colors up to and above the 3rd order.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: Possible.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+)
2V angle: 50º
Alterations: Alters to rutile with hematite, forming even pseudomorphs.
May be confused with: rutile.
5. Reflected Light Microscopy
Sample preparation: Pseudobrookite acquires a good polish. Its hardness upon polishing is lower than that of rutile. Generally, the rock containing pseudobrookite is porous and needs to be impregnated to achieve a good quality polish.
PLANE POLARIZED LIGHT – PPL
Reflection color: Blue-gray, a color with a defined and very conspicuous shade of blue, but mixed with gray tones.
The color varies with variations in chemical composition.
Compared to the color of rutile, the color of pseudobrookite is very similar.
Pleochroism: No.
Reflectivity: 17.67 – 18.32%
Bireflectance: Weak to very weak.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Subtle anisotropy in shades of gray.
Internal reflections: Brown to reddish-brown, they can tend towards orange or reddish-yellow, in some cases very bright, resembling the anisotropy color of covellite.
May be confused with: Some other minerals.
Rutile exhibits more abundant and light yellow reflections, as well as more intense anisotropy.
Ilmenite is similar, but has much stronger and more colorful anisotropy and does not exhibit internal reflections.
General Characteristics:
Grain shape: when it occurs alone, it can form very well-developed idiomorphic crystals. When it occurs intergrown with hematite, it always shows a tendency towards idiomorphic development. When recrystallized from ilmenite, its shape depends heavily on the original material. It can replace original ilmenite lamellae. Titaniferous magnetite crystals with altered ilmenite may show the magnetite portion replaced by hematite and the ilmenite portion replaced by pseudobrookite.
Inclusions 1: Pseudobrookite inclusions can occur in rutile, titanomagnetite, and goethite.
Inclusions 2: Pseudobrookite may contain rutile and hematite inclusions, sometimes through exsolution processes. This is a rather rare situation.
Alteration of pseudobrookite leads to an aggregate of rutile and hematite, forming pseudomorphs.