BIXBYITE
Bixbyite – Mn3+2O3 – is an oxide with a fairly restricted distribution, occurring only in some very specific parageneses. It is part of some Mn ores, but is never the main ore mineral in the deposit. In the collector’s mineral market, pieces with bixbyite reach high prices due to the generally well-developed crystal shapes and their high luster.
It can contain up to 25 mol% of Fe2O3. In addition, it may contain Ti, Al, Si, and Mg. The bixbyites from Thomas Range (Utah, USA) may exhibit exomorphically exsolved jacobsite crystals on their surfaces. Bixbyite is often slightly magnetic.
1. Characteristics
Crystal system: Cubic diploidal.
Color: Black.
Habit: Cubes, rarely combined with octahedra and diplohedra, up to 8 cm. Massive.
Cleavage: {111},in traces.
Tenacity: Brittle.
Twinning: By {111}, of interpenetration.
Fracture: Irregular.
Mohs Hardness: 6 – 6.5
Parting: No.
Streak: No information available.
Lustre: Metallic bronze.
Diaphaneity: Opaque.
Density (g/cm³): 4.9
2. Geology and Deposits
Bixbyite occurs in metamorphosed Mn ores, being generated as a reaction product of braunite + hematite. These are sedimentary rocks with Mn that have undergone regional or contact metamorphism.
It also occurs in lithophysical cavities of rhyolites, being of exhalative-pneumatolytic origin at high temperatures, above 850ºC.
3. Mineral Associations
Due to the contrast between the parageneses in which it occurs, the associated minerals are very varied.
In the type locality (rhyolites of Thomas Range, Utah, USA) it is associated with quartz, sanidine, topaz, garnet (spessartine), pseudobrookite, hematite and beryl (red beryl).
In Mn ores it is evidently associated with other Mn minerals such as braunite, manganite, pyrolusite, hausmannite, jacobsite, hollandite and diaspore-(Mn). It is one of the minerals that forms “psilomelane”.
Also with fluorite, magnetite, cassiterite and barite.
4. Transmitted Light Microscopy
This does not apply, as bixbyite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Bixbyite acquires an excellent polish with some effort. Its polishing hardness is high, similar to the hardness of braunite and higher than the hardness of hausmannite. Hollandite can be more or less hard, depending on the orientation of the section.
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray with a creamy to brownish-yellow or olive-yellow hue.
Compared to the color of braunite, the color of bixbyite is very similar, but slightly more yellowish; this yellow tone is diagnostic, as braunite tends towards reddish tones.
Compared to the color of jacobsite, the color of bixbyite is much lighter and more yellowish.
Compared to the color of hematite, the color of bixbyite is brownish.
Compared to the color of hollandite, the color of bixbyite is much darker, brownish; the color of hollandite is white.
Compared to the color of hausmannite, the color of bixbyite is more yellow, lighter.
Pleochroism: No.
Reflectivity: 23,07%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, but may exhibit anomalous anisotropy if observed with non-exactly crossed nicols.
Internal reflections: No.
May be confused with: Other Mn minerals.
Braunite is less yellow and more reddish.
Jacobsite is extremely similar, but is slightly more yellow and frequently shows internal reflections, which does not occur in bixbyite.
General Characteristics:
Grain shape: generally occurs in idiomorphic crystals, cubic or in cubes combined with octahedra and diplohedra, which is very characteristic. It can occur as granular aggregates.
Cleavage (111) may be visible, but is usually absent.
Twinning, cross-shaped, may occur, exhibiting a checkerboard (“parquet”) texture. The crystals are covered by twin lamellae that form up to four distinct sets, each with its own orientation, resembling leucite twinning. Occasionally this checkerboard (“parquet”) texture shows two distinct generations. The twin lamellae have variable widths and may show internal reflections due to the exsolution of silicates parallel to the lamellae. Weathering can make the twinning more visible.
Zoning may be visible in larger crystals.
Exsolution of silicates may occur along the twin lamellae.
Replacement of twin lamellae by very fine-grained masses of Mn oxides may occur.
Alteration of bixbyite produces an intergrowth of hematite and braunite.
Inclusions may occur, of hollandite, pyrolusite, lamellar braunite or lamellar hausmannite, the latter parallel to (100).
Substitutions: bixbyite is replaced by braunite, very fine-grained hausmannite (following crystallographic directions) and hematite (forming a network in bixbyite).
