HOLLANDITE
Hollandite – Ba(Mn4+6,Mn3+2)O16 – is a relatively common oxide that is part of some Mn ores.
In 2012, the IMA revised the nomenclature of the Hollandite Supergroup. Now hollandite is redefined as the Ba-Mn3+ end of the Coronadite Group. Classical hollandite was redefined as ferrihollandite, a name that designates the Ba-Fe3+ end of the Group. It is chemically similar to romanechite and may contain Pb, U, V, Na, K, and Fe.
Macroscopically, it is indistinguishable from several other Mn oxides, such as cryptomelane, manganite, and coronadite.
Conclusive identification of many Mn oxides is obtained only or preferably with X-ray Diffraction or Infrared Spectroscopy.
1. Characteristics
Crystal system: Monoclinic prismatic, pseudotetragonal.
Color: Silvery gray to grayish black, black.
Habit: Short prismatic with a flattened pyramidal termination. Fibrous to coarsely radial, massive.
Cleavage: (110) distinct.
Tenacity: Brittle.
Twinning: Common, on {101} or {10-1}.
Fracture: Fibrous.
Mohs Hardness: 4 – 6
Parting: On {001}.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.95
2. Geology and Deposits
Hollandite occurs as a primary mineral in Mn ores formed by contact metamorphism of pre-existing Mn ores.
As a secondary mineral, it occurs as a weathering product of Mn minerals.
It can occur associated with kimberites and carbonatites.
3. Mineral Associations
It is associated with some common minerals such as quartz, calcite, hematite, goethite, limonite, and barite.
It is also associated with several other Mn minerals such as psilomelane, bixbyite, jacobsite, pyrolusite, braunite, and cryptomelane.
It also occurs with nontronite, piemontite, vanadinite, and wulfenite.
4. Transmitted Light Microscopy
This does not apply, as hollandite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: The polishing of hollandite is tolerable, sometimes poor. The polishing hardness varies in different directions: (110) breaks easily due to cleavage and is more difficult to polish than (100) or (001). Its polishing hardness in all sections is slightly less than that of bixbyite. Most sections have a much higher hardness than braunite, but some sections have a slightly lower hardness than braunite. Sections perpendicular to the “c” axis exhibit the highest hardness.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray with a yellowish tint. Basal sections show a less vivid (more dull) grayish-white color, very similar to the color of psilomelane.
Compared to the color of magnetite, the color of hollandite is very similar.
Compared to the color of pyrolusite, the color of hollandite is very similar or somewhat darker, depending on the quality of the pyrolusite polish.
Compared to the color of hematite, the color of hollandite is more yellowish and without the bluish tint.
Compared to the color of braunite and bixbyite, the color of hollandite is much lighter, without brownish or olive tones.
Pleochroism: Weak in shades of gray.
Reflectivity: 26.47 to 31.23%
Bireflectance: Weak. Basal sections are the darkest and show no bireflectance.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy, with gray colors having brownish hues, which also can be yellowish, pinkish-white, bluish, or gray-violet. Basal sections are practically isotropic. Undulating extinction is quite common.
Internal reflections: No.
May be confused with: Several other Mn oxides and other minerals from the paragenesis.
Braunite and hausmannite never form fibrous or acicular crystals.
Hematite shows a different reflection color, weaker bireflectance, and often internal reflections in blood-red.
General Characteristics:
Grain shape: hollandite can form coarse, compact masses with a radial structure. It also occurs as bundles of isolated crystals of various sizes included in the gangue, which is usually quartz. It can form idiomorphic prismatic or tabular crystals. Fibrous aggregates or acicular crystals occur. It can be compact and botryoidal.
Cleavage is usually distinct, but not visible in basal sections.
Twinning is common, as simple and repeated lamellae, resembling the twinning of hausmannite and hematite (= polysynthetic twinning of plagioclase).
Inclusions OF hollandite occur in psilomelane, bixbyite, and jacobsite.
Inclusions IN hollandite can be braunite and hausmannite.
Substitutions: hollandite can be replaced by pyrolusite, braunite, bixbyite, hematite, and psilomelane, often along cleavage planes. It can also form intergrowths with these minerals.
