Hollandite

Hollandite is a mineral in the oxide category and the principal member of the hollandite group. It is a tunnel-structured manganese oxide with the ideal formula BaMn8O16, in which manganese occurs in mixed oxidation states (approximately Mn3+ and Mn4+). In natural samples, the barium in the tunnels can be partially substituted by other large cations such as potassium or lead, giving related end-members and solid-solution with other manganese-oxide minerals.

Structure: The crystal framework is built from edge-sharing MnO6 octahedra that form parallel 2x2 tunnels along the c-axis. These tunnels accommodate the large cations and help stabilize the framework. The result is a dense, dark mineral commonly found as prismatic or tabular crystals, or in fibrous aggregates, with a vitreous to dull luster.

Occurrence and associations: Hollandite forms in oxidized zones of manganese-rich hydrothermal veins and metamorphosed Mn ore deposits. It is frequently found together with other manganese oxides such as cryptomelane, romanechite, and pyrolusite.

Naming and classification: The mineral was named after the Netherlands (Holland). It lends its name to the hollandite group, which comprises a family of tunnel-structured Mn-oxide minerals with various large cations in the channels, including cryptomelane (KMn8O16) and coronadite (PbMn8O16). Hollandite is primarily of mineralogical interest and is not a major ore mineral.