Hollandite

 ( Barium Minerals ) 

Hollandite (chemical formula: Ba(Mn⁴⁺₆Mn³⁺₂)O₁₆) is a manganese oxide mineral. Its structure consists of double chains of MnO₆ octahedra delimiting 2 × 2 tunnels. The electrostatic charge created by the Mn³⁺ for Mn⁴⁺ substitution is balanced by cations in the tunnels. Their nature determines the mineral species: Ba for hollandite, K for cryptomelane, Pb for coronadite, Sr for strontiomelane, Mindat.org Tl for thalliomelane, Mindat.org and Na for manjiroite. Mindat.org Pure species are rare and most 2 × 2 tectomanganates contain mixtures of several types of tunnel cations.

A mineral, with the chemical composition BaMn⁴⁺₆Fe³⁺₂O₁₆, that was first found in the Kajlidongri mine in the Jhabua district of Madhya Pradesh, India, had the name hollandite until it was reclassified as ferrihollandite by the International Mineralogical Association in 2012. Ferrihollandite is the barium-iron (III) endmember of the coronadite group.

File:Hollandite_star_in_quartz.jpg|Hollandite star in rare quartz inclusion File:Hollandite.jpg|Polyhedral representation of the 2 x 2 tunnel structure of hollandite. The black atoms represent Ba.Post JE, Von Dreele RB, Buseck P (1982) Acta Crystallographica B38: 1056-1065

• Manganese dioxide