| Hollandite | |
| Category: | Oxide minerals |
| Formula: | Ba(Mn4+6Mn3+2)O16 |
| Imasymbol: | Hol[1] |
| Strunz: | 4.DK.05a |
| Dana: | 7.9.1.1 |
| System: | Monoclinic |
| Class: | Prismatic (2/m) |
| Symmetry: | Monoclinic H-M symbol: (2/m) Space group: I2/m |
| Color: | Silvery-grey to greyish |
| Cleavage: | Distinct / Good, Prismatic |
| Mohs: | 4 to 6 |
| Density: | 4.95[2] |
Hollandite (chemical formula: Ba(Mn4+6Mn3+2)O16) is a manganese oxide mineral.[2] 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,[3] Tl for thalliomelane,[4] and Na for manjiroite.[5] 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,[6] that was first found in the Kajlidongri mine in the Jhabua district of Madhya Pradesh, India,[7] 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.[2]