Dana classification 07.05.03.01 | Strunz classification 4.FL.45 Crystal system Monoclinic | |
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Formula(repeating unit) (Na0.3Ca0.1K0.1)(Mn,Mn)2O4·1.5H2O Crystal class Prismatic (2/m)(same H-M symbol) | ||
Birnessite (Na0.3Ca0.1K0.1)(Mn4+,Mn3+)2O4 · 1.5 H2O is an oxide mineral of manganese along with calcium, potassium and sodium. It has a dark brown to black color with a submetallic luster. It is also very soft, with a Mohs hardness of 1.5. Birnessite is formed by precipitation in lakes, oceans and groundwater and is a major component of desert varnish and deep sea manganese nodules.
Contents
History
It was first described in 1956 and named for an occurrence in Birness, Aberdeenshire, Scotland. Birnessite is found as an oxidation product of several other minerals, including rhodonite, rhodochrosite, and as a weathering product of franklinite-wilminite ore. It has also been found as a coating along joint planes and fractures within a trachyte sill. However, it has been most commonly seen as a constituent of oceanic nodules of manganese.
A recent study found that the mineral is able to break down prions via oxidation. How well this process works outside the laboratory is unclear.
Geologic occurrence
Levinson noted the presence of Birnessite in one or more mines from the region around Zacatecas, Mexico, while other notations have been made in Canada, at Cummington, Massachusetts, from the aforementioned nodules in both the Atlantic and Pacific Oceans, from the Tachaki Mine in Japan, the Treburland Mine in Cornwall, England, and from a bog in Norway.
Composition and structure
Birnessite is a phyllomanganate, which is a type of ferroalloy that contains a high proportion of manganese. While natural forms generally contain foreign ions (i.e. Na, Ca, K) they are considered non-essential and synthetic forms of the mineral can be produced without them. However, most of the synthetic versions of the mineral undergo significant water loss at temperatures below 100 °C. Its structure is thought to be similar to that of chalcophanite, and has been modeled as such by Burns. The structure itself consists of sheets of water molecules found between sheets of edge-sharing molecules of MnO6 octahedra, and repeated on an average of every 7.2 Å, doing so along the c-axis. Of the six octahedral sites in the MnO6 octahedral layer, one is left unoccupied; Mn2+ and Mn3+ lie above each vacant slot on the octahedral. These Mn ions are low-valence, and associate with O, in both the octahedral and in the water sheets.
Properties and application
Research results published in 2015 show that 2.50eV band gap of Birnessite could be used to harvest sunlight to split water into hydrogen and oxygen.