Jacobsite is a manganese

Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

 iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 oxide mineral

Oxide mineral

The oxide mineral class includes those minerals in which the oxide anion is bonded to one or more metal ions. The hydroxide bearing minerals are typically included in the oxide class...

. It is in the spinel

Spinel

Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...

 group and forms a solid solution

Solid solution

A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...

 series with magnetite

Magnetite

Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

. The chemical formula is MnFe₂O₄ or with oxidation states

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 and substitutions:
(Mn²⁺,Fe²⁺,Mg)(Fe³⁺,Mn³⁺)₂O₄.

It occurs as a primary phase or as alteration of other manganese minerals during metamorphism

Metamorphism

Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

 of manganese deposits. Typical associated minerals include hausmannite

Hausmannite

Hausmannite is a complex oxide of manganese containing both di- and tri-valent manganese. The formula can be represented as Mn2+Mn3+2O4. It belongs to the spinel group and forms tetragonal crystals...

, galaxite

Galaxite

Galaxite is a spinel mineral with the chemical formula 2O4. It has a vitreous luster and is brownish red. It also leaves a brownish red streak...

, braunite

Braunite

Braunite is a silicate mineral containing both di- and tri-valent manganese with the chemical formula:Mn2+Mn3+6[O8|SiO4]. Common impurities include iron, calcium, boron, barium, titanium, aluminium, and magnesium....

, pyrolusite

Pyrolusite

Pyrolusite is a mineral consisting essentially of manganese dioxide and is important as an ore of manganese. It is a black, amorphous appearing mineral, often with a granular, fibrous or columnar structure, sometimes forming reniform crusts. It has a metallic luster, a black or bluish-black...

, coronadite, hematite

Hematite

Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

 and magnetite.

It was first described in 1869 and named for the Jakobsberg Mine, Nordmark

Nordmark

Nordmark can mean:*Nordmark Hundred - a district of Värmland in Sweden*the German name for the Northern March - a territorial organisation in Holy Roman Empire*Arbeitserziehungslager Nordmark a work camp from the Nazis...

, Filipstad

Filipstad

Filipstad is a locality and the seat of Filipstad Municipality, Värmland County, Sweden with 6,177 inhabitants in 2005.Filipstad was granted city privileges in 1611 by Charles IX of Sweden, who named it after his son Duke Carl Philip .After a major fire destroyed forest and town in 1694, Filipstad...

, Värmland

Värmland

' is a historical province or landskap in the west of middle Sweden. It borders Västergötland, Dalsland, Dalarna, Västmanland and Närke. It is also bounded by Norway in the west. Latin name versions are Vermelandia and Wermelandia. Although the province's land originally was Götaland, the...

, Sweden

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

.