Jacobsite

{{Infobox mineral

| name = Jacobsite

| category = Oxide minerals
Spinel group
Spinel structural group

| boxwidth =

| image = Jacobsite-cktsr-15a.jpg

| imagesize = 260px

| caption =

| formula = iron(II,III) manganese oxide, (Mn,Mg)Fe₂O₄

| IMAsymbol = Jcb{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| molweight =

| strunz = 4.BB.05

| system = Isometric

| class = Hexoctahedral (m{{overline|3}}m)
H-M symbol: (4/m {{overline|3}} 2/m)

| symmetry = Fd{{overline|3}}m (no. 227)

| unit cell = a = 8.457 Å; Z = 8

| color = Black to brownish black

| habit = Disseminated to massive, rarely as octahedral crystals

| twinning = Spinel law, flattened on {111} or lamellar

| cleavage = {111}, probably a parting

| fracture = Conchoidal

| mohs = 5.5–6.5

| luster = Metallic

| refractive = ~2.3

| opticalprop = Isotropic

| birefringence =

| pleochroism =

| streak = reddish black to brown

| gravity = 4.76

| density =

| melt =

| fusibility =

| diagnostic =

| solubility =

| diaphaneity = Opaque

| other = weakly magnetic

| references = [http://rruff.geo.arizona.edu/doclib/hom/jacobsite.pdf Handbook of Mineralogy][http://www.mindat.org/min-2061.html Mindat.org]

}}

Jacobsite is a manganese iron oxide mineral. It is in the spinel group and forms a solid solution series with magnetite. The chemical formula is (Mn,Mg)Fe₂O₄ or with oxidation states and substitutions:

(Mn²⁺,Fe²⁺,Mg)(Fe³⁺,Mn³⁺)₂O₄.[http://www.webmineral.com/data/Jacobsite.shtml Webmineral data]

File:Jacobsite-d05-134a.jpg, Kalahari manganese fields, Northern Cape Province, South Africa. Size 3.8 x 3.5 x 3.2 cm]]

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It occurs as a primary phase or as alteration of other manganese minerals during metamorphism of manganese deposits. Typical associated minerals include hausmannite, galaxite, braunite, pyrolusite, coronadite, hematite and magnetite. It is a ferrimagnetic substance, which is weakly attracted by a magnet.

It was first described in 1869 and named for the Jakobsberg Mine, Nordmark, Filipstad, Värmland, Sweden.