# Possible explanations to a negative correlation between iron (Ferromagnetic) and magnetic field observations from orbit

When looking at magnetic field observation of a planet with no current magnetic field, we can observe magnetic anomalies, which is basically when ferromagnetic minerals acquire remanent magnetization through a couple of varying processes. My question is if there are any suggestions or explanations to having a negative correlation between iron concentration and the magnetic field observations from orbit. i.e As the iron concentration increases the magnetic field observations seem to be decreasing and points with a high magnetic anomalies are those with lowest iron concentration.

• Can you give specific examples of this? Jun 24 '20 at 10:12
• I've been looking at iron concentrations and magnetic observations of Mars and I noticed that there is a negative correlation between iron and remanent magnetic anomalies observed from orbit in regions with the highest magnetic field observations Jun 24 '20 at 10:38
• @AhmedAbdulla this is a really interesting question! Do these maps say anything about the chemical form of the iron? Can you include some specific examples of a few sites that show this negative correlation? How do we know this is even true? Thanks, and welcome to Space!
– uhoh
Jun 24 '20 at 10:57
• Info like that should go in the question. Jun 24 '20 at 11:21
• Some searching shows BD530 seems to be related to ferric iron oxide...a non-magnetic iron oxide that results from weathering. You'd get a strong signature wherever this material is accumulating on the surface as it gets eroded from rocks and blown around...not in the iron-rich geological formations that froze in magnetic fields as they formed. Not giving as an answer as I don't know what the others are, but this gives an example of how these might not mean what you think. Jun 24 '20 at 12:15

Not all iron minerals are magnetic. Their magnetic properties vary from weak, through medium to strong. The Curie point determines whether an allotrope of iron is magnetic. If the Curie point is below 770 oC the allotrope will be ferromagnetic.

What you are asking about will be determined by the mix of iron minerals present at the specific location you are studying.

Magnetite, Fe3O4, is a ferrimagnetic iron oxide and can be magnetized to be a permanent magnet.

It is the most magnetic of all the naturally-occurring minerals on Earth.

The relationships between magnetite and other iron oxide minerals such as ilmenite, hematite, and ulvospinel have been much studied; the reactions between these minerals and oxygen influence how and when magnetite preserves a record of the Earth's magnetic field.

Magnetite also has the highest content of iron, at 72.4%.

Pyrrotite is an iron sulfide classified as a magnetic mineral. Its ...

magnetism decreases as the iron content increases

It is a nonstoichiometric variant of FeS, the mineral known as troilite.

The ideal FeS lattice, such as that of troilite, is non-magnetic. Magnetic properties vary with Fe content. More Fe-rich, hexagonal pyrrhotites are antiferromagnetic. However, the Fe-deficient, monoclinic Fe7S8 is ferrimagnetic. The ferromagnetism which is widely observed in pyrrhotite is therefore attributed to the presence of relatively large concentrations of iron vacancies (up to 20%) in the crystal structure. Vacancies lower the crystal symmetry. Therefore, monoclinic forms of pyrrhotite are in general more defect-rich than the more symmetrical hexagonal forms, and thus are more magnetic. Monoclinic pyrrhotite undergoes a magnetic transition known as the Besnus transition at 30 K that leads to a loss of magnetic remanence. The saturation magnetization of pyrrhotite is 0.12 tesla.

The nature of the ferromagnetic properties of pyrrotite are confirmed elsewhere.

Ilmenite is a medium magnetic mineral (paramagnetic). It is a iron-titanium oxide, FeTiO3.

Hematite is an iron oxide, Fe2O3, that is antiferromagnetic. Its iron content is 70%.

Hematite is not magnetic and should not respond to a common magnet. However, many specimens of hematite contain enough magnetite that they are attracted to a common magnet. This can lead to an incorrect assumption that the specimen is magnetite or the weakly magnetic pyrrhotite.

• Fantastic answer! +1
– uhoh
Jun 24 '20 at 23:46