What is magnetism in mineralogy ?

Magnetism : definition

The vast majority of minerals does not react to the magnetic field of a small steel magnet. Only a small handful of them, among which magnetite, pyrrhotite, iron-bearing platinum, maghemite, are attracted to these magnets, which has earned them the nickname "natural magnets" ; for scientists it is ferromagnetic minerals.

However, when placed in the strong magnetic field of an electromagnet, minerals acquire magnetic properties. They then behave in two different ways : some are attracted, these are the paramagnetic minerals (amphiboles, pyroxenes, biotite...) due to a magnetic permeability "m" greater than 1. Others are repelled, these are diamagnetic minerals (carbonates, halides, etc...), characterized by a magnetic permeability "m" less than 1. These different behaviors in the magnetic field of an electromagnet are used to carry out mineral separations and to obtain pure minerals for fine mineralogy manipulations (geochronology, geochemistry, etc...).

The magnetic properties of minerals are expressed by their magnetic susceptibility (k) or by their specific magnetic susceptibility (c = k / r) where "r" is the density of the mineral. These values are given in 10-6 CGSEM units on a scale from 100,000 (iron) to negative values. In this range, the values of ferromagnetic minerals are very strong (greater than 1500), those of paramagnetic minerals weaker but positive (usually between 1 and 50 with a maximum of 290 for hematite), while those of diamagnetic minerals are negative. These values are only determined for common minerals.