Magnetic susceptibility of the rock matrix related to magnetic fabric studies

The effects of serpentinization on density and magnetic susceptibility : a petrophysical model

The anisotropy of magnetic susceptibility in biotite, muscovite and chlorite single crystals

Magnetic anisotropy of some phyllosilicates

Magnetization, susceptibility and M6ssbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of the Tzg triplet of 1,120 K. Predominant exchange interactions in the iron-rich samples are ferromagnetic with J= 1.2 K, as for other trioctahedral ferrous minerals. A peak in the sus- ceptibility of thuringite occurs at Tm = 5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the M6ssbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4~ 1). A factor favouring this random ferromagnetic ground state over the planar antiferromag- netic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illus- trate how long range ferromagnetic order may be destroyed by vortices forming around the Fe 3 +--Fe 3 + pairs. tral in chlorites. Substitution of trivalent ions for Mg in the brucite layers is compensated by replacement of Si by A1 in the tetrahedral sheets of the talc layers (Bailey 1980). The general formula for a trioctahedral chlorite is

Magnetic properties of sheet silicates; 2:1:1 layer minerals

The magnetic behavior of eight samples of biotite and a lepidomelane was studied in the temperature range 15–300 K and analyzed in relation to their chemical composition The platy morphology of biotite facilitates magnetic measurements with the applied field either in the plane of the sheets (Xe⊥ ) or perpendicular to the plane (Xe∥) At high temperature, the samples obey Curie‐Weiss laws, showing typical paramagnetic behavior which is due to the presence of both Fe2+ and Fe3+ ions However, the susceptibilities are anisotropic (χe⊥/χe∥≂125), because of a trigonal contribution to the crystal field which affects only the ferrous ions Formulae are given which permit the ferrous Curie‐Weiss parameters to be derived from the measured susceptibility At low temperature (T<10 K), three of the biotite samples containing high concentrations of iron (17 to 27 wt %) undergo an antiferromagnetic transition The magnetic structure consists of octahedral sheets of iron ions with their moments ferromagnetically co

Magnetic properties of biotite micas

Silicates are the most common Earth materials. Many iron‐rich solid solution series order magnetically but ordering temperatures rarely exceed 100 K, even for the iron end‐members. Minerals with sheet and chain structures are discussed, with emphasis on the magnetic properties of greenalite, cronstedtite, biotite, and crocidolite determined by magnetization, susceptibility, and Mossbauer measurements and by magnetic neutron diffraction. The principal magnetic interactions in the ferrous minerals are ferromagnetic, due to near 90° Fe–O–Fe bond angles for edge‐sharing octahedral sites, yet all of them order antiferromagnetically as the ferromagnetic sheets or chains are coupled by relatively weak antiferromagnetic interactions. A spin flop transition occurring in fields of 1–15 kOe at 4.2 K is analyzed in terms of effective anisotropy and exchange fields. Magnetocrystalline anisotropy arises from the Fe2+ ion which is in an orbital singlet 5A1g, lz=0 ground state in an effectively trigonal crystal field. Th...

Magnetic order in silicate minerals (invited)

Ferrous talc - A planar antiferromagnet

We have examined a series of sheet silicates, both in single crystal form in an applied magnetic field (biotite, chlorite, vermiculite) and as powders in the magnetically ordered state (greenalite, berthierine). It is found that VZZ is negative in the 1 : 1 minerals, that η ≤ 0,2 and that the Z-axis is close to the c-axis of the silicate. These results indicate a |0> ground state for the Fe2+ ion and a c-plane of easy magnetization. In the 2 : 1 minerals however, η = 0.9(1). The sign of VZZ was not determined, but the Z axis is again close to the c-axis.

https://hal.archives-ouvertes.fr/jpa-00218469/document

O. Ballet

Papers

Magnetic properties of sheet silicates; 2:1:1 layer minerals

Magnetic properties of biotite micas

Magnetic order in silicate minerals (invited)

Ferrous talc - A planar antiferromagnet

ELECTRIC FIELD GRADIENT AT Fe2+ SITES IN TRIOCTAHEDRAL LAYER SILICATES