Magnetite Exsolution Research Articles

Magnetite exsolution in almandine garnet

AbstractThree almandine-rich metamorphic garnets have been studied by analytical electron microscopy and electron microprobe analysis. Electron microprobe analyses with total Fe calculated as Fe2+ show that there are no significant departures from stoichiometry due to the presence of Fe3+ in any of the garnets studied. However, in the transmission electron microscope (TEM) all the garnets were found to contain myriad spherical, iron-rich particles up to 400 Å in diameter. Microdiffraction techniques have revealed that the particles are a cubic spinel phase, consistent with magnetite. There is no crystallographic relationship between the host garnet and the particles, a rare situation for exsolution processes. The presence of such particles is interpreted in terms of the exsolution of magnetite from almandine garnet during cooling. This can apparently occur at temperatures below 55°C. The size of the particles is a qualitative indicator of the cooling rate of the rock, but is also dependent on the original Fe3+ content of the host garnet.

Magnetic properties of rocks from the Geitafell gabbro complex, SE Iceland

The southern portion of Geitafell gabbro, SE Iceland, has been sampled in detail along a horizontal profile of 1.5 km length for rock magnetism analysis. Within this profile a ea. 200 m .wide zone exists with anomalously strong NRM of reversed polarity. Magnetite enrichment in this zone- up to 5 vol% - cannot alone account for the large NRM values. Oxidation exsolution of ilmenite in titanomagnetites and, less important, reduction exsolution of magnetite in ilmenites as well as formation of magnetite from silicates are considered the cause of the strong magnetization. During these processes at high temperatures small magnetite grains have been formed with »pseudo single domain• (psd) magnetic properties. Small sta­bility of NRM during AF demagnetization rules out »single domain« grains and is interpreted as result of interaction between psd- and »multi domain« magnetic moments. The rocks studied have preserved their paleomagnetic directions since cooling. It is suggested that such rocks might contribute to marine magnetic anomalies.

ChemInform Abstract: EFFECT OF GADOLINIUM ON MAGNETITE EXSOLUTION IN A GARNET MATRIX

AbstractAus einer Fe‐ Y‐ Granat‐Matrix (Y3FesOn) wird Magnetit nicht mehr über einen Y3 ′‐Diffusions‐ Mechanismus, sondern nach einem Keimbildungsmechanismus herausgelöst, wenn Y" durch Gd3+ substituiert wird.

Effect of Gadolinium on Magnetite Exsolution in a Garnet Matrix

Magnetite exsolves from a garnet matrix via a pretransformed nucleus when gadolinium is substituted for yttrium in yttrium iron garnet. The effect of gadolinium on the exsolution mechanism was studied by X‐ray microanalysis using a transmission electron microscope; a strong decrease in gadolinium concentration was found within the pretransformed nuclei. The ionic structure of the garnet lattice indicates that Gd3+ ions allow a shear process which leads to the pretransformed nuclei, whereas with Y3+ ion diffusion is the only process for magnetite exsolution.

Analysis and lattice imaging of a transitional event on garnets

The exsolution of magnetite from a substituted Yttrium Iron Garnet, containing an iron excess may lead to a transitional event. This event is characterized hy the formation of a transitional zone at the center of which the magnetite nucleates (Fig.1). Since there is a contrast between the matrix and these zones and since selected area diffraction does not show any difference between those zones and the matrix in the reciprocal lattice, it is of interest to analyze the structure of the transitional zones.By using simultaneously different techniques in electron microscopy, (oscillating crystal method microdiffraction and X-ray microanalysis)one may resolve the ionic process corresponding to the transitional event and image this event subsequently by high resolution technique.

Inclusions in muscovite of iron oxides and hydroxides

Muscovites from Rajasthan often contain numerous inclusions of magnetite, maghemite, hematite, lepudocrocite and geothite. Magnetite shows polygonization structure which is often preserved in its transformation products, maghemite and hematite. Optical, chemical and X-ray studies of single crystal of green muscovite showing zones of inclusions indicate that magnetite was exsolved from initial Fe rich muscovite. Subsequent oxidation of magnetite gave rise to maghemite and hematite. Lepidocrocite and goethite are formed by the hydration of hematite. The exsolution of magnetite took place below 500°C and the oxidation to maghemite and hematite at about 375°C.