ALBERT

Description: Synthetic polycrystalline samples of Fe-Ti oxides (titanomagnetite, Tmtss; ilmenite-hematitess, Ilmss; pseudobrookitess, Psbss) are very sensitive to changes in the redox conditions at high temperatures, either during synthesis experiments or during thermomagnetic measurements. For instance, exposure to air for a few seconds at the end of a synthesis run at 1300°C of a Tmtss-Ilmss sample produces surficial oxidation down to a depth of some 100 μm. This oxidation zone is well visible on backscattered electron images of polished sections through the sample pellet. It is characterized by so-called trellis “oxyexsolution” textures, i.e., fine lamellae of Ilmss within the Tmtss crystals and lamellae of Psbss within the Ilmss crystals. In this oxidation zone the newly grown Ilmss lamellae and the surrounding Tmtss are more Fe rich than the original crystals. The presence of trellis textures in the crystals of both coexisting phases, Tmtss and Ilmss, show that only short-scaled elemental transport within the crystals was involved and that equilibrium was not attained. Even though the oxidation zone is very narrow, the imprint of the new Tmtss compositions is well recognizable in temperature-dependent magnetic susceptibility curves. In temperature-dependent saturation magnetization (MS-T) curves, however, the contribution of more Fe-rich Tmtss from the oxidation zone can be easily overseen. However, surficial oxidation of Tmtss does occur during MS-T measurements with a variable field translation balance, apparently in relation with insufficient Ar flowing around the sample. Further examples of rapid surficial oxidation of Fe-Ti oxide samples are also discussed.

Description: [1]  Shock experiments with pressures ranging from 3 to 30 GPa have been conducted on a mixed assemblage of hexagonal and monoclinic pyrrhotite. All samples were studied with respect to their particular shock-induced microstructures and magnetic properties at high and low temperatures. Up to 8 GPa, microstructures in shocked pyrrhotite are characterized by mechanical deformation producing a damage of the crystal structure. At pressures of 20 GPa and upward, amorphization and mechanical twinning are the dominant structural features induced by shock. Within the lower-pressure range coercivity, saturation isothermal remanent magnetization and coercivity of remanence increase with shock pressures, in agreement with more single-domain (SD)-like behavior. Simultaneously, the λ-peak of hexagonal pyrrhotite decreases and the 34 K transition of monoclinic pyrrhotite broadens and is depressed. Magnetic hardening is triggered by grain-size reduction, but also by the formation of SD within discrete multidomain grains. Planar deformation features subdivide such multidomain grains into lath-shaped domains with average sizes lying in the SD range. The planar deformation features disappear at 20 GPa and irregular, nanometer-sized “amorphous domains” occur instead. Pressure release from 30 GPa finally triggers partial melting of pyrrhotite. The sharp interfaces between molten and crystalline pyrrhotite document a rapid change of thermal conditions. Within molten pyrrhotite, quenched iron crystals occur. The presence of native iron strongly influences the magnetic properties, depending on the particular amount in the studied sample and likely affects the magnetic properties of impact lithologies on Earth and extraterrestrial material.

Description: [1]  This study presents low-temperature SIRM, susceptibility and hysteresis data of magnetite that have been combined with mineralogic observations. It aims to unravel the origin of two different Verwey transition temperatures observed in different rocks from drill cores of the Chesapeake Bay impact structure (CBIS). We distinguish three types of magnetite in these rocks. (1) “ primary magnetite ” (mt prim) in granites from allochthonous and autochthonous crystalline basement. It has average grain sizes up to some hundreds µm, shows typical multidomain magnetic behaviour and a regular Verwey transition at ~120 K. (2) “Shocked magnetite” occurs in fragments of crystalline rocks and as single grains in the suevite and impact breccia. These rocks show two Verwey transitions - a regular transition at ~120 and a “low–temperature Verwey transition” (LT V ) at around 95 K. The shocked grains are strongly deformed and partially molten. These grains have been strongly alterated during a post-impact phase. The LT V originates from an oxidized magnetite fraction with small grain sizes (some tens of µm). Compared to the larger grain size fraction (some hundreds of µm), the high surface/volume ratio of the small grains allows a more pervasive development of the resulting non-stoichiometry that finally controls T V . (3) “Secondary” magnetite, which formed after the impact from hydrothermal fluids, also shows two Verwey transitions. Besides the regular one, an additional LT V appears between 90 and 100 K. “Secondary” magnetite forms clusters consisting of numerous needle-shaped magnetite crystals, which range from few nm to about 10 µm. This grain size variation is in agreement with hysteresis properties and first-order reversal curves, which indicate superparamagnetic to multidomain magnetic behaviour. The LT V in this population is also the of result of a thin oxidized surface layers that make up an effectual percentage of the bulk volume of the small grains. The Verwey transition is often strongly broadened because this type of magnetite holds a broad range of grain sizes with varying ratios between oxidized surface and bulk volume. This study shows that especially the Verwey transition of small magnetite grains reacts very sensitive to surface oxidation. As the Verwey transition temperature of small grains is very sensitive to post-impact alteration it cannot be used as a reliable shock pressure indicator.