Abstract– We describe the geological, morphological, and climatic setting of the San Juan meteorite collection area in the Central Depression of the Atacama Desert (Chile). Our recovery activities yielded 48 meteorites corresponding to a minimum of 36 different falls within a 3.88 km² area. The recovery density is in the range 9–12 falls km⁻² depending on pairing, making it the densest among meteorite collection areas in hot deserts. This high meteorite concentration is linked to the long‐standing hyperaridity of the area, the stability of the surface pebbles (> Ma), and very low erosion rates of surface pebbles (approximately 30 cm Ma⁻¹ maximum). The San Juan meteorite population is characterized by old terrestrial ages that range from zero to beyond 40 ka, and limited weathering compared with other dense collection areas in hot desert. Chemical weathering in San Juan is slow and mainly controlled by the initial porosity of meteorites. As in the Antarctic and other hot deserts, there is an overabundance of H chondrites and a shortage of LL chondrites compared with the modern falls population, suggesting a recent (< few ka) change in the composition of the meteorite flux to Earth.

We present a description of opaque minerals, opaque mineral compositions, magnetic properties, and paleomagnetic record of the Tissint heavily shocked olivine‐phyric shergottite that fell to Earth in 2011. The magnetic mineralogy of Tissint consists of about 0.6 wt% of pyrrhotite and 0.1 wt% of low‐Ti titanomagnetite (in the range ulvöspinel 3–15 magnetite 85–97). The titanomagnetite formed on Mars by oxidation‐exsolution of ulvöspinel grains during deuteric alteration. Pyrrhotite is unusual, with respect to other shergottites, for its higher Ni content and lower Fe content. Iron deficiency is attributed by an input of regolith‐derived sulfur. This pyrrhotite has probably preserved a metastable hexagonal monosulfide solution structure blocked at temperature above 300 °C. The paleomagnetic data indicate that Tissint was magnetized following the major impact suffered by this rock while cooling at the surface of Mars from a post‐impact equilibrium temperature of approximately 310 °C in a stable magnetic field of about 2 µT of crustal origin. Tissint is too weakly magnetic to account for the observed magnetic anomalies at the Martian surface.

We have developed a scanning magneto-impedance (MI) magnetic microscope to image surface stray magnetic fields of room-temperature geological samples with submillimeter resolution. The instrument consists of a small, 30 μm diameter, 5 mm length amorphous wire-based magneto- impedance (MI) sensor without any cooling mechanisms. The spacing between the sensor head and the sample was less than 300 μm. The length of the amorphous wire and sample-to-wire distance limits the spatial resolution. We have achieved a spatial resolution of 400 μm with a magnetic resolution of 10 nT. This instrument enables us to map a two-dimensional out-of-page component of a stray magnetic field of a natural remanent magnetization over a millimeter-thick slab of a primitive ordinary chondrite meteorite, documenting dipolelike features. A comparison of element mapping images with the stray field of the meteorites reveals what individual metals carry the dipolar remanences in the meteorites. These results suggest that the scanning MI microscope offers a room-temperature operable, small, low-maintenance alternative to the scanning SQUID microscope, and can aid in the interpretation of the magnetic remanence acquisition process of a meteorite.

The decomposition process of the precursor amorphous phase in ${\mathrm{Nd}}_{\mathrm{5}}{\mathrm{Fe}}_{\text{77−x}}{\mathrm{B}}_{\mathrm{18}}{\mathrm{Cr}}_x$ ($\text{x=0}$ and 3) has been studied by means of x-ray diffractometry and Mössbauer spectroscopy with particular attention paid to the formation of ${\mathrm{Nd}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{14}}\mathrm{B}$ induced by Cr. Amorphous ${\mathrm{Nd}}_{\mathrm{5}}{\mathrm{Fe}}_{\mathrm{74}}{\mathrm{Cr}}_{\mathrm{3}}{\mathrm{B}}_{18}$ was confirmed to decompose to stable $\mathrm{\alpha -}{\mathrm{Fe/Fe}}_{\mathrm{2}}{\mathrm{B/NdFe}}_{\mathrm{4}}{\mathrm{B}}_4$ via metastable ${\mathrm{Fe}}_{\mathrm{3}}{\mathrm{B/Nd}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{14}}\mathrm{B}$ during isothermal annealing at 953 K. However, the metastable formation of magnetically hard ${\mathrm{Fe}}_{\mathrm{3}}{\mathrm{B/Nd}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{14}}\mathrm{B}$ in the Cr-added system is spoiled by a preannealing treatment at 873 K which alters the metastable decomposition products from ${\mathrm{Fe}}_{\mathrm{3}}{\mathrm{B/Nd}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{14}}\mathrm{B}$ to ${\mathrm{Fe}}_{\mathrm{3}}{\mathrm{B/Nd}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{23}}{\mathrm{B}}_3.$ We discuss the Cr-induced formation of ${\mathrm{Nd}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{14}}\mathrm{B}$ from the viewpoint of kinetics.