![]() ![]() The dimensions of the seifertite crystals appear to become coarser near the shock vein. TEM images show that the seifertite crystals are rhomboid or spindle in shape and have dimensions of 50–200 nm by 100–600 nm ( Fig. ![]() In some cases, a small amount of stishovite accompanies seifertite. The lattice parameters coincide with the seifertite contained in Shergotty 16. The XRD patterns indicate that the silica grains consist of seifertite ( Fig. Blocks of the tweed-textured silica grains (~20 × 10 × 5 μm 3) were extracted by a focused ion beam (FIB) system and scanned with a synchrotron X-ray beam. Raman and CL spectroscopy revealed that some silica grains with lamellae-like textures contain stishovite. Those grains have well-developed fractures, different from silica grains that have coesite in their shock veins and the tweed-like grains in and around the shock veins. Silica grains with a lamellae-like texture become dominant as the distance from the shock vein increases ( Fig. Most of the tweed-like silica grains in NWA 4734 lie close to shock veins. Aoudjehane and Jambon 19 also observed similar silica grains in NWA 4734, and proposed that they might be seifertite based on their cathodoluminescence (CL) spectrum, although there is no standard spectrum to identify seifertite. 1c), similar to silica grains in a Martian meteorite known as Shergotty, which contains seifertite 15, 16, 17. We found many silica grains with a tweed-like texture ( Fig. TEM images show that nano-size coesite crystal assemblages exist in the silica grains (Fig. Although most of silica grains in the shock veins are amorphous, some near the shock vein wall contain coesite ( Fig. Such impurities also exist in the silica of Martian and lunar meteorites 15, 18. There are many silica grains in NWA 4734 that are about 100 μm in size, and they contain small amounts of Al 2O 3 (0.59–0.99 wt%) and Na 2O (0.21–0.26 wt%) ( Supplementary Table S1). We also clarify the nature, occurrence and formation mechanism of seifertite. We report robust evidence for the existence of seifertite, which was found only in a shocked Martian meteorite so far 15, 16, 17, in a shocked lunar meteorite, NWA 4734, along with coesite and stishovite confirmed by X-ray and electron beam analyses. NWA 4734 showed many shock features such as shock veins and the presence of maskelynite, indicating that it was heavily shocked. We investigated a lunar meteorite, Northwest Africa 4734 (hereafter, NWA 4734), and particularly a silica grain contained in the meteorite using a field-emission scanning electron microscope, an X-ray diffractometer (XRD), and a transmission electron microscope (TEM). These issues are not only important for understanding the dynamic events that have occurred on the Moon but they are also clues for exploring the Earth’s interior dynamics, because post-stishovite phases are expected to exist in the deep lower mantle and D″ layer in the core-mantle boundary of the Earth 12, 14. The mechanism through which cristobalite transforms to its high-pressure polymorph has not been explained. Notably, most silica grains in lunar meteorites and Apollo samples are cristobalite. However, the occurrence and nature of high-pressure polymorphs of silica are still enigmatic in lunar meteorites. There is still no clear evidence for the existence of post-stishovite phases in lunar meteorites. Synthetic experiments indicate that silica transforms to coesite, stishovite, CaCl 2-type, α-PbO 2-type (seifertite) and pyrite-type forms as pressure and temperature rise 11, 12, 13. A recent study, however, revealed that a lunar meteorite, Asuka 881757, contains high-pressure polymorphs of silica, coesite and stishovite formed by meteorite bombardment 3.800 Ma ago 10. ![]() Several previous studies have proposed that few high-pressure polymorphs are contained in lunar meteorites 8, 9 although many craters and thick regoliths of the moon imply that it has experienced heavy meteorite bombardments. The existence of a high-pressure polymorph in a meteorite suggests that its parent body has gone through a dynamic event 1, 2, 3, 4, 5, 6, 7. ![]()
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