3DED As a Robust Technique for Decoding Phyllosilicate Frameworks

The layer-stacking characteristics of phyllosilicate minerals serve as critical indicator of crystallization conditions and metallogenic environments, making their precise identification and description highly significant. Conventional structural characterization method such as powder/single-crystal X-ray diffraction has fundamental limitations when analyzing the Martian phyllosilicates with fine grain size (mostly less than 2 mu m), multiple coexisting phases, and restricted sample availability. Transmission electron microscopy (TEM) can provide nanoscale microstructural analysis, while the destructive, labor-intensive sample preparation is required to expose the [hk0] zone axis and reveal the layer-stacking sequences of phyllosilicates. Here, we demonstrate that three-dimensional electron diffraction (3DED) enables rapid, accurate phase and polytype analysis of phyllosilicates in minutes, without specialized sample preparation. We implemented continuous rotation electron diffraction method to systematically collect the diffraction data of biotite, phlogopite, kaolinite, illite and chrysotile specimens. The 1M, 2M1, and 1Md polytypes of biotite, along with the 1A and 1Ad polytypes of kaolinite, as well as 2M1 polytypes of illite, were successfully identified through reconstructed diffraction patterns along the [100], [010] and [001] zone axes. Additionally, we obtained a ring diffraction pattern exhibiting higher-order Laue zones along the [100] zone axis of chrysotile, a characteristic feature of its tubular silicate structure. These results demonstrate that 3DED can effectively resolve complex stacking sequences and phase mixtures in phyllosilicates without precise zone-axis alignment. This advancement provides a robust framework for reconstructing mineralization histories and elucidating geological processes through nanoscale structural fingerprints in phyllosilicates. By identifying the phases and structure of samples in a non-destructive and intuitive manner, 3DED can facilitate in situ analysis of rare or microcrystalline samples, such as extraterrestrial materials (e.g. Martian materials), contributing to understanding of complex origins and evolution of extraterrestrial bodies.