An unusual material that has been called the 'poorly characterized phase' (PCP)1 is abundant in CM2 carbonaceous chondrites. Its identity has been one of the more difficult problems of carbonaceous chondrite mineralogy. Its puzzling compositions, petrographic textures and X-ray diffraction patterns do not fit any known mineral1,2. The high-resolution transmission electron microscopy (HRTEM) described here reveals an Fe-Ni-S-O layered mineral occurring as a major component of PCP in the Mighei CM2 carbonaceous chondrite. The new mineral exhibits extremely unusual structures, with curved, rolled and wavy layers resembling those of serpentine minerals. It also occurs in a coherent intergrowth with phyllosilicates in various ordered and disordered sequences. The intergrowth is the most common form of the Fe-Ni-S-O mineral and is prevalent throughout the matrix; therefore, this new mineral should be considered an important phase that is genetically related to phyllosilicates in the CM2 carbonaceous chondrites.
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