Partially serpentinized enstatite porphyroblasts and entirely serpentinized matrix grains from the pyroxenite at Pemes, France, were studied with high-resolution and analytical transmission electron microscopy. The enstatite alteration resulted in two different textures: (1) topotactic aluminous lizardite, with five lizardite layers replacing two enstatite cells, (001)liz ║ (100)en; and (2) randomly oriented packets of lizardite mixed with polygonal serpentine, chrysotile, curved serpentines, chlorite, and mixed-layer silicates. The first texture is due to a reaction involving Al-Si tetrahedral substitutions, Mg-Al and Fe2+-Al octahedral substitutions, a net gain of approximately 40% A1 per unit volume, and moderate structural modifications. The lizardite apparently inherits part of the pyroxene structure. In the second texture, pyroxene does not control the orientation of the alteration products. The pyroxenes are dissolved, and the serpentines and other sheet silicates crystallize from the fluid. Topotactic lizardite occurs in relatively closed microenvironments (areas of restricted fluid flow), whereas the randomly oriented products tend to occur in more open environments such as the matrix, areas cut by fractures, and regions of concentrated defects at the interfaces of augite exsolution lamellae. These textures suggest that the degree of openness of the system may be significant in determining the mechanisms of the alteration reactions.

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