Formation of regularly interstratified serpentine-chlorite minerals by tetrahedral inversion in long-period serpentine polytypes

Serpentinite from Lancaster County, Pennsylvania, consists of a variety of fine-grained serpentine minerals, chlorite, randomly interstratified serpentine-chlorite, and a series of phases based on regular interstratification of serpentine and chlorite (S_xC_y, where x and y are integers). Within the resolution of the AEM technique, all layer silicates have the same Mg-rich, Al-rich, Cr-rich, and Fe-poor compositions. Regularly interstratified serpentine-chlorite minerals are frequently intimately intergrown with serpentines that have repeat distances identical to those of the regular interstratifications. Thus, dozyite (S₁C₁, β = 90°) is intimately associated with serpentine with three-layer octahedral order (I,I,II). Longer period polysomes (S₂C₁, S₁C₂, S₂C₂, S₁C₃, S₃C₂, and S₁C₄, all with β = 90°) are each accompanied by serpentines with equivalent c-axis periodicities. S_xC_y phases apparently form by selective growth of Ibb chlorite units from I,II octahedral sequences in long-period serpentines. All microscopic structural evidence is consistent with the formation of regular interstratifications by tetrahedral inversion within existing serpentine. Atomic resolution images reveal that the tetrahedral sheet is displaced by a/3 where it inverts to form the 2:1 layer. A ±a/3 shift is required for hydrogen bonding between OH of the newly formed brucite-like interlayer and O atoms of the 2:1 layer. The sense of the shift is determined by the strong interactions between the octahedral cations in the brucite-like interlayer and the Si in the 2:1 layer (direct superimposition, previously described as a type-a interaction, is strongly unfavorable). Distortion at the inversion point probably lengthens Si-O bonds in the next tetrahedra, facilitating relocation of Si on the other side of the basal O plane. Reversal of the octahedral slant in the 2:1 layer occurs because the +a/3 tetrahedral shift necessitates repositioning of O and OH coordinating octahedral cations, requiring movement of octahedral cations from type-II to type-I positions. Except in the 2:1 layers, the stacking and octahedral slants are inherited. The result is a series of regular interstratifications characterized by a single octahedral slant (specifically, Ibbb,I) and b/3 stacking disorder. This analysis reveals the importance of cation-cation interactions in determining the relative stability of pairs of 1:1 layers and in controlling the detailed structures of layer silicates formed in solid-state serpentine-to-chlorite reactions. Because similar constraints apply to formation of serpentine from chlorite by direct structural modification, the common 1T lizardite polytype may be produced from both IIbb and Ibb chlorites.