FORMATION OF REGULARLY INTERSTRATIFIED SERPENTINE-CHLORITE MINERALS BY TETRAHEDRAL INVERSION IN LONG-PERIOD SERPENTINE POLYTYPES

Serpentinite from Lancaster County, Pennsylvania, consists of a variet y of fine-grained serpentine minerals, chlorite, randomly interstratif ied serpentine-chlorite, and a series of phases based on regular inter stratification of serpentine and chlorite (SxCy, where x and y are int egers). Within the resolution of the AEM technique, all layer silicate s have the same Mg-rich, Al-rich, Cr-rich, and Fe-poor compositions. R egularly interstratified serpentine-chlorite minerals are frequently i ntimately intergrown with serpentines that have repeat distances ident ical to those of the regular interstratifications. Thus, dozyite (S1C1 , beta = 90 degrees) is intimately associated with serpentine with thr ee-layer octahedral order (I,I,II). Longer period polysomes (S2C1, S1C 2, S2C2, S1C3, S3C2, and S1C4, all with beta = 90 degrees) are each ac companied by serpentines with equivalent c-axis periodicities. SxCy ph ases apparently form by selective growth of Ibb chlorite units from I, II octahedral sequences in long-period serpentines. All microscopic st ructural evidence is consistent with the formation of regular interstr atifications by tetrahedral inversion within existing serpentine. Atom ic 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 requir ed for hydrogen bonding between OH of the newly formed brucite-like in terlayer and O atoms of the 2:1 layer. The sense of the shift is deter mined by the strong interactions between the octahedral cations in the brucite-like interlayer and the Si in the 2:1 layer (direct superimpo sition, previously described as a type-a interaction, is strongly unfa vorable). Distortion at the inversion point probably lengthens Si-O bo nds 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 reposit ioning 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 res ult is a series of regular interstratifications characterized by a sin gle 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 cont rolling the detailed structures of layer silicates formed in solid-sta te serpentine-to-chlorite reactions. Because similar constraints apply to formation of serpentine from chlorite by direct structural modific ation, the common 1T lizardite polytype may be produced from both IIbb and Ibb chlorites.