Infrared spectroscopy and X-ray diffraction are used to evaluate the OH and H2O environments in 10 Å phase (“TAP”), nominally Mg3Si4O10(OH)2·H2O. Two partially deuterated samples of TAP synthesized under different conditions have very similar IR spectra, indicating that the phase has a reproducible structural state. IR spectra were also collected of samples of fully Ni-substituted and partially deuterated TAP, and of samples heated for 1–2 h at 500 °C to remove structural H2O/D2O and leave behind bands due to OH/OD of the 2:1 layer. A high-pressure study of the Ni-TAP sample confirmed that the behavior of its H2O and OH/OD bands was analogous to that observed in previous studies of Mg-TAP. Comparison of the IR spectra of unheated, heated, and compressed samples has allowed three different types of Mg-OH (Mg-OD) stretching bands to be identified, two of which are further split, indicating subtle complexities in the TAP structure. The third band is identical to the band in talc. Two interlayer H2O stretching bands have been identified. The presence of an absorption feature that is broader than these interlayer H2O bands suggests that there is a second type of more weakly bonded H2O. On heating to 500 °C, the main interlayer H2O bands are lost, the talc-like band is unchanged, and shifts in the other Mg-OH band frequencies indicate a change in environment following the loss of the interlayer H2O. At the same time the signature of a silanol group is possibly revealed from the coincidence of band positions in the Mg-TAP and Ni-TAP spectra. The recognition of three distinct Mg-OH (Ni-OH) environments in Mg-TAP (Ni-TAP) is consistent with the structural model of TAP proposed by Welch et al. (2006) and Phillips et al. (2007), in which the transformation from talc to TAP involves a key change from hydrophobic to hydrophilic character that enables hydration of the interlayer. A final level of complexity is indicated by the identification of a 3c trigonal superstructure from single-crystal XRD, implying a structure analogous to that of the 3T phengite polytype, with interlayer H2O fulfilling the role of K. The formation of additional OH groups when talc transforms to 10 Å phase increases the amount of water contained in 10 Å phase and may also occur in closely related phyllosilicates in the Earth’s mantle, such as intergrowths of chlorite with 10 Å phase. Moreover, the reproducibility of the key features of the IR spectra for different samples implies that this water content is fixed.

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