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Further complexities of the 10 Aa phase revealed by infrared spectroscopy and X-ray diffraction

Alison R. Pawley and Mark D. Welch
Further complexities of the 10 Aa phase revealed by infrared spectroscopy and X-ray diffraction
American Mineralogist (April 2014) 99 (4): 712-719


Infrared spectroscopy and X-ray diffraction are used to evaluate the OH and H (sub 2) O environments in 10 Aa phase ("TAP"), nominally Mg (sub 3) Si (sub 4) O (sub 10) (OH) (sub 2) .H (sub 2) O. 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 degrees C to remove structural H (sub 2) O/D (sub 2) O 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 H (sub 2) O 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 H (sub 2) O stretching bands have been identified. The presence of an absorption feature that is broader than these interlayer H (sub 2) O bands suggests that there is a second type of more weakly bonded H (sub 2) O. On heating to 500 degrees C, the main interlayer H (sub 2) O 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 H (sub 2) O. 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 H (sub 2) O fulfilling the role of K. The formation of additional OH groups when talc transforms to 10 Aa phase increases the amount of water contained in 10 Aa phase and may also occur in closely related phyllosilicates in the Earth's mantle, such as intergrowths of chlorite with 10 Aa phase. Moreover, the reproducibility of the key features of the IR spectra for different samples implies that this water content is fixed.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 99
Serial Issue: 4
Title: Further complexities of the 10 Aa phase revealed by infrared spectroscopy and X-ray diffraction
Affiliation: University of Manchester, School of Earth, Atmospheric, and Environmental Sciences, Manchester, United Kingdom
Pages: 712-719
Published: 201404
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 24
Accession Number: 2014-030307
Categories: General mineralogy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: Natural History Museum, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201419
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