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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2017: 2.31

SCImago Journal Rank (SJR) 2017: 1.440
Source Normalized Impact per Paper (SNIP) 2017: 1.059

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1945-3027
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Volume 100, Issue 1

Issues

Near-infrared investigation of folding sepiolite

Maria Tsampodimou
  • Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, Athens, 11635, Greece
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/ Vanessa-Jane Bukas
  • Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, Athens, 11635, Greece
  • Present address: Department of Theoretical Chemistry, Technische Universität München, Lichtenbergstr. 4, Garching 85747, Germany
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/ Elizabeth T. Stathopoulou
  • Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, Athens, 11635, Greece
  • Present address: Department of Historical Geology and Palaeontology, Subfaculty of Geology & Geoenvironment, University of Athens, Panepistimiopolis, 15784, Zografou, Athens, Greece
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/ Vassilis Gionis
  • Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, Athens, 11635, Greece
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/ Georgios D. Chryssikos
  • Corresponding author
  • Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, Athens, 11635, Greece
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Published Online: 2015-01-10 | DOI: https://doi.org/10.2138/am-2015-4988

Abstract

Sepiolite is an industrially important clay mineral of the palygorskite-sepiolite group with alternating 2:1 ribbons and hydrated tunnels. Dry sepiolite, Mg8Si12O30(OH)4(OH2)4, loses half of its OH2 content upon further heating and undergoes a structural collapse known as folding. This treatment is considered essential for enhancing the absorptive properties of the clay. In this paper, the folding process is studied by near-infrared (NIR) spectroscopy, mid-infrared attenuated total reflectance (ATR), and thermogravimetric analysis (TGA). The folded state, Mg8Si12O30(OH)4(OH2)2, reveals a new spectrum of fundamental and higher-order OH2 vibrations, as well as systematically split doublets of structural and surface O-H vibrations. Detailed assignments for the stretching, combination, and overtone O-H modes are proposed on the basis of the two non-degenerate populations of Mg3OH (and SiOH) present in the folded state. It is demonstrated that NIR is of particular diagnostic value in monitoring conveniently and non-invasively the folding process, which appears as a simple transition between well-defined dry- and folded-structures. At the level of elementary sepiolite particles (laths), folding is described as a cooperative process requiring the integrity of the ribbons and the inter-ribbon linkages (moderately acid-leached sepiolite does not fold). This is opposed to the skewed and sometimes complex OH2 desorption trace observed by high-resolution TGA, which appears to indicate a multimodal distribution of laths. It is proposed that the rate-determining step for a sepiolite (also, palygorskite) lath to fold is the creation of a critical zone at mid-particle length, which is OH2-deficient and contains unstable, fivefold-coordinated Mg2+.

Keywords: Sepiolite; palygorskite; folding; acid activation; near-infrared spectroscopy; ATR; highresolution TGA

About the article

Received: 2014-03-26

Accepted: 2014-07-19

Published Online: 2015-01-10

Published in Print: 2015-01-01


Citation Information: American Mineralogist, Volume 100, Issue 1, Pages 195–202, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-4988.

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© 2015 by Walter de Gruyter Berlin/Boston.

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