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Pressure-induced stabilization of ordered paranatrolite; a new insight into the paranatrolite controversy

Yongjae Lee, Joseph A. Hriljac, John B. Parise and Thomas Vogt
Pressure-induced stabilization of ordered paranatrolite; a new insight into the paranatrolite controversy
American Mineralogist (January 2005) 90 (1): 252-257

Abstract

The origin and stability of paranatrolite (approximate formula Na (sub 16-x) Ca (sub x) Al (sub 16+x) Si (sub 24-x) O (sub 80) .4H (sub 2) O), a naturally occurring zeolite with the natrolite topology, has long been debated, with its detailed structure unknown. When taken from an aqueous environment and exposed to the atmosphere, paranatrolite is reported to irreversibly lose water and transform to gonnardite/tetranatrolite, Na (sub 16-x) Ca (sub x) Al (sub 16+x) Si (sub 24-x) O (sub 80) .nH (sub 2) O. Since the latter has a disordered Al/Si distribution over the framework tetrahedral sites, it is believed the same is true for paranatrolite. Natrolite itself (Na (sub 16) Al (sub 16) Si (sub 24) O (sub 80) .16H (sub 2) O) has Al/Si ordering, and as recently shown, undergoes a reversible volume expansion ( approximately 2.5%) due to pressure-induced hydration (PIH) above 1.2 GPa to a superhydrated phase (Na (sub 16) Al (sub 16) Si (sub 24) O (sub 80) .32H (sub 2) O). During this process, an intermediate phase with an even larger volume expansion of approximately 7.0% has been detected in a narrow pressure range near 1.0 GPa. We report here that this intermediate phase has a unit-cell compatible with the one reported for paranatrolite at ambient conditions with the same 24 water molecules per formula unit and propose that it is paranatrolite with an ordered Al/Si distribution. An unusual water-sodium chain is observed in the ordered paranatrolite structure: a sevenfold coordination of sodium cations provided by alternating two water bridges along the expanded elliptical channels. The density of the ordered paranatrolite is lower than those of the 16 and 32 water phases, with its channel openings far more circular than in the low-and high-pressure analogs. The atomistic details of the ordered paranatrolite provide a structural model for the naturally occurring paranatrolite and a complete understanding of this intriguing pressure-volume-hydration mechanism in natrolite, demonstrating the unique role of pressure in controlling the chemistry of microporous materials.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 90
Serial Issue: 1
Title: Pressure-induced stabilization of ordered paranatrolite; a new insight into the paranatrolite controversy
Affiliation: Brookhaven National Laboratory, Physics Department, Upton, NY, United States
Pages: 252-257
Published: 200501
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 27
Accession Number: 2005-023547
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: University of Birmingham, GBR, United KingdomState University of New York, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200512
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