Original paper

Scapolites: variation of structure with pressure and possible role in the storage of fluids

Comodi, Paola; Mellini, Marcello; Zanazzi, Pier Francesco

European Journal of Mineralogy Volume 2 Number 2 (1990), p. 195 - 202

23 references

published: Apr 17, 1990
manuscript accepted: Dec 12, 1989
manuscript received: Jun 3, 1989

DOI: 10.1127/ejm/2/2/0195

BibTeX file

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Abstract The structural evolution of meionite (Me) and marialite (Ma)-rich scapolites with pressure has been studied for Me68 and Mel7 compositions. The unit cell parameters decrease from a = 13.033(2), c = 7.569(2) Å (1 bar pressure) to a = 11.810(5), c = 7.506(5) Å (29 kbar), in Mel7, and from a = 12.172(2), c = 7.563(2) Å (1 bar) to a = 11.970(5), c = 7.470(5) Å (41 kbar) in Me68. Me68 behaves almost isotropically, with a bulk modulus of 860 kbar, but Me 17 is more anisotropic, with a bulk modulus of 600 kbar. These differences in behaviour are determined by different rigidities of the Na-O and Ca-O bonds. Most of the structural variation can be explained in terms of polyhedral tilting. Cooperative rotation of the four-membered tetrahedral rings reduces the cross-section of the [001] channels where cations are located. Pressure does not significantly affect the cages containing the chlorine and carbonate anions, so scapolite may well behave as a potential reservoir of fluids under deep crustal conditions. The comparative analysis of high-pressure and high-temperature data also indicates the expected structural stability of scapolite within the granulitic P-T field. A comparison of compressibility data reported for scapolites and other framework silicates shows that bulk moduli depend both on the cation type (for instance, they increase with substitution of Ca for Na) and also on the framework topology (i.e. moduli increase along the series analcite, sodalite, scapolite, plagioclase