Crystal Structure of Moganite and Its Anisotropic Atomic Displacement Parameters Determined by Synchrotron X-ray Diffraction and X-ray/Neutron Pair Distribution Function Analyses

Seungyeol Lee,Huifang Xu,Joerg Neuefeind,Hongwu Xu

doi:10.3390/min11030272

Abstract

The crystal structure of moganite from the Mogán formation on Gran Canaria has been re-investigated using high-resolution synchrotron X-ray diffraction (XRD) and X-ray/neutron pair distribution function (PDF) analyses. Our study for the first time reports the anisotropic atomic displacement parameters (ADPs) of a natural moganite. Rietveld analysis of synchrotron XRD data determined the crystal structure of moganite with the space group I2/a. The refined unit-cell parameters are a = 8.7363(8), b = 4.8688(5), c = 10.7203(9) Å, and β = 90.212(4)°. The ADPs of Si and O in moganite were obtained from X-ray and neutron PDF analyses. The shapes and orientations of the anisotropic ellipsoids determined from X-ray and neutron measurements are similar. The anisotropic ellipsoids for O extend along planes perpendicular to the Si-Si axis of corner-sharing SiO4 tetrahedra, suggesting precession-like movement. Neutron PDF result confirms the occurrence of OH over some of the tetrahedral sites. We postulate that moganite nanomineral is stable with respect to quartz in hypersaline water. The ADPs of moganite show a similar trend as those of quartz determined by single-crystal XRD. In short, the combined methods can provide high-quality structural parameters of moganite nanomineral, including its ADPs and extra OH position at the surface. This approach can be used as an alternative means for solving the structures of crystals that are not large enough for single-crystal XRD measurements, such as fine-grained and nanocrystalline minerals formed in various geological environments.

Highlights

Moganite (CNMMN no. 99–035) is a silica polymorph that takes the name from its locality: the Mogán formation on southern Gran Canaria in the Canary Islands, first described by Flörke et al (1976) [1]
This study reports for the first time the atomic displacement parameters (ADPs) of natural moganite
The sample contains moganite 75.3(5) wt.%, quartz 14.2(3) wt.%, gypsum 6.6(2) wt.% and halite 3.7(2) wt.% determined by Rietveld refinement (Figure 1)

Summary

Introduction

Moganite (CNMMN no. 99–035) is a silica polymorph that takes the name from its locality: the Mogán formation on southern Gran Canaria in the Canary Islands, first described by Flörke et al (1976) [1]. Moganite has been observed ubiquitously in silica sinters, volcanic rocks, carbonate turbidites and deep-sea sediments around the world and is used as an important indicator for evaporitic paleoenvironments [1,2,3,4,5,6,7]. It has been identified as a common constituent in silica varieties such as chert, agate, flint, and chalcedony and commonly coexists with fine-grained quartz [1,2,8,9,10]. The presence of water in moganite has been reported in several studies [12,17,18,19], suggesting that water mainly occurs as free water and silanol species (Si–OH)

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Discussion

Conclusion