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Short-Range Order in Amphiboles

Department of Geological Sciences, University of Manitoba Winnipeg, Manitoba, R3T 2N2, Canada, clhawth@ms.umanitoba.ca

Giancarlo Della Ventura

Dipartimento di Scienze Geologische, Terza Universitá di Roma via Ostiense 169, I-00154 Roma, Italy, dellaven@uniroma3.it

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Extensive spectroscopic, diffraction and theoretical work in the last ten years has shown that short-range order (SRO) of both cations and anions is a common feature of amphiboles (Hawthorne et al. 2005). From an experimental perspective, the problem is that SRO does not obey the translational symmetry of the structure in which it occurs, and hence SRO is difficult to detect and decipher directly by standard diffraction methods. There is information on SRO resident in diffuse scattering from a crystal, but this information is quite difficult to extract, particularly when the SRO is complicated, and this approach has only been used for simpler materials. SRO can be detected by several spectroscopic techniques [e.g., infrared spectroscopy, magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy], but the problem is that only part of the local arrangement is derived, and the complete picture of SRO is often not clear. Here, we examine SRO of cations and anions in monoclinic amphiboles, and show that detailed information on SRO can be derived by a combination of infrared spectroscopy and local bond-valence theory, augmented by results from Rietveld and single-crystal diffraction.

	LONG-RANGE ORDER

 
Let us first examine the issue of LRO for a simple (two-dimensional) crystal of general composition M₂SiO₄ where M = Mg, Fe²⁺, and consider the composition Mg_0.8 Fe²⁺_1.2 SiO₄ for the case where there are two distinct sites, M(1) and M(2), in the unit cell (Fig. 1). Maximum order involves complete occupancy of one site by Fe²⁺ and occupancy of the other site by a mixture of Mg and Fe²⁺. Either M(1) or M(2) can be fully occupied by Fe²⁺, leading to the two long-range ordered arrangements indicated in Figure 1. Long-range disorder occurs when Mg and Fe²⁺ occur in equal amounts at the . . . [Full Text of this Article]

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