American Mineralogist, Volume 90, pages 16931704, 2005 0003-004X/05/11121693$05.00/DOI: 10.2138/am.2005.1829 1693 INTRODUCTION Pyrite framboids constitute one of the most remarkable forms of inorganic self-assembly found in nature. Ohfuji and Rickard (2005) dened the texture in terms of three specic attributes: (1) spheroidal to sub-spheroidal microscopic clusters (up to 250 μm) of (2) 10 3 to 10 6 discrete microcrystals of pyrite (FeS 2 ), and (3) which are basically all equant, equidimensional, and equimorphic (Fig. 1). Framboidal pyrite is ubiquitous in various geological environments, such as ancient sedimentary rocks of Archaean age (e.g., Hallbauer 1986), recent marine and lacustrine uncon- solidated sediments (e.g., Sweeney and Kaplan 1973; Perry and Pedersen 1993), anoxic water columns (e.g., Ross and Degens 1974; Muramoto et al. 1991; Wilkin and Barnes 1997; Suits and Wilkin 1998), hydrothermal ore deposits (e.g., Kanehira and Bachinski 1967; Ostwald and England 1977; England and Ostwald 1993), and volcanic rocks (Love and Amstutz 1969). Similar framboid-like aggregates of microcrystalline pyrite have been produced experimentally (e.g., Sweeney and Kaplan 1973; Graham and Ohmoto 1994; Wilkin and Barnes 1996; Butler and Rickard 2000a; Ohfuji 2004; Ohfuji and Rickard 2005). Pyrite framboids display two contrasting internal structures made up of constituent microcrystals: (1) an ordered structure (Figs. 1a and 1b) composed of the microcrystals that are ar- ranged into an almost uniform morphological array and (2) a disordered structure (Fig. 1c) with no obvious internal ordering. In two-dimensional (2D) framboid sections, the microcrystal orderings are usually observed as cubic and hexagonal patterns (Love and Amstutz 1966; Kalliokoski and Cathles 1969; Rick- ard 1970; Ohfuji and Akai 2002) and as parallel linear patterns (Love and Amstutz 1966; Ohfuji 2004) in which the individual microcrystals have the same or very similar morphological orien- tations (Fig. 1a). The three-dimensional (3D) architecture of the regular microcrystal arrangements has been demonstrated from the morphological point of view as either (1) cubic close packing (ccp) or (2) icosahedral packing (Ohfuji and Akai 2002). Interest- ingly, these two packing structures are commonly observed in synthetic nano-scaled metal clusters of Au, Ag, and Pt particles (Spiegelmann and Poteau 1992; Lu and Tanaka 1997; Yacaman et al. 2001), which consist of densely packed atoms. A major, and hitherto unanswered, question is what framboids (especially highly ordered framboids) actually are in crystal- lographic or materials science terms. There are no published studies on the crystallography of framboids. Butler (1994), in an unpublished Ph.D. study, reported the results of a single crystal X-ray diffraction (XRD) investigation of framboids. He reported sharp powder XRD patterns from individual pyrite framboids from the Rammelsberg deposit. The important result from Butlerʼ s work was that even framboids displaying well- dened internal microcrystal organization (ccp structure) were not simple single crystals in XRD terms. This was conrmed by a recent single-crystal XRD study of pyrite framboids (Ohfuji 2004). Both Butler (1994) and Ohfuji (2004) concluded that *Present address: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Ehime, Japan. E-mail: ohfuji@sci.ehime-u.ac.jp Structure of framboidal pyrite: An electron backscatter diffraction study HIROAKI OHFUJI, 1, * ALAN P. BOYLE, 2 DAVID J. PRIOR , 2 AND DAVID RICKARD 1 1 School of Earth, Ocean, and Planetary Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, U.K. 2 Department of Earth Sciences, University of Liverpool, Liverpool L69 3GP, U.K. ABSTRACT The detailed crystallography of natural pyrite framboids has been determined for the rst time using electron backscatter diffraction techniques. The crystallographic ordering of microcrystals correlates positively with morphological ordering; the crystallographic orientations are random in morphologically disordered framboids and are almost ordered in morphologically ordered framboids. Morphologically ordered framboids involve two types of systematic misorientations across the microcrystal boundar- ies: low-angle (ca. <20) and high-angle (ca. 7090) misorientations. The low-angle misorientation probably reects slight physical misalignment of microcrystals in the packing structure, whereas the high-angle misorientation is considered to result from the dichotomy of the pyrite microcrystals having fourfold morphological symmetry but only twofold crystallographic symmetry about <100>. Thus, the crystallographic orientation of microcrystals is not uniform, even in highly ordered framboids. This suggests that the self-organization of microcrystals in pyrite framboids is not crystallographically controlled, for example by sequential replication of existing microcrystals, since this would not result in high lattice misorientation angles between adjacent microcrystals. Presumably, the self-organization process is a consequence of the aggregation of multiple equidimensional and equimorphic microcrys- tals that have nucleated in a xed volume. We suggest that the regular arrangement of microcrystals occurs by the physical rotation (reorientation) of individual microcrystals, driven by the reduction in surface free energy between neighbors.