Abstract
Atomic-scale high angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging and electron diffractions are used to address the complexity of lattice-scale intergrowths of REE-fluorocarbonates from an occurrence adjacent to the Olympic Dam deposit, South Australia. The aims are to define the species present within the intergrowths and also assess the value of the HAADF STEM technique in resolving stacking sequences within mixed-layer compounds. Results provide insights into the definition of species and crystal-structural modularity. Lattice-scale intergrowths account for the compositional range between bastnäsite and parasite, as measured by electron probe microanalysis (at the µm-scale throughout the entire area of the intergrowths). These comprise rhythmic intervals of parisite and bastnäsite, or stacking sequences with gradational changes in the slab stacking between B, BBS and BS types (B—bastnäsite, S—synchysite). An additional occurrence of an unnamed B2S phase [CaCe3(CO3)4F3], up to 11 unit cells in width, is identified among sequences of parisite and bastnäsite within the studied lamellar intergrowths. Both B2S and associated parisite show hexagonal lattices, interpreted as 2H polytypes with c = 28 and 38 Å, respectively. 2H parisite is a new, short hexagonal polytype that can be added to the 14 previously reported polytypes (both hexagonal and rhombohedral) for this mineral. The correlation between satellite reflections and the number of layers along the stacking direction (c*) can be written empirically as: Nsat = [(m × 2) + (n × 4)] − 1 for all BmSn compounds with S ≠ 0. The present study shows intergrowths characterised by short-range stacking disorder and coherent changes in stacking along perpendicular directions. Knowing that the same compositional range can be expressed as long-period stacking compounds in the group, the present intergrowths are interpreted as being related to disequilibrium crystallisation followed by replacement. HAADF STEM imaging is found to be efficient for depiction of stacking sequences and their changes in mixed-layer compounds, particularly those in which heavy atoms, such as rare-earth elements, are essential components.
Highlights
Advanced electron microscopy of foils prepared by focused ion beam-scanning electron microscopy (FIB-SEM) allows atomic-scale insight into patterns of element distribution in minerals, with significance for both petrogenesis and the minerals industry [1]
REE-fluorocarbonates appear concentrated within a range of deposits that are loosely affiliated to the iron-oxide copper gold (IOCG) clan, including the giant
We use two FIB-prepared transmission electron microscopy (TEM) foils obtained from bastnäsite-synchysite group (BSG) intergrowths (Figure 2a–c) which occur as filling of a mm-sized grain of replaced feldspar [13]
Summary
Advanced electron microscopy of foils prepared by focused ion beam-scanning electron microscopy (FIB-SEM) allows atomic-scale insight into patterns of element distribution in minerals, with significance for both petrogenesis and the minerals industry [1]. Ca-poor side of the group (parisite, bastnäsite and their disordered lattice-scale intergrowths) were reported in a preliminary TEM study of samples from mineralised granite in the vicinity of the Olympic Dam deposit [12] In this case, the BSG minerals, intergrown with molybdenite and sericite, form an assemblage interpreted to pseudomorphically replace pre-existing plagioclase within red-stained granite [13]. The BSG minerals, intergrown with molybdenite and sericite, form an assemblage interpreted to pseudomorphically replace pre-existing plagioclase within red-stained granite [13] They are illustrative of the link between early iron-metasomatism and REE-Mo-mineralisation at Olympic Dam since sub-μm-scale Fe-oxides are documented within pits along the parisite-dominant lamellae and within K-feldspar in the same samples [12]. The aims are firstly to define all species present within the intergrowths relative to the compositional range within the BSG minerals, and secondly, to assess the value of the HAADF STEM technique in resolving stacking sequences within long-period mixed layer compounds
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