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Minerals 2017, 7(11), 227; https://doi.org/10.3390/min7110227

Short-Range Stacking Disorder in Mixed-Layer Compounds: A HAADF STEM Study of Bastnäsite-Parisite Intergrowths

1
School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia
2
Adelaide Microscopy, The University of Adelaide, Adelaide, SA 5005, Australia
3
BHP Olympic Dam, Adelaide, SA 5000, Australia
*
Author to whom correspondence should be addressed.
Received: 3 October 2017 / Revised: 11 November 2017 / Accepted: 13 November 2017 / Published: 20 November 2017
(This article belongs to the Special Issue Rare-Earth Carbonates)
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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. View Full-Text
Keywords: HAADF STEM; REE-fluorocarbonates; short-range stacking disorder; mixed-layer compounds HAADF STEM; REE-fluorocarbonates; short-range stacking disorder; mixed-layer compounds
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Ciobanu, C.L.; Kontonikas-Charos, A.; Slattery, A.; Cook, N.J.; Wade, B.P.; Ehrig, K. Short-Range Stacking Disorder in Mixed-Layer Compounds: A HAADF STEM Study of Bastnäsite-Parisite Intergrowths. Minerals 2017, 7, 227.

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