Ferro-tschermakite with polysomatic chain-width disorder identified in silician magnetite from Wirrda Well, South Australia: A HAADF STEM study

Abstract

Silician magnetite within ~1.85 Ga lithologies hosting the ~1.6 Ga Wirrda Well iron oxide copper gold (IOCG) prospect, South Australia, was examined at the nanoscale. The magnetite is oscillatory-zoned with respect to the density and orientation of nanometer-scale inclusions, among which Si-Fe-nanorods and Al-rich amphibole (as much as hundreds of nanometers long and tens of nanometers wide) form swarms along <111> directions in magnetite. The amphibole is identified as ferro-tschermakite (Ftsk) with the crystal-chemical formula: A(K0.06Na0.01)0.07B(Ca1.65Na0.35)2C(Fe2.072+Al1.64Mg1.15Ti0.06Fe0.043+Mn0.04)5T(Si6.48Al1.52)8O22W(OH)2. This contains single and double rows of a triple-chain silicate attributed to clinojimthompsonite (Cjt) as coherently intergrown (010) zippers along the entire length of the grains. High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) imaging and simulation of Ftsk and Cjt on the [001] zone axis provide direct visualization of crystal structures. These are defined by the 7- and 10-atom octahedron strips (B+C sites) and flanked by double- and triple-pairs of Si atoms (T sites). Remarkably, the sites for light cations and/or vacancies are clearly imaged as single and double, darkest, diamond-shaped motifs separating the octahedron strips showing that A cavities known in amphibole are readily depicted in the wider-chain silicate. I-beam models show that nanoscale intergrowths among the two silicates are coherent along zigzag chains of cations at the edges of the octahedron strips, with single and double rows of the triple-chain silicate corresponding to 1 and 1.5 unit cells of Cjt (27 and 41 Å intervals along the b axis). This type of polysomatic chain-width disorder is widely reported in Mg-rich pyriboles but is shown here in an Al-Fe-rich amphibole. The lack of planar defects and/or reaction fronts at mutual contacts between three-chain zippers and host amphibole indicates primary co-crystallization growth, promoted by the formation of the Si-Fe-nanorods. Co-crystallizing plagioclase is also preserved in close vicinity to the amphibole hosted by magnetite (from a few nanometers to micrometers apart). In contrast, the replacement of amphibole by phyllosilicates is recognizable as irregular swells along the (010) zippers and results in extensive chloritization of the amphibole during an overprinting event. Pressures of ~11.5 kbar are estimated using Al-in-hornblende nano-geobarometry and calculated Al content in Ftsk (3.16 apfu). Assuming the amphibole-plagioclase association buffered by host magnetite fulfills the textural equilibration criteria required for application of this barometer, we interpret the Ftsk nanoinclusions in magnetite as preserved evidence for amphibolite facies metamorphism affecting host lithologies at Wirrda Well with subsequent retrograde alteration during the ~1.6 Ga IOCG mineralizing event. Magnetite records petrogenetic processes by accommodating variable ranges of nanomineral inclusions and preserving them over geological time scales. HAADF STEM imaging is ideally suited to the depiction of crystal-structural modularity and also provides insights into the evolution of geological terranes with protracted histories.

Micron- to atomic-scale investigation of rare earth elements in iron oxides

“…Some domains show sets of Si-bearing precipitates along (111) planes within host magnetite [hereafter called (111) Si-Fe rods; Figure 4B]. Whereas the (111) Si-Fe rods have widths corresponding to 2d 111 of magnetite and comparable atom arrangements although with darker appearance due to replacement of Fe atoms by Si (Xu et al, 2014;Ciobanu et al, 2019Ciobanu et al, , 2022b, some are much darker and notably lack the middle arrays of atoms (inset on Figure 4B). Complex relationships between the two Fe-oxides and crystal-structural modification of hematite are observed at the nanoscale (Figure 4).…”

Tin-bearing magnetite with nanoscale Mg-Si defects: Evidence for the early stages of mineralization in a skarn system

“…The structure and composition of magnetite is adept at preserving evidence of inherited protoliths, stages of alteration, and superimposed events, information that is critical for genetic interpretation in terranes with intensively altered lithologies. Expanding on the information obtained from traditional micronscale techniques, an extraordinary level of detail can be accessed using HAADF STEM imaging (Xu et al, 2014;Ciobanu et al, 2019Ciobanu et al, , 2022Huang and Beaudoin, 2021;Verdugo-Ihl et al, 2021), a technique that provides a more direct visualization of atoms in the crystal structures than traditional TEM methods.…”

“…Recent nanoscale studies of magnetite from IOCG and IOA systems have used high angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging to better understand the physical nature of these trace elements. It has been shown that such elements, notably Si, are not always lattice-bound, rather, they can occur as sub-micron scale nanoparticles (Deditius et al, 2018;Ciobanu et al, 2019Ciobanu et al, , 2022Verdugo-Ihl et al, 2020;Huang and Beaudoin, 2021;Cook et al, 2022b).…”

“…The versatility of Z-contrast imaging techniques such as HAADF STEM for elucidating the relative roles of solid solution and inclusions in a host mineral is emphasized by studies of 'silician magnetite' initially defined as containing >1 wt% SiO 2 (Shiga, 1988) and first synthesized at 6-10 GPa and 1,200 °C (Ohtaka et al, 1997). Silician magnetite is typified by presence of Si-Fe nanoprecipitates (γ-Fe 1.5 SiO 4 ) as lamellae along <111*> directions of width 2d 111 in host magnetite in both BIF and IOCG deposits (Xu et al, 2014;Ciobanu et al, 2019Ciobanu et al, , 2022Cook et al, 2022b). Corroborated by the presence of other silicate inclusions, silician magnetite is recognized as forming at a wide range of temperatures, spanning from ≥400 °C in IOCG breccias with alkali-calcic alteration (Ciobanu et al, 2019;Verdugo-Ihl et al, 2020) to ≥600 °C in granites metamorphosed at amphibolite facies (Ciobanu et al, 2022), thus well above the low-temperature depositional environment of BIFs (≤250-300 °C; Huberty et al, 2012).…”