Revisiting the Bøggild Intergrowth in Iridescent Labradorite Feldspars: Ordering, Kinetics, and Phase Equilibria

Abstract

The enigmatic Bøggild intergrowth in iridescent labradorite crystals was revisited in light of recent work on the incommensurately modulated structures in the intermediated plagioclase. Five igneous samples and one metamorphic labradorite sample with various compositions and lamellar thicknesses were studied in this paper. The lamellar textures were characterized with conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The compositions of individual lamellae were analyzed with high-resolution energy-dispersive X-ray spectroscopy (EDS) mapping and atom probe tomography (APT). The average structure states of the studied samples were also compared with single-crystal X-ray diffraction data (SC-XRD). The Na-rich lamellae have a composition of An44–48, and the Ca-rich lamellae range from An56 to An63. Significant differences between the lamellar compositions of different samples were observed. The compositions of the Bøggild intergrowth do not only depend on the bulk compositions, but also on the thermal history of the host rock. The implications on the subsolidus phase relationships of the plagioclase feldspar solid solution are discussed. The results cannot be explained by a regular symmetrical solvus such as the Bøggild gap, but they support an inclined two-phase region that closes at low temperature.

Thermodynamics of ordering and mixing in plagioclase feldspars: atomistic modelling in favour of Landau theory

“…In this case, the success of near-ideal mixing models would probably be explained by overall correct symmetry of these models and absence of solvus, with the difference (Gibbs mixing energy insufficiently negative and/or solvi ignored) being minor for petrological applications to high-temperature rocks, and where modelling the equilibrium structure and stability gaps of lower-temperature plagioclase such as in the 'e' domain may be perceived as unnecessary. Yet the analysis of Jin et al (2021) sets the temperature above which stability gaps become significant well above 800°C, and it appears that using Landau theory would be much more satisfying to reconcile crystallographic data to phase equilibria. Overcoming apparent incompatibilities between thermodynamic formalisms is required, possibly by decoupling Al-Si order parameters from structural parameters.…”

“…Compositional continuity along the albite-anorthite solid solution is suggested by available data above at least 900°C, and definitive evidence for a solvus above the 'e 1 ' field of plagioclase is still missing. A limited miscibility gap has been suggested around the C 1 ⇀ ↽ I 1 transition near 800°C (e.g., McConnell, 1974;Jin et al, 2021) but remains uncertain. In any case, negative Margules parameters in the C 1 domain are implied, as in Carpenter (1992), and smooth variations of∆ G mix around the phase transition.…”

“…Deriving a complete phase diagram for plagioclase has been a challenging task and the version proposed by Carpenter (1994) has not been significantly modified. The low-temperature part remains mysterious as the incommensurately modulated 'e' structures and their thermodynamic stability are poorly understood, although evidence for the low-temperature closure of the Bøggild gap is growing (Jin and Xu, 2017;Jin et al, 2021). The main features of the phase diagram at lithospheric temperatures are briefly outlined below and the reader is referred to Carpenter (1994) for more details.…”

Order–Disorder Diversity of the Solid State by NMR: The Role of Electrical Charges

“…This specimen has been analyzed in an 27 Al{ 29 Si} HMQC NMR experiment at 17.6 T (750 MHz) as shown in Figure 17. In the 27 Al spectrum at 11.7 T, labradorite produces only a single broad resonance centered at ~60 ppm [98]; at 17.6 T, however, a doublet is resolved reflecting the modularity of the structure at ~48 ppm for p 1 -plagioclase ~An 60 and at ~53 ppm for p 2 -plagioclase ~An 40 , a compositional range for the domains that has already been suggested in the literature [99][100][101]. The 29 Si spectra of p 1 -and p 2 -plagioclases have been simulated with six components, whereas the total spectrum can be reproduced by the sum of about 50% of each component (Table 7).…”