Abstract
We used atom probe tomography to complement electron microscopy for the investigation of spinodal decomposition in alkali feldspar. To this end, gem-quality alkali feldspar of intermediate composition with a mole fraction of a_{text {K}}=0.43 of the K end-member was prepared from Madagascar orthoclase by ion-exchange with (NaK)Cl molten salt. During subsequent annealing at 550,^circ hbox {C} and close to ambient pressure the ion-exchanged orthoclase unmixed producing a coherent lamellar intergrowth of Na-rich and K-rich lamellae. The chemical separation was completed, and equilibrium Na–K partitioning between the different lamellae was attained within four days, which was followed by microstructural coarsening. After annealing for 4 days, the wavelength of the lamellar microstructure was approx 17,hbox {nm} and it increased to approx 30,hbox {nm} after annealing for 16 days. The observed equilibrium compositions of the Na-rich and K-rich lamellae are in reasonable agreement with an earlier experimental determination of the coherent solvus. The excess energy associated with compositional gradients at the lamellar interfaces was quantified from the initial wavelength of the lamellar microstructure and the lamellar compositions as obtained from atom probe tomography using the Cahn–Hilliard theory. The capability of atom probe tomography to deliver quantitative chemical compositions at nm resolution opens new perspectives for studying the early stages of exsolution. In particular, it helps to shed light on the phase relations in nm scaled coherent intergrowth.
Highlights
Alkali feldspar is a widespread phase in magmatic, metamorphic, and sedimentary rocks
A perfect match between bulk chemical analysis by electron probe micro analysis and the bulk composition calculated from lamella compositions and modal proportions as obtained from Atom probe tomography (APT) analysis was achieved testifying to the high accuracy that can be attained with APT analysis of nm-scaled lamellar intergrowth
The lamellar compositions of aK = 0.24 and aK = 0.59 are interpreted as binodal points pertaining to the Na-rich and K-rich limbs of the coherent solvus at 550 ◦C and ambient pressure
Summary
Alkali feldspar is a widespread phase in magmatic, metamorphic, and sedimentary rocks. It forms a solid-solution between a Na- ( NaAlSi3O8 ) and K- ( KAlS3O8 ) end-member. The resulting intracrystalline microstructures reflect the conditions and mechanisms of magmatic and metamorphic crystallization and may potentially be used as petrogenetic indicators (Parsons et al 2013, 2015; Parsons 1978; Parsons and Lee 2005, 2009; Fitz Gerald et al 2006; Sanchez-Munoz et al 2016). Exsolved alkali feldspar is common in both magmatic and metamorphic rocks, and the resulting microstructure is referred to as perthite. Once formed, the lamellae coarsen and adapt their compositions to changing P-T conditions until they are gradually frozen in with decreasing temperature
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