Chemically induced fracturing in alkali feldspar

Scheidl, Katharina S.; Schaeffer, A. K.; Petrishcheva, Elena; Habler, Gerlinde; Fischer, F.D.; Schreuer, J.; Abart, Rainer

doi:10.1007/s00269-013-0617-1

Cited by 26 publications

(30 citation statements)

References 19 publications

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“…exsolution lamellae in alkali feldspar) can be coherently intergrown so that the bonded interfaces of the two phases are each constrained by the partner phase (e.g. Salje et al ., ; Scheidl et al ., ). Because the compressibilities and thermal expansion of the two phases will differ, a change in pressure and temperature would lead to free grains of the free phases undergoing different amounts of strain.…”

Section: Introductionmentioning

confidence: 97%

How large are departures from lithostatic pressure? Constraints from host–inclusion elasticity

Angel

Nimis

Mazzucchelli

et al. 2015

Journal Metamorphic Geology

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Minerals trapped as inclusions within other host minerals will develop non-lithostatic pressures during both prograde and retrograde metamorphism because of the differences between the thermo-elastic properties of the host and inclusion phases. There is only a single possible path in P-T space, the entrapment isomeke, along which no residual pressure would be developed in a host/inclusion system; non-lithostatic pressures are developed in inclusions as a result of the external pressure and temperature deviating from the isomeke that passes through the entrapment conditions. With modern equation of state and elasticity data for minerals now available it is possible to perform precise calculations of the isomekes for mineral pairs. These show that isomeke lines are not straight lines in P-T space at metamorphic conditions. We show that silicate inclusions in silicate hosts tend to have flat isomekes, with small values of dP/dT(isomeke), because of the small range of thermal expansion coefficients of silicate minerals. As a consequence, the general behaviour under decompression is for soft silicate inclusions in stiffer hosts to develop excess pressures, whereas a stiff silicate inclusion in a softer matrix will experience lower pressures than lithostatic pressure. The opposite effects occur for compression after entrapment on the prograde path. The excess pressures in inclusions, including allowance for mutual elastic relaxation of the host and inclusion, are most easily calculated by using the isomeke as a basis. Analysis of the simplest possible model of a host-inclusion system indicates that deviations from lithostatic pressure in excess of 1 GPa can be readily produced in quartz inclusions within garnets in metamorphic rocks. For softer host minerals such as feldspars the pressure deviations are smaller, because of greater elastic relaxation of the host. The maximum pressure deviation from lithostatic pressure in the host phase around the inclusion is one-third of the pressure deviation in the inclusion. Routines for performing these calculations have been added to the EosFit7c software package

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Section: Introductionmentioning

confidence: 97%

How large are departures from lithostatic pressure? Constraints from host–inclusion elasticity

Angel

Nimis

Mazzucchelli

et al. 2015

Journal Metamorphic Geology

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“…Abundant cracks are visible that are aligned in a subparallel manner and run at high angles relative to both the (010) and the (001) cleavage planes. These cracks have been described earlier and are related to the anisotropic contraction of the crystal lattice during a compositional shift towards more Na-rich compositions (Neusser et al 2012;Scheidl et al 2014;Petrishcheva et al 2019).…”

Section: Methodsmentioning

confidence: 52%

Spinodal decomposition in alkali feldspar studied by atom probe tomography

et al. 2020

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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 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 • 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 ≈ 17 nm and it increased to ≈ 30 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.

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“…Scheidl et al. (2013) used single crystals of sanidine from the Eifel (Germany) machined to cuboidal plates with either polished (001)- or polished (010)-surfaces. They applied chemical shifts towards more sodium-rich compositions by similar cation-exchange experiments and obtained systems of regularly spaced parallel cracks emanating from the polished (010)- or (001)-surfaces and extending approximately perpendicular to the crystallographic a -direction.…”

Section: Introductionmentioning

confidence: 99%

“…Although instructive, the study of Scheidl et al. (2013) is limited due to some simplifying assumptions. It is strictly valid only for isotropic material, and any application to monoclinic alkali feldspar introduces a systematic error.…”

Section: Introductionmentioning

confidence: 99%

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Diffusion-controlled crack propagation in alkali feldspar

et al. 2018

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The chemically driven propagation of interacting parallel cracks in monoclinic alkali feldspar was studied experimentally. Single crystals of potassium-rich gem-quality sanidine were shifted towards more sodium-rich compositions by cation exchange with a NaCl–KCl salt melt at a temperature of and close to ambient pressure. Initially, a zone with elevated sodium content formed at the crystal surfaces due to the simultaneous in-diffusion of sodium and out-diffusion of potassium, where the rate of cation exchange was controlled by sodium–potassium interdiffusion within the feldspar. A chemical shift of potassium-rich alkali feldspar towards more sodium-rich compositions produces highly anisotropic contraction of the crystal lattice. This induced a tensile stress state in the sodium-rich surface layer of the crystals, which triggered the formation of a system of nearly equi-spaced parallel cracks oriented approximately perpendicular to the direction of maximum shortening. Crack propagation following their nucleation was driven by cation exchange occurring along the crack flanks and was controlled by the intimate coupling of the diffusion-mediated build-up of a tensile stress state around the crack tips and stress release by successive crack propagation. The critical energy release rate of fracturing was determined as 1.8–2.2 from evaluation of the near-tip J-integral. The mechanism of diffusion-controlled crack propagation is discussed in the context of high-temperature feldspar alteration.

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Chemically induced fracturing in alkali feldspar

Cited by 26 publications

References 19 publications

How large are departures from lithostatic pressure? Constraints from host–inclusion elasticity

How large are departures from lithostatic pressure? Constraints from host–inclusion elasticity

Spinodal decomposition in alkali feldspar studied by atom probe tomography

Diffusion-controlled crack propagation in alkali feldspar

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