Experimental Na/K exchange between alkali feldspar and an NaCl–KCl salt melt: chemically induced fracturing and element partitioning

Neusser, Gregor; Abart, Rainer; Fischer, Franz Dieter; Harlov, Daniel E.; Norberg, Nicholas

doi:10.1007/s00410-012-0741-9

Cited by 44 publications

(51 citation statements)

References 22 publications

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“…Gem-quality, near end-member orthoclase crystal fragments (Or90Ab10, 0.35 wt% Fe 2 O 3 ) from Itrongay, Madagascar, were exchanged in a Na 0.8 K 0.2 Cl melt at 850 °C and 0.1 MPa according to the method described by Neusser et al (2012) to produce a homogeneous (Ab60Or40), ion-exchanged alkali feldspar. The natural orthoclase was first crushed in ethanol in an agate mortar, dried, and sieved.…”

Section: Methodsmentioning

confidence: 99%

“…The natural orthoclase was first crushed in ethanol in an agate mortar, dried, and sieved. 200 mg of the crushed orthoclase (grain size fraction 100 < x < 200 µm) was placed into a quartz glass tube together with 20 molar excess of a NaCl-KCl salt mixture (Neusser et al 2012). After sealing the glass tube under near vacuum conditions (0.1 bar) it was put into an 1 atm oven and heated to 850 °C, which is well above the melting temperature of the salt mixture.…”

Section: Methodsmentioning

confidence: 99%

“…The crack pattern (red boxes) results from strain induced by the exchange of K by Na within the inert tetrahedral framework during diffusive ion exchange of the original, natural orthoclase with the salt melt (Na 0.8 K 0.2 Cl) in a sealed quartz glass tube at 850 °C under a partial vacuum (cf. Neusser et al 2012). This substitution reaction is accompanied by a decrease in molar volume (∼5 vol%) that is compensated for by the formation of cracks in the exchanged feldspar.…”

Section: Methodsmentioning

confidence: 99%

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Experimental development of patch perthite from synthetic cryptoperthite: Microstructural evolution and chemical re-equilibration

Norberg

Harlov

Neusser

et al. 2013

American Mineralogist

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In this study, deuteric coarsening of a lamellar cryptoperthite to a patch perthite has been experimentally induced for the first time. An homogeneous alkali feldspar, bulk composition Ab60Or40, was produced by molten salt ion-exchange with a gem quality orthoclase from Madagascar. This was then annealed isothermally for 32 days at 550 °C. This resulted in a coherent lamellar cryptoperthite (periodicity 28 nm) probably by spinodal decomposition and subsequent coarsening. The cryptoperthite was then reacted with 20 wt% H 2 O or 0.1 M HCl at 400 and 500 °C at 200, 400, and 1000 MPa for 96 h and for 192 h at 1500 MPa. At 1000 and 1500 MPa and 500 °C, the cryptoperthitic crystal fragments were partially replaced by a mosaic of albite and sanidine subgrains in the form of a patch perthite. Other than the Gibb's free energy of reaction as derived from the chemical potentials of the reactant and product phases, the second principle mechanism responsible for driving this reaction is interface-coupled dissolution-reprecipitation driven by a reduction in the coherency strain energy stored within the initial cryptoperthite. The main indicator of this is a change in the microtexture due to the destruction of the coherent lamellar cryptoperthite and its replacement by an assemblage of incoherent subgrains. Detailed mass balancing, based on XRD and EPMA data, indicate that conversion of cryptoperthite to patch perthite is not isochemical. Experimental formation of patch perthite appears to take place in two steps. Between the original cryptoperthite and the patch perthite, TEM investigation identified a "transition zone," characterized by a significant coarsening of the preexisting lamellar microstructure, without any sign of a loss in lattice coherency. This zone is probably the result of hydrous species diffusing into the cryptoperthite causing the H 2 O-enhanced diffusion of Na and K and hence lamellar coarsening. In the second step, this coarsened microstructure is replaced by the patch perthite. The polycrystalline patch perthite is characterized by a very complex distribution of albite and sanidine sub-grains. Even the larger albite or sanidine patches are polycrystalline. This inevitably results in a three-dimensional network of grain boundaries characterized by high defect densities resulting from the structural misfit between the albite and sanidine sub-grains. An increase in average grain size with distance from the interface indicates that secondary coarsening of the initially, very fine-grained intergrowth occurred. This was driven by the reduction of interface and surface energy similar to the coarsening observed in multiphase ceramics. In addition, the newly formed patch perthite is highly porous despite the fact that the results obtained in this study show that the replacement of cryptoperthite by patch perthite is nearly isovolumetric (∆V ∼-0.6%). The observed reaction-induced porosity results from differences in relative solubilities between the cryptoperthite and the patch perthite. Porosity abundance has bee...

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Experimental development of patch perthite from synthetic cryptoperthite: Microstructural evolution and chemical re-equilibration

Norberg

Harlov

Neusser

et al. 2013

American Mineralogist

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“…Experiments show that tourmaline also forms when pure water is utilised. In this case, Na required for its crystallisation is not necessarily furnished directly by the fluid, but most likely comes from a Na/K exchange process affecting feldspars ( [43] and references therein), where K in the solution is provided by a biotite reaction. As a matter of fact, XRD spectra reveal tourmaline and K feldspar as the main neo-formed phases.…”

Section: Experiments Performed Using Pure Watermentioning

confidence: 99%

Experimental Investigation of Biotite-Rich Schist Reacting with B-Bearing Fluids at Upper Crustal Conditions and Correlated Tourmaline Formation

et al. 2017

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Fluid-rock interaction experiments between a biotite-rich schist (from Mt. Calamita Formation, Elba Island, Italy) and B-bearing aqueous fluids were carried out at 500-600 • C and 100-130 MPa. The experiments have been carried out in order to reproduce the reaction, which would have produced tourmalinisation of the biotite schist, supposedly by circulation of magmatic fluids issued from leucogranitic dykes. The reacting fluids were either NaCl-free or NaCl-bearing (20 wt %) aqueous solutions, with variable concentration of H 3 BO 3 (0.01-3.2 M). The experimental results show that tourmaline (belonging to the alkali group) crystallise under high-temperature and upper crustal conditions (500-600 • C, 100-130 MPa) when H 3 BO 3 concentration in the system is greater than 1.6 M. The composition of tourmaline is either dravitic (Mg-rich) or schorlitic (Fe-rich), depending if an NaCl-bearing or NaCl-free aqueous solution is used. In the first case, a significant amount of Fe released from biotite dissolution remains in the Cl-rich solution resulting from the experiment. By contrast, when pure water is used, Na/K exchange in feldspars makes Na available for tourmaline crystallisation. The high concentration of Fe in the residual fluid has an important metallogenic implication because it indicates that the interaction between the saline B-rich fluid of magmatic derivation and biotite-rich schists, besides producing tourmalinisation, is capable of mobilising significant amounts of Fe. This process could have produced, in part or totally, the Fe deposits located close to the quartz-tourmaline veins and metasomatic bodies of the Mt. Calamita Formation. Moreover, the super-hot reservoir that likely occurs in the deepest part of the Larderello-Travale geothermal field would also be the site of an extensive reaction between the B-rich fluid and biotite-bearing rocks producing tourmaline. Thus, tourmaline occurrence can be a useful guide during deep drilling toward a super-hot reservoir.

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“…Values of 9 and 20 kbar were obtained for a coherent albite precipitate in microcline (Pryer & Robin, ). Moreover, the coherency stresses in alkali feldspar may be even higher when updated elastic constants are used (Brown et al ., ; Neusser et al ., ). These results are not surprising if the volume difference between the orthoclase‐rich feldspar and albite‐rich precipitate (up to 10 vol.%) is taken into account.…”

Section: Examplesmentioning

confidence: 99%

Grain‐scale pressure variations and chemical equilibrium in high‐grade metamorphic rocks

Tajčmanová

Podladchikov

Powell

et al. 2014

Journal Metamorphic Geology

104

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In the classical view of metamorphic microstructures, fast viscous relaxation (and so constant pressure) is assumed, with diffusion being the limiting factor in equilibration. This contribution is focused on the only other possible scenario – fast diffusion and slow viscous relaxation – and brings an alternative interpretation of microstructures typical of high‐grade metamorphic rocks. In contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals, a multi‐anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time‐scales in a polycrystalline material. In such a mechanical context, exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses (>10 kbar) at geological time‐scales. Furthermore, it is argued that the existence of grain‐scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims. Assuming zero net flux across the microstructure, an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning. This new barometric method is applied to plagioclase rims around kyanite in felsic granulite (Bohemian Massif, Czech Republic), yielding a grain‐scale pressure variation of 8 kbar. In this approach, kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade.

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Experimental Na/K exchange between alkali feldspar and an NaCl–KCl salt melt: chemically induced fracturing and element partitioning

Cited by 44 publications

References 22 publications

Experimental development of patch perthite from synthetic cryptoperthite: Microstructural evolution and chemical re-equilibration

Experimental development of patch perthite from synthetic cryptoperthite: Microstructural evolution and chemical re-equilibration

Experimental Investigation of Biotite-Rich Schist Reacting with B-Bearing Fluids at Upper Crustal Conditions and Correlated Tourmaline Formation

Grain‐scale pressure variations and chemical equilibrium in high‐grade metamorphic rocks

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