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

Abstract

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...

Diffusion-controlled and replacement microtextures in alkali feldspars from two pegmatites: Perth, Ontario and Keystone, South Dakota

“…The periodicities, ∼20 μm, are much smaller than the coarse vein perthites in the present study, but Klokken cooled comparatively rapidly and has not been subject to long periods of annealing (see Parsons and Fitz Gerald, 2011). The Klokken patch perthites are strikingly similar to those produced experimentally by Norberg et al (2013).…”

Textural and compositional complexities resulting from coupled dissolution–reprecipitation reactions in geomaterials

“…7b), the volume of reaction-produced porosity is larger than can be explained by the change in molar volume. In the case of patch perthitization (V m ~ -0.6%) porosities of up to 4.75% in natural samples (Parsons and Lee 2009;Walker et al 1995), and 11% in experimental samples (Norberg et al 2013) have been observed. This porosity is related to the increased solubility of cryptoperthite as a result of stored strain energy associated with coherent exsolution.…”

“…We illustrate the influence of the interactions between solid-state and CDR mechanisms on the textural and compositional development by considering two examples: perthitization (Norberg et al 2013), and dealloying of sylvanite (Zhao et al 2013). Cryptoperthite, i.e., strain-controlled intergrowths of plagioclase and K-feldspar (KAlSi 3 O 8 ), have been observed to coarsen by up to three orders of magnitude in the presence of a melt or an aqueous fluid (Norberg et al 2013;Parsons and Lee 2009;Smith 1974;Worden et al 1990).…”

Focused Ion Beam and Advanced Electron Microscopy for Minerals: Insights and Outlook from Bismuth Sulphosalts

“…These range from biomineralisation [17,18], to rock-forming silicates such as feldspars [11,19,20], natural and synthetic garnets (e.g., [21,22], or minerals of secondary origin such as phosphates and phyllosilicates [23][24][25][26]. Rare materials such as carbonado diamonds and meteorites have also been addressed (e.g., [27][28][29][30][31][32]).…”

“…More often, FIB-SEM instrumentation serves in the selection of slices for TEM-foil preparation. Other applications of dual-beam techniques include FIB-Electron Back Scatter Diffraction (high-resolution micro-structural sample characterisation for small grains, such as the correlation of chemical heterogeneity and lattice distortion in uraninite [10]), and 3D-tomographic imaging (e.g., dealing with porosity, inclusion distribution, or phase intergrowths throughout a sample volume [11,12], or FIB-X-ray microtomography for analysis of tiny fluid inclusions [13]). …”