The Evolution of Twin Patterns in Perthitic K-Feldspar From Granitic Pegmatites

Sánchez-Muñoz, L.; García-Guinea, Javier; Zagorsky, V.Ye.; Juwono, Tjipto; Modreski, Peter J.; Cremades, Ana; Tendeloo, Gustaaf Van; Moura, Odulio J.M. de

doi:10.3749/canmin.50.4.989

Cited by 17 publications

(20 citation statements)

References 28 publications

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“…These pits form in pairs at sites where misfit edge dislocation loops encircling the albite lenses intersect the cleavage surface, and demonstrate that the lamellae are semicoherent. Contrary to the interpretation of Sánchez-Muñoz et al (2012, their fig. 14) dislocations develop during coherent coarsening at T well below the C2/m-C 1 shearing transformation in albite, when twinning first appears (see Brown and Parsons, 1984b).…”

Section: Perth: Crypto-and Micro-perthitic Microclinecontrasting

confidence: 42%

“…This low-T growth habit is a hall-mark of subgrains produced by replacement reactions in feldspars by deuteric fluids in igneous rocks (Worden et al, 1990;Walker et al, 1995;Lee, 2000, 2009;Parsons et al, 2013), during retrogression of high-grade metamorphic rocks (Waldron et al, 1993) and at near-surface T during diagenesis . It is hard to see how 'diamond' albite subgrains in figure 12f of Sánchez-Muñoz et al (2012), which are chemically zoned in the ultraviolet using CL, could have grown by solid-state diffusion. Lee et al (2007), Parsons et al (2008) and have illustrated and discussed the complex, sometimes oscillatory zoning that can be observed in replacive subgrains using CL.…”

Section: Keystone: Cryptoperthitic Microclinementioning

confidence: 99%

“…Some semicoherent lamellae adjacent to fractures have been naturally dissolved, a feature also seen in Perth (Figs 6, 8). Sánchez-Muñoz et al (2012) provide TEM micrographs (their fig. 14) showing semicoherent film lamellae from a pegmatitic feldspar, in one case FIG.…”

Section: Keystone: Cryptoperthitic Microclinementioning

confidence: 99%

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

Lee

Parsons

2015

Mineral. mag.

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Macro-and micro-perthitic microclines from pegmatites from Perth, Ontario (Wards catalogue 46 E 0510) and Keystone, South Dakota (Wards 46 E 5125) have been studied using light and electron microscopy. A sample of the type perthite from Perth, Ontario (Hunterian Museum, Glasgow, M2361) was compared using light microscopy. It differs in bulk composition and microtexture from the Wards sample. The Perth sample from Wards is a mesoperthite, with sub-periodic ∼mm-thick albite veins near (100), with irregular surfaces. The microcline has regular tartan twins and formed from orthoclase by a continuous process. The Keystone sample is a microperthite, with non-periodic albite veins mainly in {110}. Irregular tartan twins, volumes of irregular microcline and subgrains suggest that the microcline formed by dissolutionreprecipitation. Microcline in both samples contains semicoherent cryptoperthitic albite films that formed after the development of tartan twins. The bulk compositions of these intergrowths imply exsolution below ∼400°C. Diffusion parameters imply sustained heating for between 0.11 My at 400°C, 1.5 GPa and 8.4 My at 300°C, 1 GPa. Unrealistic times are required at 200°C. Subsequently, the crystals reacted with a fluid leading to replacive growth of the vein perthites. Unusually, Albite twin composition planes in replacive subgrains have sub-periodic dislocations, formed by coalescence of advancing growth twins. Processes that might lead to periodic, replacive intergrowths are discussed. The Perth and Keystone feldspars have been used for experimental work on dissolution during weathering and on anomalous thermoluminescence fading. Their microtextures make them unsuitable for obtaining properties that can be extrapolated to feldspars in general.

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Section: Perth: Crypto-and Micro-perthitic Microclinecontrasting

confidence: 42%

Section: Keystone: Cryptoperthitic Microclinementioning

confidence: 99%

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

Lee

Parsons

2015

Mineral. mag.

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“…Specimen 9544 has a XRD pattern very close to that of low microcline but shows regular albite-pericline twinning at both the optical and transmission electron microscopy (TEM) scales (Sánchez-Muñoz et al 2012). Specimens 176 and 116 show heterogeneous optical extinction and "irregular twinning" (as described by Bambauer et al 1989) at the TEM scale (Sánchez-Muñoz et al 2006a).…”

Section: Microclinementioning

confidence: 94%

Medium-range order in disordered K-feldspars by multinuclear NMR

Sánchez-Muñoz

Sanz

Sobrados

et al. 2013

American Mineralogist

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The structures of K-rich feldspar, (K>Na)AlSi 3 O 8 , are currently described as "ideal" crystals with periodic average structures from Bragg diffraction maxima obtained by reciprocal-space techniques. Polymorphism is explained by variable substitutional disorder of framework Si and Al cations in tetrahedral T sites, and positional disorder of cavity alkali cations in a single M site. Here, highresolution magic angle spinning multinuclear magnetic resonance spectroscopy, leading to 29 Si, 27 Al, and 23 Na spectra at 9.4 as well as 27 Al, 39 K, and 23 Na spectra at 19.6 T, has been used to investigate the "real" structures along the order-disorder series of K-feldspar crystals. The "ideal" and "real" structure coincides only in the perfectly long-range ordered triclinic end-member of the low-microcline structure. Long-range disordered structures (either with monoclinic or triclinic symmetry by X-ray diffraction) show non-random disorder at the medium-range scale, triclinic-like distortions with four sets of T sites for framework atoms, two sets of M sites for alkali atoms, and Al-O-(K,Na) multi-site correlations by NMR spectroscopy. The K-feldspar structures can be described by a medium-range structure using the number of Al atoms per four-membered rings of tetrahedra, with "…-2-0-2-0-…" chains for microcline and orthoclase where the Al-occupancies t 1 O > t 2 m > t 2 O ≈ t 1 m, and with "…-1-1-1-1-…" chains for valencianite and sanidine, in which t 1 O > t 2 m ≈ t 2 O ≈ t 1 m. Framework cations respect Loewenstein's rule (Al-O-Al avoidance), as well as some additional constraints of charge dispersion involving deficiency of Si atoms in Q 4 (4Si,0Al), (1Si,3Al), and (0Si,4Al) environments, constraints which are particularly strong in valencianite. These "real" structure features cannot be described by "ideal" structures owing to the lack of resolving power of the reciprocal-space techniques.

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“…Other workers, some cited below, have reported comparable features. Each alkali feldspar can potentially record a different history, especially with respect to replacive events, and many other types of microtexture are known, particularly in pegmatites (Smith and Brown 1988;Cerný 1994;Sánchez-Muñoz et al 2012) and granulite facies rocks (Waldron et al 1993;Cayzer 2002;Abart et al 2009b;Tajčmanová et al 2012). Many perthitic intergrowths are described by Smith and Brown (1988, Ch.…”

Section: Introductionmentioning

confidence: 99%

Routine characterization and interpretation of complex alkali feldspar intergrowths

Parsons

Gerald

Lee

2015

American Mineralogist

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Almost all alkali feldspar crystals contain a rich inventory of exsolution, twin, and domain microtextures that form subsequent to crystal growth and provide a record of the thermal history of the crystal and often of its involvement in replacement reactions, sometimes multiple. Microtextures strongly influence the subsequent behavior of feldspars at low temperatures during diagenesis and weathering. They are central to the retention or exchange of trace elements and of radiogenic and stable isotopes. This review is aimed at petrologists and geochemists who wish to use alkali feldspar microtextures to solve geological problems or who need to understand how microtextures influence a particular process. We suggest a systematic approach that employs methods available in most well founded laboratories. The crystallographic relationships of complex feldspar intergrowths were established by the 1970s, mainly using single-crystal X-ray diffraction, but such methods give limited information on the spatial relationships of the different elements of the microtexture, or of the mode and chronology of their formation, which require the use of microscopy. We suggest a combination of techniques with a range of spatial resolution and strongly recommend the use of orientated sections. Sections cut parallel to the perfect (001) and (010) cleavages are the easiest to locate and most informative. Techniques described are light microscopy; scanning electron microscopy using both backscattered and secondary electrons, including the use of surfaces etched in the laboratory; electron-probe microanalysis and analysis by energy-dispersive spectrometry in a scanning electron microscope; transmission electron microscopy. We discuss the use of cathodoluminescence as an auxiliary technique, but do not recommend electron-backscattered diffraction for feldspar work. We review recent publications that provide examples of the need for great care and attention to pre-existing work in microtextural studies, and suggest several topics for future work.

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The Evolution of Twin Patterns in Perthitic K-Feldspar From Granitic Pegmatites

Cited by 17 publications

References 28 publications

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

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

Medium-range order in disordered K-feldspars by multinuclear NMR

Routine characterization and interpretation of complex alkali feldspar intergrowths

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