The nature of potassium feldspar, exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars

Brown, William L.; Parsons, Ian

doi:10.1007/bf01187138

Cited by 41 publications

(61 citation statements)

References 19 publications

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“…Recent TEM work on perthites has shown a complicated interrelationship between turbidity and exsolution microtextures (Worden et al, 1990;Guthrie and Veblen, 1991;Fitz Gerald and Harrison, 1993). The little previous TEM work on Or-rich crypto-and micro-perthites has shown that the regular microstructures are coherent or periodically semicoherent (Aberdam, 1965;Gandais et al, 1974;Brown and Parsons, 1984). As we show below, the extremely varied microstmctures may be used to obtain information on the thermal history of the crystals, and on fluid-feldspar interactions during cooling.…”

Section: Introductionmentioning

confidence: 87%

“…Because albite and orthoclase have similar cell parameters and X-ray work shows that in perthites the two lattices are usually strained, which means that their cell parameters are even closer, we consider that it is reasonable to assume that in a [001] lattice image, (020) fringes for albite and orthoclase at a given defocus would be continuous across a coherent interface. Furthermore, as Brown and Parsons (1984) have demonstrated, defects such as dislocations rapidly damage under the electron beam in the TEM. No such areas of beam damage are evident in Fig.…”

Section: Transmission Electron Microscopymentioning

confidence: 99%

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Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

1995

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Alkali feldspar phenocrysts (bulk composition Or75.oAb24.6Ano.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto-and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ~< 590~ by heterogeneous nucleation of thin albite films which coarsened to > 1 ~tm length. Cryptoperthites developed at < 400~ by homogeneous nucleation of sub-~tm long platelets between films. Platelets are coherent, but the coarser microperthite lameUae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite--orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous Ixm to sub-~tm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lameUae. Albite was selectively dissolved and micropore-rich irregular microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystaUization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at < 400~ the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of cryptoand micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular microcline.

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Section: Introductionmentioning

confidence: 87%

Section: Transmission Electron Microscopymentioning

confidence: 99%

Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

1995

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“…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). Unlike the semicoherent lamellae in an orthoclase matrix imaged by Lee et al (1995, figs 2, 3, 4) and by Fitz Gerald et al (2006, fig.…”

Section: Perth: Crypto-and Micro-perthitic Microclinementioning

confidence: 94%

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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“…Brown and Willaime (1974) imaged dislocations in a ternary cryptoperthite from the type larvikite, and Gandais et al (1974) observed dislocations in a microperthitic moonstone of unknown provenance. Brown and Parsons (1984) found periodic dislocations (actually tiny holes thought to be caused by electron beam damage) in a ternary mesoperthite from a syenogabbro in the Klokken intrusion. They provided a lattice-fringe image (their Fig.…”

Section: Misfit Dislocations On Perthite Lamellaementioning

confidence: 99%

Nanotunnels and pull-aparts: Defects of exsolution lamellae in alkali feldspars

Gerald

Parsons

Cayzer

2006

American Mineralogist

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The nature of potassium feldspar, exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars

Cited by 41 publications

References 19 publications

Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

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

Nanotunnels and pull-aparts: Defects of exsolution lamellae in alkali feldspars

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