Abstract
AbstractMacro- 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 fromorthoclase 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 dissolution–reprecipitation. Microcline in both samplescontains 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 growthtwins. 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 propertiesthat can be extrapolated to feldspars in general.
Highlights
ALKALI feldspars are a common and striking constituent of granitic pegmatites
Coherent cryptoperthitic albite lamellae occur in both microclines and we show that their character and periodicity are consistent with existing experimental work on perthite exsolution kinetics by volume diffusion (Yund, 1974; Yund and Davidson, 1978), extrapolated to the range 400–300°C
Turbidity in alkali feldspars is caused by large numbers of μm-scale micropores that form at the junctions between the incoherent subgrain mosaics of which replacement perthites are composed (Worden et al, 1990; Walker et al, 1995)
Summary
ALKALI feldspars are a common and striking constituent of granitic pegmatites. Perhaps because they often form crystals 100s of mm across they have been little studied at the sub-μm scales accessible by Scanning (SEM) and Transmission Electron Microscopy (TEM). Laves and Soldatos (1963) include a micrograph of a (010) section of a perthite from Perth, (reproduced by Smith, 1974, fig 19-4) which shows both microperthitic μm-scale film lamellae and mm-scale macroperthitic lamellae and lenses in the microcline matrix, with a small angular difference between them This too is similar to the Wards specimen. Sánchez-Muñoz et al (2012) provide LM and TEM micrographs of feldspars from a large number of pegmatites including some beautiful LM images of twinning in microcline, and HRTEM images illustrating the coherency of twins and albite films. Perthites formed by replacement are usually several orders of magnitude coarser than film perthites (Parsons and Brown, 1984) Both types of intergrowth occur in the feldspars described here. Components in the bulk crystal was unambiguously demonstrated in feldspars from the Shap granite (Lee and Parsons, 1997), and we show below that the same conclusion can be reached for Perth and Keystone
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