Academic journal article Journal of Geoscience Education

A Web Resource for the Study of Alkali Feldspars and Perthitic Textures Using Light Microscopy, Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy

Academic journal article Journal of Geoscience Education

A Web Resource for the Study of Alkali Feldspars and Perthitic Textures Using Light Microscopy, Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy

Article excerpt

ABSTRACT

A Web site for laboratory activities focused on alkali feldspars and the development of perthitic textures is available at http://www.geosci.ipfw.edu/sem/semedx .html. Backscatter and secondary electron images, plane- and cross-polarized light images, energy dispersive X-ray data (including spectra and results files with data expressed as weight and atomic percents) and X-ray diffraction data are available. Care has been taken to match the areas examined by light and electron methods, allowing students to directly compare the data available from each technique. The information is targeted at undergraduate-level laboratories in mineralogy and petrology.

INTRODUCTION

Research in mineralogy and petrology often relies on data from electron microscopy and microchemical methods. It is important that undergraduates gain a rudimentary understanding of these fundamental techniques if they are to read the literature and critically evaluate the information it contains. Unfortunately, the equipment is expensive and often beyond the resources of smaller, undergraduate-oriented departments. Even when available, electron microscopes do not easily lend themselves to use by the large number of students that would normally populate an undergraduate laboratory in mineralogy or petrology. As a consequence, undergraduates rarely gain meaningful experience with electron microscopy and related methods.

A Web resource at http://www.geosci.ipfw.edu/ sem/semedx.html facilitates undergraduate engagement with electron microscopy, energy dispersive X-ray spectroscopy, light microscopy and X-ray diffraction analysis. The site is suitable as an image source for lectures, or as a data source for laboratory exercises. Available information includes backscatter and secondary electron images, plane- and cross-polarized light images, energy dispersive X-ray (EDX) data and X-ray diffraction (XRD) data. The EDX data are presented as the raw spectra, as results files for each of 44 analyzed points (with data expressed as weight and atomic percents) and as a summary file in an Excel database. all images and data are from the identical areas of two analyzed samples, an anorthoclase phenocryst and a perthite, helping students make direct comparisons regarding the information available by light, electron and X-ray methods.

SAMPLES

The analyzed materials are a perthite from Perth, Ontario and an anorthoclase from Mt. Erebus, Antarctica. Both samples were purchased as stock items from Wards Scientific (http://wardsci.com/).

Anorthoclase is an alkali feldspar of composition Or^sub 40^Ab^sub 60^ to Or^sub 10^Ab^sub 90^ that forms part of the high-temperature solid solution between sodium-rich and potassium-rich feldspar end members. Upon cooling, the solid solution becomes incomplete with anorthoclase and the sodium-rich sanidines exsolving into two separate Na- and K-rich feldspar phases (albite and either orthoclase or microcline) resulting in the development of perthitic (or antiperthitic) textures.

The particular anorthoclase used here occurs as phenocrysts in phonolite lavas recovered on the slopes of Mt. Erebus. The phenocrysts are zoned and contain inclusions of ulvospinel, clinopyroxene, apatite, olivine and glass. There have been several published descriptions of the Mt. Erebus anorthoclase phenocrysts (e.g., Kyle, 1977; Mason et al., 1982; Kyle et al., 1992; Caldwell and Kyle, 1994; Dunbar et al., 1994).

The sample of perthite is a characteristically twinned microcline feldspar displaying large exsolution lamellae easily visible in the hand sample and in the petrographie microscope. Standard textbooks routinely provide information on the development of perthitic texture. see Yund (1983) for additional discussion.

METHODS

A scanning electron microscope uses electrons from a filament (typically a heated tungsten wire) accelerated down a column through a voltage potential. …

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