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Smithsonian National Museum of Natural History
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Department of Mineral Sciences

Izalco Volcano
SEM
 

Overview of the microscope.

An FEI Nova NanoSEM 600 variable pressure ultra high resolution field emission gun scanning electron microscope resides in the Department of Mineral Sciences of the Smithsonian Institution. In addition to secondary, backscattered and scanning transmission electron imaging to the nanometer scale, the instrument is fitted with a ThermoFisher energy dispersive x-ray detector (EDS) and a Gatan cathodoluminescence (CL) detection system. Variable pressure capability allows imaging and analysis of samples that are not conductively coated. The large sample chamber can hold specimens up to 15cm in diameter and several cm thick.

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SEM
 

Inside view of the microscope.

The pole piece of the Nova NanoSEM showing several detectors. The tiny cone at the center of the backscatter detector is a pressure-limiting aperture that allows for the introduction of water vapor into the sample chamber for working at low vacuum on samples that are not conductively coated. The non-magnetic collimator on the end of the EDS probe allows for the use of ultra high resolution mode in which the sample is immersed in a magnetic field.

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SEM
 

Unusual samples being loaded for imaging and analysis. On the left is a tooth and on the right is a metal camel charm from Egypt. Textural and chemical data were collected from both specimens that revealed important information answering specific questions about the objects.

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SEM
 

Imaging

Backscattered electron images collected of a variety of minerals using low vacuum mode on uncoated samples.

SEM
 

Backscattered electron images of a polished section of scoria from Hawai`i. Black areas are vesicles. The large crystal in the image at left is olivine containing tiny chromite grains which appear white. In the image at right are elongate gray crystals of plagioclase and smaller more equant lighter gray pyroxene crystals contained in glass matrix.

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Energy Dispersive X-ray Spectroscopy

When a material is bombarded with electrons, x-rays are emitted from the sample. The EDS detector collects x-ray spectra that allows for rapid element identification and quantification . X-ray spectra can be collected over an area, pixel by pixel, creating a hyperspectral data cube.

SEM
 
Element Oxide weight # of cations
Line percent based on 13 oxygen
Si K 33.96 3.127
Al K 15.45 1.676
Fe K 20.09 1.391
Mg K 0.58 0.08
Ca K 11.47 1.131
La L 5.32 0.181
Ce L 13.13 0.443
Total 100 8.028

An EDS spectrum of the mineral allanite with a standardless quantitative analysis and stoichiometry. The carbon seen in the spectrum is from the conductive coating on the sample.

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Hyperspectral X-ray imaging

SEM
 

Backscattered electron mosaic of a CV3 carbonaceous chondrite called Leoville, taken using the Scanning Electron Microscope in the Department of Mineral Sciences. This image is a mosaic of eight separate images. This is a thin section of a Calcium, Aluminium rich inclusion (CAI), with a small amount of matrix material visible at the right hand side (the lighter grey material). The bright white veins running through the CAI are iron oxide, formed during terrestrial alteration of this meteorite. The rectangle in the upper left is the region shown in the images below.

SEM
 

X-ray maps for magnesium (red), aluminium (blue) and calcium (green). In the lower right is an overlay of all 3 elemental maps that reveals the distribution of the mineral phases in the meteorite. The veins of iron oxide now appear black, since they contain no Mg, Ca or Al.

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Cathodoluminescence detection system.

SEM
 

The Gatan MonoCL spectrometer has a high sensitivity photomultiplier tube (HSPMT) detector and a Xiclone liquid nitrogen-cooled CCD array. A parabolic mirror is inserted between the sample and the pole piece. The electron beam passes through a hole in the mirror which collects the light, called cathodoluminescence or CL, that is emitted from the sample. The HSPMT detector is used for full spectrum (panchromatic) imaging of the CL from a sample ranging from the ultraviolet to the infrared. Separate colored filters can be inserted to collect red, green and blue filtered images for recombination into a color image. The HSPMT is also used for collecting spectral data serially from a moving diffraction grating. The CCD array can collect data from a range of spectral wavelengths simultaneously. The range is dependent upon the position and the density of grooves in the interchangeable gratings of the spectrometer. The CCD array can also be used to collect large area hyperspectral images that contain CL spectra for each pixel of the image. A cold stage can be installed in the microscope sample chamber for collection of low background spectra at near liquid nitrogen temperature.

SEM
 

A panchromatic cathodoluminescence image of the mineral benitoite on the left and zircon on the right. Fine details of crystal growth like this are often only visible in Cathodoluminescence. Images such as these are commonly used for guidance in further analytical work.

SEM
 

A cathodoluminescence spectrum of a diamond at near liquid nitrogen temperature. At room temperature the spectrum appears only as two broad peaks. This spectrum was collected using a CCD detector in less than 10 seconds.