by Brendan

A Jeol 6400 Scanning Electron Microscope, similar to the one I use at ANU. (Image: http://www.technicalsalessolutions.com/Instruments/SEM/jsm-6400f.htm)

As geologists we love to look at our samples but sometimes the features we want to see are too small to be visible using the naked eye or even high-powered optical microscopes, in that case our next point of call is a scanning electron microscope (SEM). We are fortunate at the ANU to have a dedicated unit that operates (and allows us to use) a wide variety of electron microscopes including a number of different SEMs. These machines (we tend to call all large pieces of analytical equipment machines) are used for two purposes, the first being to obtain high resolution, high magnification images of our samples, in my case typically experimental products or natural mineral grains, and the second being to obtain good quality analyses of the major element composition.

A Schematic diagram of a scanning electron microscope (image: http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html)

SEMs are constructed in a similar manner to conventional light microscopes with an electron source at the top (replacing a light source), with the beam of electrons being controlled by a series of lenses (magnets rather than glass) onto the sample were the reflected electrons are then exposed to a detector and then displayed on a monitor (replacing an eyepiece). All of this allows us to produce images that can show not only the shape and structure of different crystal phases but also highlight differences in composition (all in greyscale), which is why I like to use it. A good description of how a scanning electron microscopes operate can be found here.

A section of one of my experiments as seen using an electron microscope at 420 times magnification, with multiple phases visible.

My experimental products (see image at right) consist of a variety of different crystal phases, often along an amorphous (non-crystalline) pool of quenched melt and I generally want to obtain the composition of each phase. Being able to clearly see each phase makes it much easier to obtain a good spread of measurements and thus a much clearer understanding of the processes I am trying understand.

Though they don’t don’t have the big appeal of other machines that feature lasers, acid or ion beams, electron microscopes are by far one of the most commonly used and useful machines available to us.