X-ray analyses of compositions of tiny spots

Composite image of x-ray maps and spectra for granite sample

Multi-element EDS x-ray composition map (colorful) and backscatter electron image (gray) of Wanakena granite sample with x-ray spectra for individual analysis spots (2.7 Mb original image)

Colorful multi-element map of the chemical composition of a granite sample surface

Layered multi-element map of the composition of the surface of a granite sample.  Each color indicates a high concentration of a different element.  (higher-resolution 1 Mb image)

Wanakena granite

The Wanakena granite is an unusual igneous in the northwestern part of the Adirondack Mountains of northern New York because it contains the minerals pyroxene and olivine.

In addition to distinguishing different minerals by their compositions on colorful chemical composition maps, the EDS can measure the precise composition of individual spots on a sample.  This image shows how we use the backscatter image (BSD) to distinguish between light and heavy minerals (dark and bright gray, respectively), and how we see broad trends in composition using chemical maps (colorful “multi” map). 

The spikey white graphs summarize the EDS x-ray data for each of the spots analyzed on the sample (numbered white dots).  The horizontal axis of each graph corresponds to the energy level of x-rays that glow from the sample (correlates with the types of elements present in the sample).  The vertical axis records how much of each type of x-ray is emitted (correlates with concentration).  We can identify each mineral by it’s characteristic chemical composition.

These images show that the iron and titanium minerals formed at high temperature, then separated into magnetite (Fe3O4) and ilmenite (FeTiO3) as the rock cooled (that's how the bands or yellow and brown formed).  The black surrounding these grains is quartz (SiO2).  The big red and green crystals surrounding the iron-titanium minerals are feldspars that were originally homogenous when they crystallized at high temperature, but then separated into sodium-rich albite (green) and potassium-rich K-feldspar (red) as the solid rock cooled. (2.7 Mb original image)