When molten magmas freeze (crystallize) to form minerals, the minerals grow in a concentric, snow-ball-like fashion.  Because the magma composition and pressure-temperature conditions can change during crystal growth, the composition of a crystal can differ in the center compared to the outside.  Each concentric layer of new crystal records the chemical conditions at the time that layer forms, just as tree rings record changes in the environment during the growth of the tree.

This crystal has a very well-formed shape, which tells us that it crystallized early in the history of the magma, floating around in liquid like snow flakes crystallizing from a cloud.  One of the students in Kutztown University’s mineralogy class wanted to measure the chemical composition at many points across the crystal to see how the composition changed as the magma cooled.  The tiny white points on the colorful “multi” image show the locations of each little pinpoint analysis.  The big square shows where the students analyzed an average composition rather than a tiny pinpoint spot.  The white spectrum graph reports the data from that analysis.  The crystal contains iron (Fe), magnesium (Mg), silicon (Si), and oxygen (O), but little to no aluminum (Al), potassium (K), sodium (Na), or calcium (Ca), indicating it is a pyroxene called clinoenstatite with composition Mg0.63Fe0.37SiO3.

Clinoenstatite phenocryst in glassy basaltic volcanic rock.  Note also the calcium-bearing pyroxene (augite) crystal in the lower left corner. (1 Mb original file)