Etching zoned plagioclase

These photos were taken using Nomarski lenses, after the slide was etched with fluorboric acid for 75 seconds. Nomarsik lenses uses reflected light instead of refracted light (which has been used to take the pictures ppreviously seen). It splits the light beam so that there will be different travel times of the beams of light depending on the area of the slide that the light hits. This reveals a micro- topography depending on the differentially etched zones. The more calcic zones will have a lower relief than the more sodic zones.The lower the relief the longer time it takes the light to be reflected, and this is how the zoning stands out.

By using this method, we can get a qualitative observation of the history of the magma chamber through chemical variations of the plagioclase crystal. Also any truncations of zoning due to resorbtion indicate these subtle changes.