Nomarski interference contrast microscopy is useful for the study of zoning in calcic plagioclases. Crystals are sectioned, polished, etched in fluoboric acid and coated with a layer of reflecting substance. Images of the surface formed from reflected light using Nomarski interference contrast reveal its relief. Comparison with images formed with crossed polarized transmitted light and plane polarized transmitted and reflected light and with microprobe analyses indicates that surface relief corresponds to Ab/An compositional structure. Zones as thin as 0.5 μm and with compositional contrast of about 1 percent An can be resolved using the method. Optimum etching occurs on polished surfaces which are inclined more than about 40° to the growth zones and the c and/or a crystallographic axes.

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