Figure 4. This shows a lens of minerals that contains more calcium than the rest of the rock. Light colored lenses and layers like this probably were limy concretions (concentrations formed by water moving within the rock) and layers in the rock before metamorphism. The carbon dioxide in the original lime was driven off by the heat of metamorphism and the remaining calcium combined with aluminum and silica to form minerals like epidote (green speckles in this photo) and an orange garnet (grossular), not shown here but evident in other layers. Geologists often refer to layers abundant with these minerals as calc-silicate layers.
Last updated on October 6, 2005