Image credit: James St. John

Identifying Metamorphic Minerals

In this simplified treatment for the classification of the common metamorphic rocks, we will confine our identification of metamorphic minerals to only a few easily recognizable ones.

  • calcite
  • muscovite
  • garnet

In this simplified treatment, the most important minerals for identifying marble are CaCO3 (calcite, aragonite) and (Ca,Mg)CO3 (dolomite). A simple test for these minerals is the acid test, using cold, dilute HCl.

  • a strong fizzing reaction with larger bubbles demonstrates the abundance of calcite
  • a weaker, slower reaction that generates smaller bubbles demonstrates the abundance of dolomite

Mineralogy Video #1: Identifying Calcite

Calcite and the Acid Test.

What's important for a simplified identification of an unknown metamorphic rock sample is the recognition of calcite, CaCO3. As we've learned previously, calcite is a mineral that reacts strongly to dilute, cold, HCl (hydrochloric acid). So a simple acid test will distinguish a calcitic metamorphic rock from others and thus result in the identification of marble.

Watch this short video that demonstrates the HCl acid test on two metamorphic rocks.

Mineralogy Video #2: Mineral Sheen

Sheen (i. e., shiny, like satin) means a highly reflective (mirror-like) appearance caused by, for example, platy minerals such as graphite, chlorite, or muscovite mica (among others). So sheen is a proxy for a limited set of these minerals.

Sheen is contrasted with a dull appearance, in which light is much more scattered off the surface of a rock, and therefore does not appear highly reflective on a plane or curved surface.

Learn to recognize the appearance of sheen from the following examples.

Dull vs Sheen