Image credit: James St. John
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.
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.
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.
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.