Identify Rx

Image credit: Scott Brande


Identification is the action of assigning a name to an object. Identifying an unknown metamorphic rock requires the observation of its features and the matching of those features to named categories of rocks in a classification.

The identification process is illustrated by the diagram below centered on an unknown sample.

  • Observe features of the unknown. (Hint: if you have a physical sample or a printed image or model, place it over the image in the diagram.)
  • Test features of the unknown sample by comparing its features against descriptions in the boxes.
  • Identify the unknown sample by the closest match (most similar comparison) to a named rock.

Observing for Identification

To identify a metamorphic rock by name using the simplified classification, we must be able to recognize particular features of metamorphic rocks.

In this simplified treatment, characteristics will be restricted to those typically observable with the naked eye and with a few simple tools, e.g., dilute, cold, HCl (hydrochloric acid), a glass plate.

More advanced students will need to seek an alternative resource for this topic.

Features for Identification

In the simplified classification above, three features in combination lead to the name of common metamorphic rocks.

  • structure (foliation, lineation)
  • mineral composition (specifically CaCO3, or not)
  • grain size of abundance material (matrix)

Click links below for details on each feature.


Click image or link for text, images and videos.

Foliation is recognized by an alignment of material.

Image credit: Scott Brande

Mineral Composition

Click image or link for text, images and videos.

Calcite mineralogy is recognized by a vigorous reaction to cold, dilute HCl. The acid test distinguishes marble from all other metamorphic rock

Image credit: Scott Brande

Grain Size

Click image or link for text, images and videos.

Grain size is assessed without magnification aids. Particles range from microscopic to boulder-sized blocks.

Image credit: Scott Brande