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Physical Geology Helpful Lab Hints

Exercise 5: Metamorphic Rocks

Rock Identification Hints

Use the Table provided by me at the beginning of the semester (or on this web site) to identify the rocks and their characteristics. The following rocks are present: gneiss, phyllite, slate, marble, quartzite, anthracite, and four (4) schists.

Step 1:  Determine whether or not the rock is foliated.  Foliation is the alignment of minerals in the rock.  Foliation can be subtle; you may have to turn the rock and examine it carefully to see the foliation.  There are four types of foliation:

• Slaty: the rock breaks in layers, and the flat surfaces have a dull luster.
• Phyllitic: the rock breaks in layers as slaty cleavage, but the flat surfaces have a silky luster.
• Schistose: the rock has large crystals of platy minerals (such as the micas, mineral #'s 1 & 2), the platy minerals are aligned.
• Gneissic: the rock is "tiger-striped."

• If the rock is foliated, the type of cleavage will determine the name of the rock.  See the top half of Table 5.2 on page 82.
  • If the rock is schist, you should modify the name of the rock with the names of the minerals you observe.  (For example, a schist where muscovite and staurolite are visible would be a muscovite-staurolite schist.)  Each mineral has unique characteristics listed in Table 5.1 on page 81.
  • If the rock is gneiss, you may modify the name with the name of the intrusive igneous rock of the same composition (For example, a gneiss with a felsic composition would be a granite gneiss.)
• If the rock is not foliated, you will identify it based on the mineral composition.  See the bottom half of Table 5.2 on page 82.  Each mineral has unique characteristics listed in Table 5.1 on page 81.

Step 3:  Determine the metamorphic grade.  Use Figure 5.13 on page 84 and the comments in Table 5.2 on page 82.  You can also use the textbook (Chapter 8).

Back to lab 5 help.

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