Class 4 Plutonic Rock Identification Exercise & Plutonic Web Sites - Lab

Plutonic  Rock  Identification Exercise (Optional)

Browse Plutonic Rock Websites

1.  Number the rocks using some white enamel paint and a fine-point, permanent marking pen.

2.  Put the number of the specimen you are examining in the left column of the sheet.

3.  Identify the minerals comprising rock and their relative percentages.   In particular note the abundance of quartz, feldspars and ferromagnesium minerals (biotite, amphibole, pyroxene).  Use a hand lens and a mineral test kit to identify small minerals.  Note the shapes and sizes of crystals.  Note if the grains are all about the same size, or if there are two distinct grain sizes apparent. 

4.  Note any magnetism present, overall or specific minerals forming the rock.

5.  Note any effervescence indicating carbonate minerals.

6.  Note the specific gravity of the rock (generally heavy or light ?).

7.  Use the igneous classification below to name the rock. 

**Diorite:   Composition: albite + hornblende + access. min.’s. 

**Gabbro:   Composition: calcic plagioclase + olivine + pyroxene + access. min.’s.

**Granite:   Composition: quartz + potassium feldspar + plagioclase + biotite + access. min.’s.

**Peridotite:  Composition: olivine + pyroxene + access. min.’s (chromite, etc..).

**Pyroxenite:  Composition: pyroxene + olivine +/- amphibole +/- sulfides +/- garnet + access. min.’s.