B. TASK 2 Review: IDENTIFICATION OF IGNEOUS ROCKS

Igneous rocks are classified using three properties: their mineral composition, grain size, and colour.  The igneous rock classification chart posted on Learn shows how these properties interrelate.

The column on the left hand side describes the grain size or texture of the rock.  Phaneritic rocks are composed of visible crystals (generally 1-10 mm in size).  Phaneritic rocks which have very large grains are called pegmatites.  When there are two distinct grain sizes the rock is porphyritic.  The larger crystals are called phenocrysts.  Aphanitic rocks are those fine grained rocks with crystals too small to identify with the naked eye.  These rocks may be porphyritic as well.  Oftentimes they are vesicular.  Vesicles form. during rapid cooling of the lava when gas bubbles are frozen or trapped as the lava cools.  Sometimes the lava is so full of bubbles it appears frothy and the resultant solid rock looks very spongy.  Pumice is a highly vesicular extrusive rock derived from felsic lavas.  The thin vesicle walls combined with tiny air pockets yields a rock which floats on water. Basic lavas tend to produce a highly vesicular denser material which freezes and is termed scoria, which will sink in water.  The glassy rocks have no crystalline component.  They look like glass.

The shaded portions of the upper section of the chart represent the percentage abundance of quartz, feldspars, micas, and ferromagnesian minerals in the igneous rocks.  This is the most important mineralogical information used to classify igneous rocks.  Igneous rocks composed mostly of quartz, potassium feldspar, and plagioclase are called felsic and are light coloured.  Igneous rocks composed mostly of ferromagnesian minerals are called mafic and are dark coloured.  In between the felsic and mafic rocks are the intermediate rocks which contain mixtures of both mafic and felsic minerals.  Rocks composed primarily of ferromagnesian minerals are called ultramafic rocks.  They are dark coloured.

At the very top of the chart is the colour index.  This index is the visually estimated percentage of mafic minerals in a rock.

The procedure for identifying igneous rocks using this chart is as follows:

1. Determine the texture of the rock.

a. If the rock is coarse grained estimate the abundance of quartz, feldspars, micas, and ferromagnesian minerals.

i. Determine the colour index.  This identifies the rock as felsic, intermediate, mafic or ultramafic.  Suppose the rock was light coloured and contains 20% quartz, 40% K feldspar, 15% Na-plagioclase, 10% each of muscovite and biotite, with a minor amount of amphibole.  The shaded areas on the colour chart represent the relative proportions of each mineral within a rock.  This particular rock must be a granite (no other rock on the chart contains muscovite).  The composition of this particular granite lies on a vertical line about midway across “the granite column”.

b. If the rock is essentially fine grained the mineralogical composition will have to be inferred from the colour of the rock.  Felsic aphanitic rocks tend to be pink, white, light grey or buff coloured.  Mafic aphanitic rocks tend to be black, dark grey or dark brown.  Intermediate aphanitic rocks, as one might imagine, are somewhere in between.  The colours of these intermediate rocks may be moderate tones of grey, greenish grey, red, and brown or tan.

2. Use textural terms to name the rock.  A granite with large pink microcline phenocrysts would be called “porphyritic granite”.  A black fine grained rock with a few phenocrysts and some vesicles would be called a vesicular porphyritic basalt.

C. Task 2: How to describe hand samples

When describing a hand sample, make sure to comment on the following six things:

1. The overall colour of the fresh surface of the rock should be described. Note the colour of the weathered surface.

2. Grain size:

a. the rock may be equigranular (equal grain sized) or porphyritic (large phenocrysts in a finer grained groundmass or matrix).

b. Phaneritic rocks may be fine-grained (grain size < 1mm), medium grained (1-5 mm), or coarse grained (grain size >5mm).

c. Aphanitic rocks may be fine-grained to glassy.

3. Mineralogy:

a. For phaneritic and porphyrtitic  rocks, determine the percentage composition of the mafic and felsic minerals in the hand specimen. Identify the major minerals, indicate the relative %  they make up of the rock and describe their size (use dimensions) and shape (hexagonal, rectanglular, etc).

b. Based on this information, record the colour index of the rock based on your percentage of mafic minerals present.

Colour Index:

leucocratic (0-33%)

mesocratic (34-66%)

melanocratic (67-100%)

c. Aphanitic rocks are too fine grained to determine mineralogy and the colour index can’t be used. Simply describe the colour of the rock.

4. Fabric/texture (the shape, spatial and inter-relationships of the minerals):

a. Describe the textures of individual minerals as well as the overall texture of the rock. Igneous minerals may be euhedral, subhedral or anhedral. The rock as a whole may be described as homogenous, porphyritic, flow-banded, vesicular, amygdaloidal, and so on…..there are many, many terms you will learn during this lab to use to describe rocks.

5. Specific gravity:

a.  Learn to judge, and record, the specific gravity of the rock as being high, medium or low.

6.  Finally, give the rock a good petrological name.