How do We Determine the Mineral Composition of Igneous Rocks?

rock table top half.jpg

In the last lesson we started to classify igneous rocks based on their observable properties like texture, density, color, and environment of formation.   We were able to do this using the top half of the Scheme for Igneous Rock Identification in the ESRT seen above.  In this lesson we will discuss another method of identifying igneous rocks...by determining their mineral composition.  To do this we will need to use the ESRT Igneous Rock Chart in it's entirety.

minerals.jpg

We are already familiar with the top half of the Igneous Rock Chart, so for now lets focus on the bottom half (shown above).  This area shows us all of the minerals that can be found in igneous rocks.  Notice that the chart is made up of different patterns.  Each pattern represents a different type of mineral.  

 

Now if we look at both of these charts together we can begin to use the entire chart  

igneous rock chart.jpg

The first thing that we can do using the entire Igneous Rock Chart together is find which minerals are present in different types of rocks.  Upon examination it beomes apparent that this chart can be divided into three columns, or families of rock, each with different minerals. 

felsic rocks.jpg

The Granite Family

The granite family consists of all of the rocks on the left side of the chart.  These rocks are considered a family because they have many things in common:

  • Light Color
  • Low Density
  • Felsic composition

THE GRANITE FAMILY ALSO HAS THE SAME MINERALS IN COMMON.  If we trace a line straight down from these rocks into the minerals part of the chart we see that they all contain:

  • Potassium Feldspar
  • Quartz
  • Plagioclase Feldspar
  • Biotite
  • Amphibole

 

mafic rocks.jpg

The Gabbro Family

The Gabbro Family consists of all of the rocks on the right side of the table.  These rocks all have: 

  • Dark Color
  • High Density
  • Mafic Composition

THE GABBRO FAMILY ROCKS ARE ALL MADE FROM THE SAME MINERALS.  Again, if we simply look directly below them in the mineral part of the table we find that they are made from only the following minerals: 

  • Plagioclase Feldspar
  • Biotite
  • Amphibole
  • Pyroxene
  • Olivene
inbetween rocks.jpg

The Diorite Family

The Diorite Family consists of all of the rocks in the middle column of the table.  These rocks are harder to classify because they have characteristics of both of the other families.  The have average color, density, and composition. 

ALL OF THE ROCKS IN THE DIORITE FAMILY ARE COMPOSED OF THE FOLLOWING MINERALS:

  • Plagioclase Feldspar
  • Biotite
  • Amphibole
  • A tiny amount of Pyroxene

 

Using Mineral Composition to Identify Rocks

If we know the mineral composition and the texture of the rock we can identify it using the table.   

Let's Practice:  

What is the name of the coarse textured igneous rock composed of Quartz, Potassium Feldspar, Biotite, and Amphibole?

  • B/c it has a course texture it must be in the course texture row. 
  • B/c it has contains Quartz, Potassium Feldspar, Biotite, and Amphibole it must be in the Granite Family in the left column. 
  • These characteristics intersect at only one igneous rock - Granite!
practice ID 2.jpg
practice ID 3.jpg

 Let's Practice Another:  Which igneous rock has a fine vesicular texture, and is composed of Pyroxene, Olivne, Amphibole, and Plagioclase Feldspar?

  • Fine vesicular tells us it must be in the row of rocks that have a fine vesicular texture. 
  • The mineral composition tells us it must be in the Gabbro Family in the right hand column.
  • The row and column intersect at one igneous rock - Vesicular Basalt!