North Seattle Community College's
Igneous Rock Identification Scheme
@2002 -- The information contained in this document
A Systematic ApproachThe identification of igneous rocks can be approached in different ways. Your igneous specimens represent most of the rock types shown on pages 52 through 55 of your textbook. It is critical to understand the charts on pages 53 and 55. The terminology you will be asked on quizzes and the midterm is the most important aspect of the lab (much more so than the proper identifications).
As a more systematic approach to identifying your igneous rocks before stepping through the 20 multiple-choice questions in Mike's Igneous Rock Lab, I have devised the following exercise. Please step through it and use the "Week 4 -- Lab Part 1" submission form in your In Box in order to submit your answers for each of the steps in the exercise below.
Understand Igneous Rock Terminology. In other words, really know the meaning of the compositional and textural terms used to describe igneous rocks: phaneritic, aphanitic, felsic, mafic, porphyritic, pyroclastic, vesicular, glassy, etc.).
Review the Classification Chart and photographs on pages 52, 53 and 55 of your textbook. You have samples of all the igneous rocks in the photographs except for one (we don't have peridotite). Understand where these rocks fit into the classification chart and why! Look at which rocks have the most quartz, which have the most potassium K-feldspar, which have the most plagioclase feldspar, and which have the most ferromagnesian minerals (dark minerals with grey, black and green colors such as amphibole-hornblende, pyroxene and olivine). In our mineral collection, we have quartz, potassium K-feldspar, and hornblende.
Practice and Test your Identification Skills. Richard Harwood has created an interactive website which not only presents excellent photographs of various igneous rock but also lets you practice identifying them and grading your own results. Please thoroughly examine his website before attempting our own lab:
Using Composition Differences
Print out and use the handy-dandy chart at the bottom of this web page. Notice that the chart at the bottom of this web page has essentially three columns according to composition. The mafic categories are on the left, the intermediate compositions fit in the middle, and the felsic samples belong on the right.
Next separate your 9 igneous rocks (samples #13-21, with the red labels) into two groups according to their composition: Mafic (dark) and felsic (light). Do not try to identify minerals at this point but rely on the lightness or darkness of the rocks to make this preliminary separation. The light colors of "felsic" rock include pink and orange (even though these may have dark shades). I grouped 5 rock samples as "mafic" and 4 as "felsic." (Some of these will turn out to be "intermediate" in composition but this distinction will come about later.) Make your best determinations and pick out and place the mafics and the felsics into boxes 1 and 2, respectively, in the chart at the bottom of this page.
Important Note: You may determine that you have more mafic samples and less felsic samples than I have stated, or vice versa. Don't worry, specimens vary a bit from kit to kit. However, the comments below fit with the true identifications of our samples so, if possible, try to match your results with mine as we step through this scheme. Thanks.
Using Texture Differences
Now separate your mafic samples further according to their textures: Glassy, vesicular, phaneritic or aphanitic. You probably have 1 glassy mafic igneous rock, 1 vesicular mafic igneous rock, 1 aphanitic mafic igneous rock, and 2 phaneritic mafic igneous rocks. Place these 5 rocks into the appropriate Boxes 3, 4, 5 and 6.
Question 4: The number (not name!) of the one
Question 5: The number (not name!) of the one mafic sample belonging
Question 6: The numbers (not names!) of the two mafic samples belonging
Separate your felsic samples according to their textures as well: Two of the felsic samples have visible minerals with a texture being either phaneritic or porphyritic (which is a combination of phaneritic and aphanitic textures as explained in your text). Two other felsic samples have textures in which individual minerals can not be distinguished. Use your textbook to determine which ones are vesicular, pyroclastic, or aphanitic. I found 2 felsic igneous rocks with visible crystals (either phaneritic or porphyritic), 1 vesicular felsic rock and 1 aphanitic felsic igneous rock. Place the vesicular and aphanitic rocks into the appropriate Boxes 9 and 11.
Question XX: The number (not name!) of any
Question 8: The number (not name!) of the one
Your last two felsic specimens have visible minerals but different textures. One specimen is phaneritic and the other is porphyritic. It is important to understand what a porphyritic texture is and how it is formed. Distinguish these rocks according to texture and place your phaneritic sample in Box 7 and your porphyritic sample in Box 8.
Naming Our Most Distinctive Samples
Use the photographs and information in your textbook and on Richard Harwood's website to name the rocks found in boxes by themselves. We only have one glassy mafic rock in Box 3, one vesicular mafic rock in Box 4, and one aphanitic mafic rock in Box 5. According to the textbook and website, what are their names? Double check to see if they fit with the appropriate photographs (if available) in our textbook. Place these 3 rocks into the appropriate "rock name" boxes: 3A, 4A and 5A. Write their names into these boxes.
We also have 2 phaneritic mafic rocks in Box 6. We will look into how to distinguish these through identification of their visible minerals in a later step.
Of our 4 felsic igneous rocks, two have visible minerals (the phaneritic and porphyritic samples) and will be discussed below. The other two do not have minerals we can identify. Place your vesicular sample in Box 9A and your aphanitic sample in Box 11A. Box 10A is blank. According to the textbook and website, what are their names? Double check to see if they fit with the appropriate photographs (if available) in our textbook.
Minerals can be identified in the four remaining igneous rocks (the two in the "phaneritic mafic" category, the one in the "phaneritic felsic" category, and the one in the "porphyritic felsic" category): We use the lightness or darkness of igneous rocks to help determine composition but, when minerals are visible, we can be much more precise as to their mineral components. Our two samples with "mafic" compositions might actually be intermediate or mafic or ultramafic in composition depending on their mineral content. The two samples with "felsic" compositions might be actually felsic or perhaps are intermediate in composition. Shades of lightness or darkness in igneous rocks allow good conjectures on composition but, if we can identify actual minerals, we can be more definite on the names of these rocks.
Look at the chart on page 53 of our textbook to find the identities and proportions of various minerals found in igneous rocks. Let's make use of the minerals we identified last week. According to the chart, which ones might we need to identify in our samples? We have samples of the following minerals which are found in the chart: Quartz, potassium feldspar (orthoclase), plagioclase feldspar, biotite, amphibole (hornblende) and olivine. Pull out these samples to compare to the visible minerals we have in our four remaining igneous rocks. For our purposes, pyroxene and amphibole have similar features so our hornblende sample can also represent the pyroxene category in the chart.
Remember these features especially: Quartz is a felsic mineral, light grey, with conchoidal fracture; potassium feldspar (orthoclase) is pink or orange and has cleavage in two directions; plagioclase feldspar looks the same as the pink variety but is dark gray or white; biotite is a dark mineral with basal cleavage; hornblende is an example of a ferromagnesian mineral, dark green or black in color, with cleavage in two directions; and olivine is a green mineral with conchoidal fracture rather than cleavage.
Use your hand lens to try to identify the minerals you can see in these four igneous rocks. Start with the two mafic samples. As best you can, identify the minerals that are dominant in each of these rocks. Describe what you see and compare the mineral content with the proportions shown in the chart. HINT: One of these phaneritic rocks is actually "intermediate" in composition rather than "mafic" whereas the other rock falls in the "mafic" category. Place one rock in Box 6A and the other in Box 6B.
Try to identify the minerals in the two "felsic" rocks (Boxes 7 and 8): The rock with visible quartz and potassium feldspar (orthoclase) truly belongs in the "felsic" category. You should be able to identify a third mineral in this sample as well and then can place it in the appropriate "rock name" box.
The other rock has only one mineral type visible because the other minerals are aphanitic (too small to identify). That one mineral that makes up the large crystals will determine whether this rock falls in the "felsic" or "intermediate" categories. Identify that mineral and place it in the appropriate "rock name" box.
The Next Step....
You have now (hopefully!) identified all 9 specimens.
Proceed through the 20 multiple-choice questions in "Mike's Igneous Rock Lab" (Part 2 of this week's lab homework), using the information you now have already determined here.
THE IGNEOUS ROCK CHART