Igneous Rocks and Triangle DiagramsPart I: Igneous Compositions

Part II: Point CountingPart III: Normalizing Data

An original laboratory exercise byEileen Herrstrom

University of Illinois at Urbana-Champaignherrstro@illinois.edu

2018

Context

The audience for this activity is an undergraduate class on introductory physical geology or quantitative reasoning.

Students must know how to recognize minerals in rocks, have basic knowledge about using Microsoft Excel (enter formulas, fill down), and understand how to read triangle diagrams.

This activity is a laboratory exercise that follows lectures on igneous rocks and falls near the beginning of the course.

Access to a computer is an essential part of the exercise.

Goals

The content and concept goals for this activity include visually estimating the mineral compositions of rock samples in hand specimen and thin section.

Higher order thinking skills goals for this activity involve applying the method of point counting to identify mineral compositions of rocks more precisely, expressing point counts as percentages, and comparing estimates of composition derived by the two methods.

Other skills goals for this activity consist of normalizing point counting data using a spreadsheet, plotting the compositions on the correct triangle diagram, and classifying several intrusive igneous rocks based on composition.

Igneous Rocks and Triangle DiagramsPart I: Igneous Compositions

Overview

Geologists describe rocks as they do minerals – by their physical properties – but rocks have only two properties, namely, composition and texture. In this part of the exercise, you determine compositions for three rocks.

Learning Objectives

Estimate mineral compositions of rocks visually using hand specimens Estimate mineral compositions of a rock visually using a thin section Identify felsic igneous rocks using a triangle diagram

How Do Geologists Identify Igneous Rocks?

Geologists identify all rocks by emphasizing various aspects of texture (size, shape, and arrangement of grains) and composition (minerals). For igneous rocks, we begin usually by classifying the rock in two different aspects. One aspect distinguishes grain size as either coarse (mineral crystals are large enough to see with the naked eye) or fine (mineral crystals are too small to see unaided). The other aspect determines the general composition as felsic (light-colored), mafic (dark-colored) or intermediate. The combination of grain size and color is often enough information to name the rock.

Figure 1.1 Chart of igneous compositions and textures.https://commons.wikimedia.org/wiki/File:RhyoliteUSGOV.jpg

https://commons.wikimedia.org/wiki/File:Sarmizegetusa_Regia_2011_-_Large_Andesite_Sanctuary_Close_Up-6.jpghttps://commons.wikimedia.org/wiki/File:Basalt_36mw1041.jpg

https://commons.wikimedia.org/wiki/File:Granite_Yosemite_P1160483.jpghttps://commons.wikimedia.org/wiki/File:Diorite2.tif (Michael C. Rygel via Wikimedia Commons)

https://commons.wikimedia.org/wiki/File:Impala1200.jpg

Igneous Compositions and Textures

Igneous rocks are distinguished and named based on composition and grain size (one of the aspects of texture). To describe the composition of igneous rocks, use the following terms:

Felsic o High in silica (SiO2 > 65%)o Contain quartz, alkali and plagioclase feldspars, muscovite, biotiteo Light colored

Light gray, tan, pink, white Intermediate

o Less silica than felsic rocks (SiO2 ~ 55%), less iron and magnesium than mafic rocks

o Contain plagioclase, pyroxene, amphiboleo Color is usually a mixture of black and white

Coarse grains produce a speckled appearance ("salt and pepper") Fine grains blend to produce a medium gray or grayish green

Mafic

o Less silica than intermediate rocks (SiO2 ~ 50%), high in magnesium and irono Contain pyroxene, plagioclase, olivineo Dark colored

Dark gray, black, green

To describe the grain size of an igneous rock, note the following details (Fig. 1.2):

Medium to coarseo Grains are visible to the naked eyeo The technical term is phaneritic.

Fine o Grains are too small to see without a microscopeo The technical term is aphanitic.

Porphyritico Two different grain sizeso Usually a few larger grains surrounded by many smaller oneso Both large and small grains may be coarse

Or both may be fine Or only the larger grains may be coarse, and smaller ones fine

Figure 1.2. Grain sizes in a hand speciment (left to right): Coarse, fine, coarse and porphyritic, fine and porphyritic.

Identifying Igneous Rocks

All of the igneous rocks you will study in this exercise are coarse in grain size because they are intrusive. When magma solidifies underground, the surrounding rocks insulate the developing igneous intrusion. Cooling occurs slowly over a long period of time, so that crystals have time to grow large.

For specific names of felsic rocks with various mineral compositions refer to Fig. 1. 3. This is a QAP triangle diagram; the top vertex represents the amount of quartz (Q) in a rock, the left vertex shows alkali feldspar (A), and the right vertex corresponds to plagioclase (P). Alkali feldspars include orthoclase, albite, and microcline, while plagioclase is a general name for albite, anorthite, and combinations of the two. The point in the middle of the triangle, representing equal amounts of quartz, orthoclase, and plagioclase, would be classified as granite. Note that mafic rocks show up only in the lower right corner on this diagram.

Figure 1.3. QAP diagram for classification of coarse-grained felsic igneous rocks.

One way to determine mineral percentages in order to use this diagram is to estimate them visually. Figure 1.4 shows the general appearance of a rock with varying amounts of a dark mineral. Open Igneous_Triangles_Samples.pdf that accompanies this exercise. The first page shows an image labeled Sample 1. By comparing the image in the PDF with the percentages in Fig. 1.4, you can determine the approximate amounts of each of the four minerals in the sample. Minerals present in this rock include quartz (gray, glassy-looking), orthoclase (pink), plagioclase (white or tan), and biotite (black).

Figure 1.4. Reference diagrams for estimating percentages visually. Numbers refer to the percentage of the dark mineral (after Terry and Chilingar 1955).

1. Refer to Fig. 1.4 to estimate the mineral percentages in Sample 1 (PDF that accompanies this exercise). The sample is a polished rock slab with a diameter of 7.6 cm (3 inches). Approximately what percentage of quartz (gray) is contained in Sample 1?

2. Approximately what percentage of biotite (black) is contained in Sample 1?

3. Approximately what percentage of orthoclase (alkali feldspar; pink) is contained in Sample 1?

4. Approximately what percentage of plagioclase (white) is contained in Sample 1?

5. Use Fig. 1.3 to name Sample 1.

6. Refer to Fig. 1.4 to estimate the mineral percentages in Sample 2 found in the PDF. The sample is a polished rock slab with a diameter of 7.6 cm (3 inches). Approximately what percentage of hornblende (black) is contained in Sample 2?

7. The other two minerals in Sample 2 are plagioclase and pyroxene. If the rock contains equal amounts of these minerals, what is their percentage?

8. Use Fig. 1.3 to name Sample 2.

Thin Sections

Geologists often use thin sections to determine rock compositions more precisely. A thin section is a very thin slice of a rock (0.03 mm or 1/64 of an inch thick) glued to a glass slide. When a rock is cut this thin, it is translucent, i.e., light shines through it. Various rocks look quite different under a microscope, and many minerals can be identified by their properties as seen with a special type of microscope called a petrographic microscope; these scopes can be adjusted in various ways to affect the appearance of the thin section. Some microscopes are equipped with cameras that can produce images of the microscopic view; such images are known as photomicrographs (Fig. 1.5).

In the photomicrograph in Fig. 1.5, the brightly colored (orange, purple, blue, green) crystals are biotite; the rectangular gray and black striped crystals are plagioclase; the rectangular white crystals are alkali feldspar; and the irregularly shaped grains that range from white through gray to black are quartz.

Figure 1.5. Thin sections on a microscope (left) and a photomicrograph of granite (right).https://commons.wikimedia.org/wiki/File:Thin_sections.JPG

https://commons.wikimedia.org/wiki/File:Thomas_Bresson_-_Granite_vu_au_microscope_(by).jpg

9. Sample 3 in the PDF is a photomicrograph with a width of 0.25 cm (0.1 inch). The minerals in this image are labeled as follows: P = plagioclase; A = alkali feldspar; Q = quartz; B = biotite; and H = hornblende. Refer to Fig. 1.4 to estimate all the mineral percentages in Sample 3. Approximately what percentage of hornblende (orange) is contained in Sample 3?

10. Approximately what percentage of quartz (shades of gray) is contained in Sample 3?

Igneous Rocks and Triangle DiagramsPart II: Point Counting

Overview

In this part of the exercise, you learn the method of point counting to quantify the composition of igneous rocks. Point counting is a much more precise technique than visual estimation for determining composition.

Learning Objectives

Estimate mineral compositions of rocks by point counting on hand specimens Estimate mineral compositions of rocks by point counting on a thin section Identify felsic igneous rocks accurately using the QAP triangle diagram

How Do Geologists Identify Igneous Rocks?

Geologists classify different rocks by emphasizing various aspects of texture (size, shape, and arrangement of grains) and composition (minerals). For igneous rocks, we begin usually by considering mineralogy: What minerals are present? What is the percentage of each mineral? A detailed description of the mineral percentages in a rock might look like the table below. All of these rocks are crystalline; granite and peridotite are coarse grained while basalt is fine grained.

Image Quartz

Orthoclase

Plagioclase

Pyroxene

Olivine

Rock

45% 30% 20% 5% --- Granite

--- 2% 48% 46% 2% Basalt

--- --- 1% 75% 24% Peridotite

Figure 2.1. Closeup views and mineral compositions of three igneous rocks.https://en.wikipedia.org/wiki/Granite

https://commons.wikimedia.org/wiki/File:A_granite_pluton_with_a_soapstone_inclusion.jpghttps://commons.wikimedia.org/wiki/File:Perid_SanCarlos.jpg

Point Counting

A more exact way of measuring the percentages of various minerals is to use the technique of point counting. In this method, you lay a rectangular grid of perpendicular lines over your sample and identify the mineral at each crossing point. Record the number of points for each mineral, calculate percentages, and identify the rock. This method is illustrated in Fig. 2.2.

Mineral

# of Points Calculate %

8 8/20*100 = 40%

5 5/20*100 = 25%

3 3/20*100 = 15%

4 4/20*100 = 20%

Total 20 100%

Figure 2.2. Point counting to estimate percentages.

1. The image for this question in the PDF is the same Sample 1 as used in Part I, but now a grid of lines has been overlain on top of it. Identify the mineral at each intersection point, and count the number of points for each mineral. Record your answers in the second column of the table below. What is the total number of points for all minerals?

Mineral Question #1.Point Counts

Question #2. Percentages

Biotite (black)

Alkali feldspar (pink)

Plagioclase (white)

Quartz (gray)

Total

2. In the third column of the table above, change your point counts to percentages. To calculate, divide each point count by the total number of points, and multiply by 100. What is the approximate counted percentage of quartz in the sample?

3. Compare your percentages in column 3 of the table above to the percentages you estimated visually (questions #1-4 in Part I). Are the percentages mostly the same as initially determined, or are they different when point counting is used?

4. The image for this question in the PDF is the same Sample 2 as used in Part I, but now a grid of lines has been overlain on top of it. How many intersection points mark hornblende (black)? Fill in your answer in column 2 of the table below.

Mineral Question #4.Point Counts

Question #5. Percentages

Hornblende (black)

Plagioclase (gray with stripes)

Pyroxene (greenish gray)

Total

5. The total number of points is the same as you found in question #1. The number of pyroxene points and the number of plagioclase points are approximately equal. Enter these numbers in column 2 of the table above. Calculate percentages in column 3 of the table. What is the approximate percentage of pyroxene in Sample 2?

6. Compare your percentages in column 3 of the table above to the percentages you estimated visually (questions #6-7 in Part I). Are the percentages mostly the same as those determined visually, or are they different when point counting is used?

7. The image for this question in the PDF is the same Sample 3 thin section as used in Part I, but now a grid of lines has been overlain on top of it. The minerals in this image are labeled as follows: P = plagioclase; A = alkali feldspar; Q = quartz; B = biotite;and H = hornblende. Identify the mineral at each intersection point of the grid, and count the number of points for each mineral. Record your answers in the second column of the table below. How many intersection points mark hornblende (orange)?

Mineral Question #7.Point Counts

Question #8. Percentages

Plagioclase

Alkali feldspar

Quartz

Biotite

Hornblende

Total

8. Calculate percentages in column 3 of the table above. What is the approximate percentage of plagioclase in Sample 3?

9. Compare your percentages in column 3 of the table above to the percentages you estimated visually (questions #6-7 in Part I). Are the percentages mostly the same as initially determined, or are they different when point counting is used? Which method is more accurate?

10. Can you plot Sample 3 on Fig. 1.3 and give the name of the rock? Why or why not?

Igneous Rocks and Triangle DiagramsPart III: Normalizing Data

Overview

In this part of the exercise, you address details about plotting rock compositions on triangle diagrams; in particular, you consider how to handle rocks with more than three mineral components.

Learning Objectives

Normalize point counting data in order to plot compositions on a triangle diagram Choose the appropriate triangle diagram to name rocks Use Microsoft Excel to normalize point counting data for several rocks and determine rock

names

Compositional Data and Triangle Diagrams Different rock types emphasize different aspects of composition and texture, and there are so many variations that geologists have coined hundreds of different rock names. It is impossible to represent all rocks, or even all igneous rocks, on the same triangle diagram. In Part I, you used the QAP triangle, which works well for felsic rocks containing quartz, alkali feldspar, and plagioclase feldspar. Other felsic rocks that contain little or no quartz may be plotted on an extension of this triangle. A number of mafic rocks may be plotted on the triangle diagrams below, yet this figure does not account for all mafic rocks. As you can see, naming rocks quickly becomes complicated.

Figure 3.1. Triangle diagrams for igneous rocks containing plagioclase feldspar, olivine, hornblende, and two types of pyroxene (after Streckeisen 1976).http://www.atlas-hornin.sk/en/article/7/classification-principles

How to Normalize

To use the QAP diagram for felsic igneous rocks, you need data for only quartz, orthoclase, and plagioclase. Yet many rocks contain more than these three minerals, such as Sample 1 that you studied in Part I, which has four minerals.

Before you can accurately plot Sample 1 on the triangle diagram, you must recalculate the percentages without biotite. Because biotite makes up only a small percentage of the rock, ignoring it does not introduce much error. This process is called normalizing.

Below is a table showing how to normalize, taking the approximate percentages for Sample 3 as an example. This example rock contains five minerals. When we drop biotite and hornblende, which are both ≤10%, the other three minerals add up to 83%. Divide the values for quartz, alkali feldspar, and plagioclase by 83 and multiply by 100. The new normalized percentages now add up to 100%, as they must in order to show them properly on a triangle diagram. The rock name for this normalized composition is granodiorite (see Fig. 3.2). Note that the sample plots near the boundary between granite and granodiorite, which is why plotting this composition using non-normalized compositions was difficult in Part I.

Mineral Percentage Normalized %Plagioclase 40 40/83*100 = 48%Alkali feldspar 17 17/83*100 = 20%Quartz 26 26/83*100 = 32%Biotite 10 ---Hornblende 7 ---

Total 100

Open the spreadsheet file for this exercise, which contains point-counting data for several other igneous rocks. Your task is to normalize the point counts and use the appropriate triangle diagram to determine the rock name for each sample.

1. Go to the “Normalizing” worksheet of the spreadsheet for this exercise, and perform the following steps: Enter these point counts for Sample 4 in cells B5:B10 (make sure to put each value in the

correct row): Alkali feldspar = 22, Biotite = 4, Hornblende = 0, Plagioclase = 15, Pyroxene = 0, and Quartz = 34. Cell B11 gives the total point count for all six minerals.

Next, enter the following formula in cell C5: =B5/B$11*100, and fill down to cell C10. To normalize Sample 4 in the worksheet, enter this formula in cell D5: =C5/(C$5+C$8+C$10),

and fill down to cell D10. Cell D11 gives the sum, which should be greater than 100. Cell D11 gives the sum, which should be greater than 100.

Finally, decide which mineral(s) to ignore, and enter “0” in the corresponding cell(s) in column D (i.e., replace the formula from the previous step with the number “0”. Cell D11 should now show a sum of 100±1.

What is the normalized percentage of plagioclase in Sample 4?

2. Refer to Fig. 3.2 (next page) and the normalized percentages of quartz, alkali feldspar, and plagioclase to determine the name for Sample 4.

3. In the “Normalizing” worksheet, perform the following steps: Enter these point counts for Sample 5 in cells E5:E10 (make sure to put each value in the

correct row): Alkali feldspar = 0, Biotite = 2, Hornblende = 31, Plagioclase = 30, Pyroxene = 6, and Quartz = 0. Cell E11 gives the total point count for all six minerals.

Next, enter the following formula in cell F5: =E5/E$11*100, and fill down to cell E10. To normalize Sample 5 in the worksheet, enter this formula in cell G5: =F5/(F$7+F$8+F$9), and

fill down to cell G10. Cell G11 gives the sum, which should be greater than 100. Finally, decide which mineral(s) to ignore, and enter “0” in the corresponding cell(s) in

column G (i.e., replace the formula from the previous step with the number “0”. Cell G11 should now show a sum of 100±1.

What is the normalized percentage of plagioclase in Sample 5?

4. Using the normalized values of plagioclase, pyroxene, and hornblende, where does Sample 5 plot on Fig. 3.2?

Figure 3.2. QAP diagram for classification of igneous rocks containing quartz, alkali feldspar, and plagioclase feldspar.

5. Sample 5 poses a problem when using Fig. 3.2 because the sample contains only one of the minerals on the QAP triangle. Fig. 3.2 is mainly intended for naming felsic igneous rocks. Given the lack of quartz in Sample 5, is this sample felsic, intermediate, or mafic?

6. Use your percentages of plagioclase, pyroxene, and hornblende to plot Sample 5 on Fig. 3.3 (next page). What is the rock name for Sample 5?

Open the “Point Counts” worksheet of the spreadsheet file for this exercise. This worksheet contains point-counting data for several other igneous rocks. Your task is to normalize the point counts and use the appropriate triangle diagram to determine the rock name for each sample. Fig. 3.4 (next page) illustrates a triangle for rocks containing mainly plagioclase, pyroxene, and olivine. Such rocks have less silica and more magnesium and iron than mafic rocks; thus, they are called ultramafic rocks.

7. In the “Point Counts” worksheet, in cell J4, enter the formula: =B4/($B4+$C4+$D4)*100. Now highlight cell J4 and fill down to J9. What is the normalized percentage of quartz in sample 6?

8. Highlight J4:J9, place the cursor over the lower-right corner of J9, and fill right into columns K and L. What is the normalized percentage of plagioclase in sample 7?

Figure 3.3. Triangle diagram for mafic igneous rocks containing plagioclase feldspar, pyroxene, and hornblende.

Figure 3.4. Triangle diagram for ultramafic igneous rocks containing plagioclase, pyroxene, and olivine.

In the following question, plot the calculated percentages from the “Point Counts” worksheet on the appropriate triangle diagram. To decide which triangle diagram to use, look at the point counts and identify the three highest numbers; the minerals corresponding to the three highest point counts should label the vertices of the correct triangle diagram for the sample.

Use Fig. 3.2 if the values of Q, A, and P in columns J, K, and L are all non-zero. Use Fig. 3.3 if:

o At least one of Q, A, or P equals zero in columns J, K, and L. o In this case, highlight cells N4:P4, and fill down to row 9. o Plot the resulting percentages for P, Py (pyroxene), and Hb (hornblende) if all values in

columns N, O, and P are non-zero. Use Fig. 3.4 if:

o At least one of Q, A, or P equals zero in columns J, K, and L. o At least one of P, Py, or Hb equals zero in columns N, O, and P. o If the rock contains a significant amount of olivine (Ol), then highlight cells R4:T4, and

fill down to row 9. Plot the resulting percentages for P, Py, and Ol if all values in columns R, S, and T are non-zero.

9. Determine the rock name for each sample in the “Point Counts” worksheet, and record the names in the table below.

Sample # Rock Name

6

7

8

9

10

11

12