Igneous_questions-2 (2)
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Arizona State University *
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Course
101
Subject
Geology
Date
Apr 3, 2024
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Pages
5
Uploaded by BarristerRiver13217
Igneous Rocks and Triangle Diagrams (GLG 101 Spring 2024) Group Number: Read the igneous_background.pdf before starting on these questions. Print and turn in this worksheet. Write the names of the group members who participated below: Part I: Igneous Compositions Use Fig. 1.4 in the igneous_background pdf to estimate the mineral percentages in Sample 1 (in samples_igneous_rocks.pdf). The sample is a polished rock slab with a diameter of 7.6 cm (3 inches). 1.
Approximately what percentage of quartz (gray) is contained in Sample 1? Answer: 2.
Approximately what percentage of biotite (black) is contained in Sample 1? Answer: 3.
Approximately what percentage of orthoclase (alkali feldspar; pink) is contained in Sample 1? Answer: 4.
Approximately what percentage of plagioclase (white) is contained in Sample 1? Answer: 5.
Use Fig. 1.3 in igneous_background.pdf to name Sample 1. Answer: Use Fig. 1.4 to estimate the mineral percentages in Sample 2 in samples_igneous_rocks.pdf. The sample is a polished rock slab with a diameter of 7.6 cm (3 inches). 6.
Approximately what percentage of hornblende (black) is contained in Sample 2? Answer: 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? Answer: 8.
Use Fig. 3.2 in igneous_background.pdf to name Sample 2. Answer: Sample 3 in samples_igneous_rocks.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 in igneous_background.pdf to estimate all the mineral percentages in Sample 3. 9.
Approximately what percentage of hornblende (orange) is contained in Sample 3? Answer:
10.
Approximately what percentage of quartz (shades of gray) is contained in Sample 3? Answer: Part II. Point Counting Look at Sample 1 in samples_igneous_rocks.pdf where gridlines have been overlain. Identify the mineral at each of the gridline intersection point
. 1.
Using the Sample 1 with grid for point counting image in samples_igneous_rocks.pdf, fill in the point count for all minerals in the Table below. Then convert point counts to percentages (similar to Figure 2.1 in igneous_background.pdf). Mineral Point Counts Mineral Percentages (%) Biotite (black) Alkali feldspar (pink) Plagioclase (white) Quartz (gray) Total Table 1. Sample 1 point counts and mineral percentages. 2.
From the table directly above, what is the approximate percentage of quartz in the sample? Answer: 3.
Using the Sample 2 with grid for point counting image in samples_igneous_rocks.pdf, fill in the point counts and percentages. Mineral Point Counts Mineral Percentages (%) Hornblende (dark gray/black) Plagioclase (gray with white streaks) Pyroxene (greenish gray) Total Table 2. Sample 2 point counts and mineral percentages. 4.
How many intersection points mark hornblende (black)? Answer: 5.
What is the approximate percentage of pyroxene in Sample 2? Answer:
6.
Fill in the point counts for Sample 3. Identify the mineral at each intersection point of the grid and count the number of points for each mineral. Record your answers in the “point counts” column in the table (next page). Mineral Point Counts Mineral Percentages (%) Plagioclase Alkali feldspar Quartz Biotite Hornblende Total Table 3. Sample 3 point counts and mineral percentages. 7.
How many intersection points mark hornblende (orange)? Answer: 8.
What is the approximate percentage of plagioclase in Sample 3? Answer: 9.
Can you plot Sample 3 on Fig. 1.3 and give the name of the rock? Why or why not? Answer: 10.
For each sample, compare your percentages in Tables 1-3 to the percentages you estimated visually in Part 1. Are the percentages mostly the same as initially determined, or are they different when point counting is used? Which method is more accurate? Answer: Part III. Normalizing Data
Open the google 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. This file has TWO SHEETS you can move between by clicking on the tabs in the bottom left. On the sheet named “Normalizing” do the following: ●
Enter these point counts for Sample 4 in cells B5:B10 (make sure to put each value in the correct row): o
Biotite = 4 o
Hornblende = 0 o
Alkali feldspar = 22 o
Plagioclase = 15 o
Pyroxene = 0 o
Quartz = 34
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●
Cell B11 gives the total point count for all six minerals. ●
Next, enter the following formula in cell C5: =B5/B$11*100, and copy and paste this formula in each row in the percentages column. Or you can hover your mouse over the lower right-hand corner of the cell you entered the formula into and drag down when the plus icon appears. ●
To normalize Sample 4 in the worksheet, decide which three mineral(s) to keep. Enter “0” in the corresponding cell(s) in column D for each mineral you will ignore. Copy over the Point Counts for the remaining minerals you are going to include. ●
Then modify the formula you used to calculate the percentages in column C for the new normalized percentages in column E. 1.
What is the normalized percentage of plagioclase in Sample 4? Answer: 2.
Refer to Figure 1.3 and determine the name of Sample 4. Answer: FIll in the point counts for Sample 5 in column F. Select which three minerals should be used for normalization. ●
Biotite = 2 ●
Hornblende = 31 ●
Alkali feldspar = 0 ●
Plagioclase = 30 ●
Pyroxene = 6 ●
Quartz = 0 3.
What is the normalized percentage of plagioclase in Sample 5? Answer: 4.
Using the normalized values of plagioclase, pyroxene, and hornblende, where does Sample 5 plot on Figure 1.3? Answer: 5.
Given the lack of quartz in Sample 5, is this sample felsic, intermediate, or mafic? Answer: 6.
Based on your answer to question 5, which diagram would be the best fit for this rock sample? Answer: 7.
Based on the figure you chose, what is the rock name for Sample 5? Answer: Now move over to the “Point Counts” sheet. This contains point-counting data for several other igneous rocks. Your task is to normalize the point counts and use the appropriate triangle diagram to determine the rock name for each sample. In the “Point Counts” worksheet, in cell J4, enter the formula: =B4/($B4+$C4+$D4)*100. Now highlight cell J4 and fill down to J10. Highlight J4:J10, place the cursor over the lower-right corner of J10, and fill right into columns K and L. All the QAP columns should now be filled for all samples.
8.
What is the normalized percentage of quartz in Sample 6? Answer: 9.
What is the normalized percentage of plagioclase in Sample 7? Answer: Next fill out the PPyHb columns (NOP) making sure the samples are normalized to those mineral types instead of the QAP. Finally, do the same for PPyOI columns (RST). 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. 10.
Fill the remaining samples in for the Table below. Sample # Rock Name 6 7 8 9 10 11 12 Please consider the following completion checklist when reviewing your work to ensure you’ve completed all tasks: __ point counts and percentages are correct/reasonable __ normalized percentages sum to 100% for each sample __ tables are filled out correctly and completely __ spreadsheets are filled out correctly and completely __ used diagrams to determine correct rock names