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I Need Help With An Assignment I need help with an assignment that is due by tomorrow 9/11(sat) no later than 9/12(Sun) Igneous Rocks and Triangle Diagrams

Part I: Igneous Compositions

Part II: Point Counting

Part III: Normalizing Data

An original laboratory exercise by
Eileen Herrstrom
University of Illinois at Urbana-Champaign
herrstro@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 Diagrams

Part 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.jpg

https://commons.wikimedia.org/wiki/File:Basalt_36mw1041.jpg

https://commons.wikimedia.org/wiki/File:Granite_Yosemite_P1160483.jpg

https://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
· High in silica (SiO2 > 65%)
· Contain quartz, alkali and plagioclase feldspars, muscovite, biotite
· Light colored
· Light gray, tan, pink, white
· Intermediate
· Less silica than felsic rocks (SiO2 ~ 55%), less iron and magnesium than mafic rocks
· Contain plagioclase, pyroxene, amphibole
· 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
· Less silica than intermediate rocks (SiO2 ~ 50%), high in magnesium and iron
· Contain pyroxene, plagioclase, olivine
· Dark colored
· Dark gray, black, green

To describe the grain size of an igneous rock, note the following details (Fig. 1.2):
· Medium to coarse
· Grains are visible to the naked eye
· The technical term is phaneritic.

· Fine
· Grains are too small to see without a microscope
· The technical term is aphanitic.
· Porphyritic

· Two different grain sizes
· Usually a few larger grains surrounded by many smaller ones
· 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 specimen (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?

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