GEOL 345 Lab 2 A Practical Guide to Identifying Igneous Rocks in Hand Samples and Thin Sections (part 2 of an ad hoc series of self-published, slapdash handbooks) Much of the background information you need for this lab is available in Dr. BizimisÕs Lecture 4 slideshow, available on Blackboard.

I’ve 4 different rocks and their thin section. I need your help to write an observation and description of each rock and identify the minerals under the thin section. All needed info is in the docs below.

You don’t need to do part 3 (Ternary Diagram)

One doc has the rocks and thin section pictures

One has The assignment

One has a guide on how to describe the rock; color, texture,…

The Excel is to write a description of the minerals

Requirements:

GEOL 345 Lab 2 A Practical Guide to Identifying Igneous Rocks in Hand Samples and Thin Sections (part 2 of an ad hoc series of self-published, slapdash handbooks) Much of the background information you need for this lab is available in Dr. BizimisÕs Lecture 4 slideshow, available on Blackboard. Refer to it as much as you want! In this guide, I am distilling some of that information, as well as some things I have learned in my several years of experience identifying igneous rocks, into some relatively simple steps that I hope will help you do this successfully. Hand SamplesÉsince sometimes your eyeballs are your only analytical instrument. 1. Start with the grain texture. a. Is the rock phaneritic/coarse-grained, or aphanitic/fine-grained? i. Phaneritic: individual crystals <3cm and can be seen easily with a hand lens; the rock is intrusive ii. Aphanitic: too fine-grained to be distinguished with a hand lens; the rock is extrusive or subvolcanic (shallow intrusive) iii. Pegmatitic: crystals >3cm intrusive. b. Is the rock porphyritic (two distinct grain-size populations)? i. Groundmass or matrix: finer-grained material ii. Phenocryst: notably larger crystals embedded within the groundmass 2. Use color as a proxy for rock composition. This is not a perfect system but is a useful rule-of-thumb. a. Dark grey/black: mafic b. White/light grey/pink: felsic c. Approximately equal proportions of light and dark minerals: intermediate d. Green: ultramafic. i. Ultramafic rocks can also be black, in which case you may (reasonably) say they are mafic. Sometimes you need a microscope to tell the difference. 3. If the crystals are too small to identify (this can be the case in both phaneritic and aphanitic rocks), your best option is to use the texture and color-as-proxy guideline to identify the rock. Ultramafic Mafic Intermediate Felsic Intrusive Dunite, peridotite, pyroxenite Gabbro Diorite Granite Extrusive Komatiite (good luck finding one) Basalt Andesite Rhyolite Table 1: Basic igneous rock classifications

4. If the crystals are large enough to identify, even if you have to use a hand lens, this can help you more accurately determine what your rock is. Refer to the descriptions on the next page, since identifying minerals in rocks is a different beast from identifying large single-mineral hand samples as you did in GEOL 302. However, you may find it helpful to look at the 302 mineral sets. Identifying minerals in rock hand samples These brief descriptions will not apply to 100% of the rocks you see, but they are generally useful. Use a hand lens and look closely! Quartz: glassy, shadowy-looking grey areasÑoften translucent. No cleavage (conchoidal fracture, remember?). Feldspars: dark to light grey, white, or pink. Should show cleavage even if you canÕt see both directions (90¡ cleavage angle). Plagioclase: dark grey to white, with parallel striations visible on cleavage planes. May show blue-green-yellowish reflections at some angles (labradorescence). K-feldspar (microcline, orthoclase): white/grey to pink, sometimes with wavy exsolution lamellae visible. Micas: flakey and reflective; crystals may be slightly bent/curved around or between adjacent crystals. May appear sub-metallic, especially in cross-section. Biotite: black or very dark brown. Muscovite: lighter brown or silvery-grey. Pyroxenes: black or very dark green, frequently blocky/prismatic crystals. 90¡ cleavage in two directions. Amphiboles (typically hornblende): Black, very dark green, or very dark brown prismatic or needle-like crystals. Two cleavage directions (60¡/120¡) Olivine: Light to dark green, glassy-looking crystals. Usually olivine crystals are very small but may appear as larger phenocrysts in some volcanic rocks.

Accurate rock identificationsÑhand samples and thin sections If you are only able to make a basic identification (e.g., Table 1 above), so be it. But try to gather more observations whenever possible. This section applies to rock hand samples in which the mineral grains are large enough to identify, and to thin sections 1. Mineral modal percentages a. Once you have identified the minerals, visually estimate their percentages by comparing to the diagram in Figure 1. 2. Plotting on ternary diagrams a. Generally, start with a QAPF diagram (Figure 2). b. If the rock plots within the gabbro field on QAPF, use one of the gabbro-specific ternary diagrams (Figure 3). These are for rocks that contain no quartz or alkali feldspar (microcline/orthoclase). c. If the rock is ultramafic, use the Ultramafic ternary diagram (Figure 4). This is for rocks that contain essentially no feldspar, hornblende, or quartz. OR 3. Plotting on ternary diagrams a. Look at the endmembers on each of the diagrams in Figures 2-4. b. Determine which diagram would be most appropriate. For example, if your rock contains no quartz but a bunch of plagioclase and clinopyroxene, maybe you donÕt need to bother with the QAPF diagram to begin withÉ 4. Identify the rock by comparing the point you plotted to the labeled ranges on Figures 2-4. REMEMBER! When plotting on a ternary diagram, you need to normalize the composition so that the sum of the endmembers you are plotting is 100%.

Figure 1: Mineral percentages. This diagram was made for use with microscopes but can be helpful when identifying hand samples as well.

Figure 2: QAPF classification diagram for intrusive rocks.

Figure 3: Ternary diagrams for classifying gabbros, depending on whether they contain plagioclase + pyroxenes only (opx and cpx), + pyroxene and olivine, or + pyroxene and hornblende.

Figure 4: Ternary diagram for classifying ultramafic intrusive rocks.

Rock 1: NEWB

Rock2:MGR-2

Rock3: FAR03-2

Rock 4:47E3309

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