Introduction to Petrology

Reviewed by Aley El-Shazly, Professor, Marshall University on 1/5/22

Comprehensiveness rating: 2 see less

This is a work in progress, as the authors have stated explicitly. So far, the modules available cover Optical Mineralogy. Igneous and Metamorphic Petrology are being prepared, which is why I gave this work a 2. As far as optical mineralogy is concerned, the score would be a 4.5/5.

Content Accuracy rating: 4

The authors are to be commended on the extensive work they did finding all the necessary videos and animations that clearly demonstrate and clarify very challenging concepts. In terms of accuracy, my score would again be a 4.5/5. Please see written comments at the bottom of this review for the very few issues that I had (per module).

Relevance/Longevity rating: 4

Very relevant, and although designed primarily for POGIL type instruction, it is an invaluable resource for students in ALL types of Mineralogy/ Petrology classes. Longevity is more difficult to assess, particularly since the textbook relies heavily on resources available on the web (especially youtube). Some of these videos may not be available in a few years!

Clarity rating: 5

See attached comments

Consistency rating: 5

See attached comments

Modularity rating: 5

Excellent!

Organization/Structure/Flow rating: 4

The available part of the textbook follows a standard organizational framework common to most Optical Mineralogy texts. Nevertheless, see attached comments on Section 2.9

Interface rating: 5

No problems.

Grammatical Errors rating: 5

See attached comments.

Cultural Relevance rating: 5

Not applicable.

Comments

Review of “Introduction to Petrology” by Johnson & Liu
Comments by Aley El-Shazly
General Comments:
This textbook is a “work in progress”, and, as described by the authors, is intended to cover Optical Mineralogy and Igneous and Metamorphic Petrology. So far, only the “Optical Mineralogy” component has been completed. Another important feature of this textbook is that it is designed to be used for POGIL or guided learning type of delivery. As such, it is not really a full-fledged textbook like those of Winter, Philpotts & Ague, or Frost & Frost, and should never be compared to them. Nevertheless, this work is an extremely valuable resource for all students, and the authors are to be congratulated for their effort in finding these resources and designing the interactive exercises that go along with the clips and demonstrations. I therefore feel that this will be a very valuable “complementary” resource for all students, including those in a Petrology class with traditional instruction (as opposed to POGIL), as it really emphasizes the practical aspects of lab work. My comments below are therefore mostly “cosmetic” in nature.
1.1 What is Petrology
Definition of “Petrography” should not be limited to using the polarizing microscope. It is simply the descriptive component of Petrology, describing texture and mineralogical composition of igneous and metamorphic rocks in hand specimen or using the microscope.
Definition of Geochemistry should not be limited to “application of chemical analyses…..” but rather the application of principles of chemistry to solve geological problems.
1.3 Why is Petrology Useful?
The authors do a very good job listing the applications/ uses of petrology, but I feel that the skills that transfer to other careers should be more emphasized, with examples of how many petrologists were hired in various industries and government entities. After all, it is the analytical ability of petrologists to solve complex problems on the origin of igneous and metamorphic rocks, and tying such origins to many fields (as illustrated in part in Fig. 1.3.1) that makes them unique.
2.2 Thin Sections
Great effort by the authors! Note that this section is rather unusual, and is only rarely covered in a traditional Mineralogy or Petrology course! Some of the information provided in the video demonstrations is a bit too much.
2.3 Electromagnetic spectrum and Properties of Light
The authors are to be congratulated for compiling these visual resources that explain some rather complex concepts.
Section 2.3: Refractive Index, Polarized light, Birefringence:
1- "If a mineral has a refractive index that is much higher or lower than the surrounding materials, it will stand out relative to the surrounding material and will have thick or distinct" edge or outline.
2- I suggest that you add "shape" to describe the minerals with such terms as prismatic, tabular, granular, ... etc., as that is quite helpful in identifying minerals.
Section 2.7: Properties under cross polarized light
1- Extinction angle is measured against cleavage or against the direction of elongation of the crystal. Please modify the text in the section "Inclined and straight extinction" accordingly. May also wish to emphasize that no extinction angle can be measured for non-elongated (e.g. granular) crystals that lack cleavage.
2- “Other Textures Under Crossed Polarized Light” Use the word “properties” instead of “textures”.
3- I suggest that you add a section on sign of elongation as well. Also, consider discussing undulatory extinction under types of extinction. In general, I prefer not to include twinning, undulatory extinction, and exsolution lamellae in this section, as these are not diagnostic properties, but phenomena related to the conditions under which the mineral formed.
Section 2.8: Interference Figures part 2:
Figure 2.8.10 shows a centered optic axis interference figure, and is used to demonstrate how to obtain and make use of interference figures. Whereas these figures are certainly the easiest to obtain, in my opinion, they are not the best to explain the optics of biaxial minerals. Instead, I would suggest starting this section by introducing the biaxial indicatrix, then going through all types of interference figures for biaxial minerals, starting with the acute bisectrix figure. It is also important to show the orientations of the fast vs. slow ray in the bxa, and how those relate/ help identify , , and , and therefore whether the mineral is positive or negative. It is important to let the student know that bxa is the most useful among figures, although obtaining such a figure is done by trial and error (though targeting grains with low, but not the lowest, interference colors). Next comes the centered optic axis figure, then the obtuse bisectirx, the flash figure, and finally, a random cut which gives a figure with one isogyre. The latter 3 are not that useful, and students should not spend time on such grains, and should focus instead on attempting to obtain a centered optic axis figure and a bxa. It is also important to point out that the objective lenses need to be perfectly centered for interference figures to be meaningful, and then, how students can tell a uniaxial from a biaxial mineral even if the grain provided has a random orientation! Students are more likely to grasp these concepts if they are introduced in that order.
Section 2.9: Atlas of Minerals in Thin Section:
Great resources! Fantastic job. However, the optic orientation diagrams will be more useful if the authors preceded the optical mineralogy sections by a brief introduction to Crystallography. The section that authors have under “Interference Figures part 2” seems out of place. I suggest that the authors begin their work with a module on crystallography, introducing the 7 crystal systems and Miller indices (the bare minimum needed for optical mineralogy).
Section 2.10: Synthesis Exercises:
Again, great job! These resources will be very useful for most instructors, especially when the work is completed. Minor suggestions to the second set of exercises:
1. Olivine vs. Clinopyroxene vs. Epidote
2. Clinopyroxene vs. Orthopyroxene
3. Quartz vs. Plagioclase
4. Orthoclase vs. Microcline vs. Plagioclase
5. Hornblende vs. Augite
6. Biotite vs. Chlorite
7. Biotite vs. Muscovite
8. Chlorite vs. Epidote vs. Serpentine
9. Garnet vs. Spinel (Hercynite)
10. Calcite vs. Dolomite (trick question)
11. Epidote vs. Staurolite
12. Kyanite vs. Andalusite
10.13. Glaucophane vs. Tourmaline vs. Chloritoid
Bushveld Complex and SW VA Case Study:
Again, the authors are to be congratulated for making these resources available. These will be invaluable tools, especially if we are forced to teach online!