Guided Discovery and Scoring Rubric for Petrographic Analysis of a Thin Section

David Mogk
Dept. of Earth Sciences, Montana State University
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A guided discovery approach is used to "unpack" the methods and observations used by "master" petrographers in the petrographic analysis of a thin section. A series of spread sheets are used to direct students to make appropriate observations to systematically a) identify minerals in thin section, b) describe rock textures to interpret petrogenetic processes and geologic history, and c) apply this information to address questions of geologic significance.

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This activity can be done in numerous courses for geoscience majors including Mineralogy, Igneous and Metamorphic Petrology, Sedimentary Petrology, and Structural Geology.

Skills and concepts that students must have mastered

It is assumed that students have already mastered (or at least been exposed to) the basic functions required to do optical mineralogy: a) all functions of the petrographic microscope are understood (analyzer, accessory plate, Bertrand lens), b) high v. low relief, c) isotropic v. anisotropic minerals, d) interference figures can be obtained and interpreted (uni- or biaxial, optic sign, 2V), e) pleochroism, f) birefringence, g) length characteristics, h) extinction angle.... This exercise will help you practice these skills, but you need to be introduced to them to begin.

How the activity is situated in the course

The series of guided discovery exercises can be applied throughout the geoscience curriculum, initially as demonstrations of methods and eventually as an authentic assessment of students' ability to make appropriate observations and interpretations of geologic history and processes.


Content/concepts goals for this activity

Higher order thinking skills goals for this activity

The continuum of Bloom's taxonomy of cognitive skills can be developed in this set of exercises ranging from a)knowledge of petrologic phenomena, b)ability to make independent observations and interpretations in context, c)apply petrologic relations to address specific questions of geologic interest, d) analyze data and synthesize the evidence in multiple working hypotheses towards a single, coherent and internally consistent explanation, and e) evaluate these observations and interpretations in light of the larger body of geologic understanding.

Other skills goals for this activity

By following the spreadsheets as guides to petrographic analysis of a thin section:
  • A learning strategy is provided that demonstrates how to systematically observe and interpret a thin section;
  • Students should begin to ask appropriate questions of the thin sections, and begin to seek the answers using a) all the techniques available via the petrographic microscope, b) making focused and purposeful observations, and c) making interpretations that are contextual, constrained (by known geologic relations), relevant and meaningful.
  • The scientific method is used to formulate hypotheses about what minerals might be expected, and the tests are provided to confirm or reject the hypotheses; taking a Popperian approach, interpretations can be continually tested against new observations to check for validity;
  • Predictions can be made based on geologic insights and reasoning;
  • Sketching and photomicrographs: by sketching images of the thin sections, students must make critical observations, decide what is important and what to emphasize, and disregard "extraneous" information in the slide. Many students will have access to digital cameras; photomicrographs are also useful, but it's best to annotate the key features .
  • Petrographic study should be purposeful: you should have a clear idea of what questions you are trying to answer and why: a) What is the stable mineral assemblage? b) What alteration minerals are present? c) What accessory minerals are present? How can these be used to interpret e.g. provenance, or to be used for further analysis e.g. geochronology? d) What is the history of the rock based on overprints and cross-cutting relations preserved in the rock fabric?
  • Internal inconsistencies can be revealed as new evidence is brought to bear; Checks on observations and interpretations can be made by considering if the results are a) possible in Nature, b) internally consistent with known relationships, etc.

Description of the activity/assignment

A series of spread sheets have been set up to provide a framework of observations and questions as a "guided discovery" exercise to clearly demonstrate the observations that a master petrographer would make. The observation of a thin section is broken down into a series of manageable tasks: reconnaissance overview of the thin section at low power; followed by creation of a systematic inventory of the rock-forming minerals (stable mineral paragenesis), alteration phases, and accessory minerals; and finally, analysis of the textures of igneous, sedimentary and metamorphic rocks.

Comprehensive lists of a) optical determinations, and b) textural features are provided as "cues" to the student to help focus attention on the full range of observations that could or should be made towards a comprehensive petrographic analysis of the thin section. These sheets are organized to include:
  • Consideration of the geologic context of the sample: What is the geologic setting where the rock was collected? What is the rock type (if known), or at least is it igneous, sedimentary or metamorphic? This type of contextual information will help guide you to interpret what minerals are likely to be present (or excluded) in the sample
  • Mineral Optics (identification of mineral phases in thin section.
    • Observations at low power in plane and cross polarized light.
    • Systematic characterization of the (stable) rock-forming minerals
    • Identification of a) secondary or replacement minerals, and b) important accessory minerals;
  • Description and Interpretation of Rock Textures
    • Igneous rocks
    • Clastic Sedimentary rocks
    • Non-clastic Sedimentary rocks (carbonates)
    • Metamorphic rocks
  • Applications; can these minerals/assemblages/textures be used to determine source area, physical conditions (thermobarometry), geo- or thermochronology, and other useful geologic information?

Initially, use of these spread sheets will appear to be prescriptive. However, given the complexity observed in Nature, no single set of questions can be universally applied to all types of samples. So, the steps and observations represented in these spread sheets provide a general framework--a place to start--and the lists of optical properties and textures are meant to be a reminder to students about the types of observations that should be made. Students can use these spread sheets as a guide to make decisions about what is important and useful for the overall interpretation.

Metacognitive components of the activity

  1. Students derive an awareness of their own learning processes by considering "what" they are doing and "why" they are performing certain operations on the petrographic microscope.
  2. Students monitor their own progress by considering a) what they expect to find based on geologic contexts, b) are their observations and interpretations consistent with what can (or cannot) occur in Nature, and
  3. Adjust their learning strategies to accomodate new lines of evidence towards formulation of internally consistent (if not "correct") observations and interpretations of the thin section.

Metacognitive goals for this activity:

The first encounter with an unknown thin section can be both confusing and overwhelming: Where do I start? What should I look for? How should I proceed? How will I know if I'm doing the right thing, and making the right observations?....

The purpose of this exercise is to "unpack" the steps taken by a master petrographer, to describe "what" observations can be made, and explain "why" these steps should be taken, what the utility or significance of the observations is, and how these observations can be appropriately interpreted (often these observations are done instantaneously in the mind of the petrographer, but in this exercise we try to explicitly outline these steps). With practice and experience these steps will become second nature. The goal of this exercise is to help students master the art of petrography so that they can independently do petrographic analysis of any rock from any context.

Assessing students' metacognition

In the course of teaching petrography in my regular coursework, I find that I continually articulate to students (one at a time) what I am doing (and why), what I am seeing (and they may or may not be seeing the same thing), why certain relationships are to be expected or prohibited in Nature (by considering the larger geologic context). The development of these guided discovery activities is an attempt to clearly articulate to all students the steps that are routinely taken in the petrographic analysis of a thin section. The goal is to more efficiently and effectively get students past the "mechanical" stages of mineral identification and textural descriptions, and help them to begin to develop higher order thinking skills of application, analysis, synthesis, and evaluation.

Determining whether students have met the goals

These spread sheets can readily be used as a scoring rubric for assessment of student learning. For a given thin section, you should readily be able to assign a point value to any observations, measurement, interpretations that you deem to be appropriate and germane. Not all fields will necessarily require an answer--and this will also help you to see if the students are correctly doing the optical procedures; i.e. are they doing these procedures by rote (and not really understanding what they are doing or why--e.g. inappropriate, inaccurate or inconsequential optical data are reported), or are they really making appropriate observations and measurements.

I emphasize the use of sketches and am beginning to use annotated digital photomicrographs in my own instruction of petrography in my classes. This requires students to focus on the most important observations and to ignore extraneous details in the thin section. Again, this really helps to reveal what the students are seeing, and what they think is significant.

By having students explicitly write their observations in this graphical form on these spreadsheets, this should provide a good measure of their understanding of the procedures of petrography, and will also provide a window into what they are seeing and thinking.

More information about assessment tools and techniques.

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