The goal of your final lab is to prepare a two-page sample petrographic report.
You will report on the thin section sample present in your kit. Because you don’t have access to a microscope, I prepared a powerpoint for you to use in your report that contain: a full scan of your thin section (in Both PPL and XPL) as well details on 4 to 6 areas in your thin section that contains both pictures and videos.
a full scan of my thin section is here:
https://drive.google.com/drive/folders/1-ZLAc9oSrf…
Your report must contain:
1) Textural description: please describe the grain size (is it homogeneous, bimodal?), the potential (or lack of) mineral orientation, and the overall level of sample alteration (are the minerals usually fresh or are they altered/partly replaced?).
2) Mineralogy:
– Estimate the number of different minerals present in your rocks.
– List all the optical properties that you can identify for each mineral based on the information given on you powerpoint (both in PPL and XPL) Optical properties include: habit, morphology, relief (high, medium, or low and + or -), cleavage number, cleavage angles, extinction angles (for inclined extinction only) , type of extinction (parallel, inclined, symmetrical), color & pleochroism , opaque vs. isotropic vs. anisotropic, uniaxial or biaxial (if anisotropic), interference color, optic sign (+ or -), sign of elongation (+ or -), types of twinning observed.
– Identify each mineral
– Estimate the proportions of each of the minerals as a percentage.
3) Hand-drawings:
Your report must be illustrated with at least three hand-drawings. The hand-drawings can be either focused on the texture of the sample, the overall mineralogy, and/or the identification of a specific mineral.Make sure to include a scale and to use the drawing skills you developed this semester (use color, draw both in PPL and XPL and make sure to draw the diagnostic properties of the minerals: relief [thick vs fine line], cleavage vs fractures [parallel thin lines vs irregular thick line], crystal habit, twinning or zoning,…)
4) Conclusions: Must be written with full sentences.
Write a summary of your observations, your conclusions of the petrography of your sample, and a potential name for the sample:
– igneous vs metamorphic.
For igneous rocks: volcanic vs plutonic; mafic, intermediate or felsic. + any information you might learn from your observations regarding the history of the magma (cooling rate, magma reinjection, ect…).
For metamorphic rocks: metamorphic grade, evidence for deformation.
Describing a rock history requires you to make assumptions based on observations. We did not practice this during the semester. This serves as an introduction to a petrology class.**
Rubric:
Textural description: 20 points
Mineralogy: 90 points
Description of the optical properties: 60 points
Mineral identification: 10 points
Estimate proportions of each mineral: 20 points
Hand-drawings: 60 points
Conclusions: 30 points
Identification of the rock type: 10 points
Identification of the rock name: 5 points*
Magmatic/metamorphic history: 15 points*
Note: I have attached an example of a petrographic report so you can have an idea.
PLEASE DO NOT MAKE BIDS IF YOU ARE NOT AN EXPERT IN GEOLOGY.
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