ES 4971-4973 Petrology
Homework 2 with Lab
Last time we discussed Igneous Rocks.
A. Assignment - Igneous Rocks
1. Study Igneous Rock Summary.doc in your lectures folder. Print it out, place a
copy into your notes (it must be printed in landscape format) , and be prepared to
write a similar table for mafic through felsic Igneous rocks.
2. Be prepared to draw Bowen’s Reaction Series from memory.
B. Reading –Igneous Textures
1. Read Chapter 3 of Winter.
C. Rock Identifications
1. A rock hand sample is examined in thin section. The microscope has a grid
reticle. Under low magnification, the minerals are identified where every
horizontal and vertical line cross. The results are Quartz 22, Orthoclase (an Alkali
Feldspar) 55, Plagioclase (not Albite) 48. (Ignore the mafics Biotite and
Hornblende) Normalize, then classify, the rock using the IUGS diagram below.
2. Cover these photographs with the transparent grid provided. In each box, count either
Quartz Q, K-spar K, Plagioclase P, Magnetite M, or don’t know. Multiple ID’s are
allowed in each box if present, just estimate the proportions, e.g. 0.8 Q 0.2 K-spar for
each. Total, then normalize the composition for Q A P, counting the K-spar as “A” for
Alkali. Ignore Magnetite and don’t know. Then determine the composition from the
IUGS chart on the front page.
Estimate the Plagioclase with its brown weathering product Sericite, a mica that will turn
to clay if it loses its water. The Sericite is visible as brown patches in PPL.
Align the A1 box with the upper left of each photo. Do rows A and B.
Obviously, you can do a much better job with a real thin section, since you can rotate the
stage, recognize Quartz from undulose extinction, and see the minerals when they are not
extinct. Still, this exercise is good practice for project work.
Copy this grid onto transparency film
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3. This photo is from a thin section, the view is crossed polars. The feldspars
are mainly potassic consisting of orthoclase and microcline. The granite was
found on the beach near the ferry terminal of North Uist, an island in the Outer
Hebrides, to the west of Scotland.
Most of the field of view consists of potassic feldspars with generally darker
grey interference colours due to their low birefringence. Most of the rest is
slightly higher birefringence Quartz with generally brighter grey and white
colours depending on their orientation with regard to the crossed polars.
A Biotite crystal in the photo, near bottom left, has its interference colors
masked by its natural brown color.
The large crystal in the centre of the field of view is a form of potassic feldspar
known as microcline. Here it shows the characteristic cross hatched or "tartan"
twinning.
TODO. Place your grid over the photo. For each intersection that falls over the
circular photo, identify the mineral . Total the Quartz (Q, Qtz), the Alkali
Feldspars Orthoclase (Or) plus Microcline (M, Mic, Microcli), and Plagioclase
(P, Plag). Normalize your totals into percents of QAP, using Orthoclase +
Microcline for A, and then indentify the rock.