Page 168 - Laboratory Manual in Physical Geology

A CTIVIT Y 5.7 Thin Section Analysis and Bowen’s Reaction Series

Name: ______________________________________ Course/Section: ______________________ Date: ___________

A thin section of a rock can be made by grinding one side of it flat, gluing the flat side to a glass slide, and then grinding the rock
so thin that light passes through it. The thin section is then viewed with a polarizing microscope. The view in plane polarized
light is the same as looking at the thin section through a pair of sunglasses. If you place a second pair of sunglasses behind the thin
section and hold it perpendicular to the first pair, then you are viewing the thin section through cross polarized light. These images
of thin sections were made by geologist LeeAnn Srogi, West Chester University of Pennsylvania.

Ca-Na Ca-rich
plag plag edge (gray) center (white)

Thin section 2: This thin section is
plag plag shown in cross polarized light. There
Plane polarized light Cross polarized light are crystals of amphibole (brown to
green color) and plagioclase (white to
Thin section 1: This thin section has a crystal of olivine (ol) that is medium gray gray color). The large plagioclase
in plane polarized light and black in cross polarized light. The olivine crystal once crystal in the center of the image is
had a rectangular outline, but is now surrounded and partly replaced by the “zoned.” It has a calcium rich (sodium
mineral pyroxene (px), which has a different composition and crystal structure. poor) center surrounded by zones that
Pyroxene is pale brown in plane polarized light, but yellow- to orange-brown in also have progressively more and more
cross polarized light. The other white to light gray crystals in this rock are sodium. The zone at its outer edge is
plagioclase (plag). equally rich in both calcium and
sodium. 1 mm
1 mm

A. Based on Bowen’s Reaction Series ( FIGURE 5.3 ), explain as exactly as you can what may have caused the relationship between

olivine and pyroxene observed in thin section 1.

B. Based on Bowen’s Reaction Series ( FIGURE 5.3 ), explain as exactly as you can what may have caused the large plagioclase

crystal in thin section 2 to be zoned as it is.

C. REFLECT & DISCUSS Based on your work above, Olivine (ol) Bowen’s Reaction Series
Decreasing Temperature Amphibole feldspar
circle and label the parts of this Bowen’s Reaction Series
diagram to indicate the exact path of crystallization and Pyroxene (px)
reaction represented in thin section 1, and then 2. Refer Plagioclase
to FIGURE 5.6 as needed. Biotite (plag)

Potassium feldspar (K-spar) + Muscovite + Quartz

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