Academic journal article The Science Teacher

A POLARIZING VIEW SEDIMENTARY METAMORPHIC IGNEOUS: An Unconventional Use of Microscopes for Teaching High School Geology and Physics

Academic journal article The Science Teacher

A POLARIZING VIEW SEDIMENTARY METAMORPHIC IGNEOUS: An Unconventional Use of Microscopes for Teaching High School Geology and Physics

Article excerpt

For many students, the first--and sometimes only--chance to look through a microscope is in high school biology class, where they observe plant and animal cells up close. Even in college, few students use a microscope for a subject other than biology. Thus, it can be a surprise to learn that microscopes are a primary tool used to understand the chemical and physical properties of rocks (e.g., Gunter 2004, Reinhardt 2004).

Petrography--the use of microscopes to study optical properties of rocks and minerals--is a cornerstone of a geoscience education. Polarized-light microscopes show thin, polished rock slices in a beautiful and informative light. Though such microscopes are expensive, a standard high school microscope can be easily and inexpensively altered so it can identify fascinating features in rocks.

This article describes a lesson plan developed for an AP environmental science (APES) class that can be adapted for any course that addresses the rock cycle. Parts of the lab could also be used in a physics class that addresses light waves.

Concepts covered by the lesson

The concept of the rock cycle is that sedimentary, metamorphic, and igneous rocks can be transformed into one another through fundamental Earth-system processes (Figure 1). This is taught at various grade levels (see Next Generations Science Standards connections box, p. 36) and is required in APES classes, falling under the category of Earth Systems and Resources (see "On the web").

Objectives

By the end of the lesson, students should be able to differentiate igneous (intrusive and extrusive), sedimentary, and metamorphic rocks by

1. recording observations of thin sections of rock samples under the microscope,

2. correctly identifying the rock type being observed, and

3. providing evidence to support their conclusions.

Students should also be able to make inferences about how porosity, permeability, and composition relates to natural resource availability.

Rock slices under a polarized microscope

Earth scientists commonly use thin, polished slices of rocks mounted on glass slides called thin sections. A hand sample is any studied rock that can be held. Thin sections help scientists differentiate among minerals that may look the same in a hand sample or are too small to see with the naked eye. They help quantify porosity and mineral grain size.

Polarizing filters are needed for microscopes to observe some features of minerals that help identify them. Two types of polarizing light that geologists use are plane polarized light (PPL) and cross-polarized light (XPL). PPL can be achieved by placing a polarizing filter (one that only allows light to pass through in one direction) between the microscope's light source and the thin section (Figures 2, p. 35, and 3). In this view, often the most useful for viewing rocks, even the most vibrantly colored minerals can appear colorless, because the sections are too thin to significantly absorb or enhance specific wavelengths of light.

XPL, on the other hand, can be achieved by placing one polarized filter directly above and a second one below the thin section and oriented at 90 degrees from the first one. In this view, colors are often brighter and different than the true color of the mineral. These "interference colors" result from the interference of two light rays produced as polarized light passes through a mineral (Figure 3).

Interference colors depend on the type of mineral and its orientation, so the same mineral may appear as a range of colors throughout the thin section. The orientation of the mineral also affects how much light passes through, so some grains of the same type of mineral will appear darker and some lighter. Nonetheless, different minerals have sufficiently distinct optical properties that identifying and distinguishing them in thin sections is often much easier than in hand samples. …

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