What materials make a good countertop?

Objectives: Identify minerals in countertops; learn about Moh's Hardness scale.

Identify the major minerals within one or more countertops, and come up with a representative Moh's hardness for each countertop. The examples in this database are comopsed mostly of feldspar and quartz, making them fairly hard and resisant to scratching.

If good countertops are hard, then why do some folks love soapstone counters?

Textures versus mineralogy

Objective: Describe rock textures and hypothesize about their origins during magma cooling.

All of these countertops are made of similar minerals, but they look very different. Students can describe grain size and overall texture of their countertop by sketching the hand sample and labeling minerals. Students can then hypothesize about order of crystallization and how features like rapakivi or exolution lamellae might form.

Poster Project: Jigsaw Exercise

Objectives: Students review identification of minerals under the polarizing microscope and learn how to complete a petrographic description of a sample. Students also learn how to name rocks based on the IUGS classification diagrams. Geochemical classification diagrams can also be introduced. The jigsaw provides an opportunity for peer review.

Lab instructions (.doc)

Sample description worksheet template (.doc)

Grading rubric (.doc)

Poster template (an 11" x 17" .ppt slide)

Point-counting on a countertop

Objective: Learn to calculate modal abundances of
minerals.
Students point-count minerals on the countertop
using deer fencing or any type of mesh as a point grid.

Countertop geochemistry detectives: Where did this rock come from?

Objective: Learn to plot and interpret tectonic discrimination diagrams using geochemical data.

Although we may have some idea of the country or region of origin for many of these countertops (see individual data pages for each countertop), we still don't know the exact location of these rock units. Another way to trace the origin of the countertops is to plot the appropriate whole-rock trace element data on Pearce tectonic discrimination diagrams. These diagrams empirically distinguish between tectonic settings for granites.

Michel-Levy method to determine plagioclase composition

Objective: Learn the procedure and limitations of the Michel-Levy method.

Plagioclase is present in most of the countertops. This database includes electron microprobe analyses in transects across plagioclase crystals.

Students can learn how to perform the optical Michel-Levy method using the polrizing microscope using thin sections of the countertops, and then compare their answers to the electron microprobe data for that sample.

It is likely that students will encounter issues with using this technique (trouble finding correctly oriented grains; zoning within the crystal), which can become a teachable moment about the complexity of doing science on real samples.

Many optical mineralogy texts have descriptions of the Michel-Levy method. One good write-up can be found here:

Nesse, W.D. (1991) Introduction to Optical Mineralogy, 2nd edition. Oxford University Press, New York, 335 p.

Feldspar thermometry

Objective: calculate temperature using feldspar thermometry and electron microprobe analyses of the countertops.

A few of the countertops contain both plagioclase and alkali feldspars. Students can calculate temperature empirically from a phase diagram (such as those found here or here). Spreadsheets for a two-feldspar thermometer and a feldspar thermometer are found as supplementary materials for the Reviews in Mineralogy and Geochemistry volume 69. Zoning and other textural information should be used to evaluate equilibrium.