Academic journal article The Science Teacher

GROUPING MINERALS BY THEIR FORMULAS; Students Use the Periodic Table to Identify Patterns in Mineral Formulas and Develop Classification Systems

Academic journal article The Science Teacher

GROUPING MINERALS BY THEIR FORMULAS; Students Use the Periodic Table to Identify Patterns in Mineral Formulas and Develop Classification Systems

Article excerpt

Minerals are commonly taught in ways that emphasize mineral identification for its own sake or maybe to help identify rocks. But how do minerals fit in with other science content we teach? I use mineral formulas to help Earth science students wonder about the connection between elements, compounds, mixtures, minerals, and mineral formulas.

This weeklong 5E learning cycle (Bybee 2014) lesson has been used with middle school students being introduced to the periodic table as well as high school students in Earth science, physical geology, and chemistry. The unit addresses the Next Generation Science Standards (NGSS Lead States 2013; see p. 23). Below I focus on the mineral-classification-by-formula portion of the unit.

Engage

To start day 1, the teacher breaks a piece of halite (rock salt, NaCl) under a document camera using a rock hammer, then hands out small pieces to groups of students seated at tables. (Safety note: Teacher and students wear goggles.) Each table group also has a large sample of the same mineral. They compare the small broken pieces to the large unbroken sample.

Then the teacher breaks apart a sample of calcite (CaC[O.sub.3]). (The Iceland spar variety works best so that samples also can be used to explore double refraction [see below]). Samples can be purchased online from science education supply companies. Again, students compare small, broken-off pieces to a large sample.

Then students compare the halite to the calcite. They notice how both minerals broke into small bits that resembled the original sample in its shape and how both are colorless and mostly see-through (translucent to transparent diaphaneity). Students also point out the different angles at which the sides meet, with halite being boxy (~90 degree angles) and calcite sides meeting at angles that are not 90 degrees.

The students or teacher will bring up that each side of each mineral has another side parallel to it that reflects light as a sheet (sometimes a side will have what looks like stairsteps). Here, students may examine ball-and-stick models (plastic, wood, or computer models that display the three-dimensional arrangement of atoms and the bonds between them) and try to identify possible planes along which certain minerals tend to break, also known as the physical property of mineral cleavage.

Bond types play a role in cleavage. Students can compare models of graphite and diamond, both made of carbon even though graphite is soft and slippery and diamond is hard. Students find weaker planes in the graphite model, where Van der Waals bonds loosely connect sheets of carbon atoms that are covalently bonded. The weaker bonds break to allow graphite sheets to slip onto paper when writing with a pencil. Diamond does not have weaker planes in its mineral structure. Other minerals such as halite have ionic bonds.

Through internet research or direct instruction, students learn that, for a substance to be considered a mineral, it needs to occur naturally, be an inorganic solid, and have a definite chemical composition and ordered internal structure. Some minerals have special properties, influenced by their elements, atomic arrangement, and bonds, that are especially helpful in identifying minerals.

For example, a helpful chemical property to investigate is whether a mineral reacts with acid. In this "acid test," a small drop of vinegar or 10% hydrochloric acid is placed on a mineral. If students see bubbles and/or hear bubbling, this is a chemical reaction that is indicative of the mineral calcite. For chemistry students or those who previously have taken chemistry, this reaction can be explored to understand that carbon dioxide gas is released from the calcite:

CaC[O.sub.3] (s) + 2HCI (aq) [right arrow] C[O.sub.2] (g) + [H.sub.2]O (l) + [Ca.sup.2+] (aq) + 2[Cl.sup.-](aq)

Halite will not exhibit a reaction to the acid. (Safety note: Teacher and students wear goggles and gloves. …

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