Mineral Properties -- Learning through Experience

Students learn the differences between color and luster; fracture and cleavage; crystal shape and cleavage/fracture; and the fundamental properties that we use to distinguish minerals from each other. They also have to make observations, conduct comparisons, decided -- critically -- which properties are important and which aren't, and gain experience in interacting with real samples that don't match textbook perfection. Students also get to interact with each other -- asking questions and giving assistance -- developing team identity. Probably the most important thing students get out of this exercise is the futility of using color as a distinguisher amongst minerals. We later, in lecture, explain the role of ionic substitution in this property.

I use this activity at the very start of my Minerals week for Introductory Geology. Most of these students have already read the Minerals chapter and been quizzed on basic vocabulary. However, I have also used this activity in labs and workshops where the students had absolutely no background in Minerals at all. And I do the activity before any lecture or review. I have never done this activity with more than 40 students, but I imagine it could scale nicely (assuming one had enough samples).

As mentioned previously, I have individual, boxed samples of calcite, quartz, gypsum, halite, and fluorite. All samples are small enough to hold easily in the palm of the hand. I ask students to come up and pick a box and then spread out and start comparing their sample with other people around them.

For calcite, I pick samples of many colors and clarity. But I always make sure that the rhombohedral shape appears at least partially in all samples.
For quartz, I look for samples that show at least a partial crystal, preferably with some conchoidal fracture.
For gypsum, I look for samples of all crystal shapes and clarity. Since hardness it he key here, I don't worry about them always looking the same.
For halite, it's the easiest one -- the cubic form is enough.
For fluorite, I look for samples that clearly show a triangular cleavage face. Again, I look for all colors.

No other materials are necessary. The entire activity usually takes about 15 minutes tops.

During the call-out reporting portion of this activity, I usually stand up in the middle of the groups and guide them a bit. I'm looking for them to report out key aspects of their mineral, and if they don't mention one of them, I might coax it out of them. I also make sure they learn the proper names of these minerals.

Examples:
Quartz -- they often need help with the language of the crystal shape -- but I encourage them to use regular language -- a long crystal with how many sides? What happens at the top? Was color useful? When it breaks, are there flat planes? Can you tell the difference between the way it grows and the way it breaks? Does it reflect light in a similar fashion (even though colors might be different)?
Gypsum -- Color is useful here -- but it's mostly the luster of the surface that catches their eye -- plus all the scratch marks left from those who have used these minerals before. After they announce it can be scratched with a fingernail, I ask everyone else to try to do the same with their own minerals.
Halite -- Color and shape are the best characteristics here. And usually there are a few brave souls willing to taste it.
Calcite -- rhombohedron, which I refer to as a skewed cube. Often they think that they have a cube like Halite, so I have to stress the right angles of Halite, and then they see it. Color unimportant. I mention here that the clear forms display double refraction. Usually one of the samples will show that. I also mention the acid test, but don't let them use it here.
Fluorite -- I always leave this to the last. The luster is very similar amongst all the samples, while the color varies widely. But it's the triangular cleavage faces that bring it home. I also mention the black light.

When all is done, I have them bring their boxes back to the front and put them on a table in their groups, so folks can come up later and review the variations. (Boxes are clear plastic.)

There will always be some students who are challenged by this activity. I often have to hold a few hands (figuratively speaking) and encourage them to check out particular groups. And when it reaches the end and there are only a few stragglers left, I might have to tell them which groups to go to and then tell them in those groups to figure out why he/she belongs.

Students get the point of the activity when they have to call out in their groups. Here's where I get a chance to assess their understanding. Later in my Introductory Geology classes, they have to identify basic rock-forming minerals. I encourage them to pick these same 5 minerals out of their boxes first, and that's a way they and I see how well they got it. And finally, I do test them, eventually, on their ability to use the language of mineral properties to correctly identify various minerals.