This lesson was designed to give hands on experience to Earth Science (NY Regents) students in the formation of Igneous rocks, but done so at environmental conditions safer than melting rocks would require. Through Observation of crystallization in different conditions of temperatures, students can make models of the processes which lead to different crystal size ranges, and the rate at which they form. A microscope for real time viewing of the crystal formation is required (however as you can see in the picture above the difference in size can be seen with the unaided eye) and at my school all of the students have school issued Ipads, so I decided to build a class set of Smartphone Microscopes (shout out to fellow instructables user Yoshinok), because the Biologists in my building hog all of the microscopes to look at their sticky and icky stuff. Using a digital device allows students to record real time video of the phenomenon they observe and not just the result. I would note also that if using an Ipad opposed to a smartphone , you may want to play with the dimensions as it took some finesse to fit an Ipad on the dimensions reported.

Supplies

Phenyl Salicylate (source of our "lava" melt) aka "salol"
Microscope or the aforementioned Smartphone microscope
Microscope slides
- Optional: I photocopied graph paper onto overhead transparency paper and glued it to my slides for crystal size referencing. This is sold from Science suppliers, but if your on a budget my quick and dirty style will save you $40 if you already have blank slides. I used glue sticks to adhere the transparency, and it did not dry clear, but it did not negatively effect the outcome.
Hot Plate
Test tube for the melt
Droppers or Syringes or Pipettes
Beaker for hot water bath
Freezer or Ice (you need to chill one of the microscope slides so that it has a different crystallization rate)
Suite of Igneous Rock Samples
PPE: Goggles, Aprons, Gloves

Step 1: The Hook: Low Confidence Confectioner

In order to introduce the topic and spark up student interest, I tell them I made them a treat over our winter break. Rock Candy! C'mon Earth Science Teachers can't resist gratuitous physical puns! Although I'm not the best cook, and I wasn't sure how to make them, so some blue rock candy got put in the freezer to cool, and some red rock candy was left on the counter for days. So I ask students to brainstorm ideas of why they ended up with different textures, to get them thinking about the time taken to solidify.

Step 2: Prepare Melt

Just before your students come in you should melt the Phenyl Salicylate, in the hot water bath. (this chemical is not super dangerous and the MSDS can be found here, however the more time it spends as a liquid and the more vaporization occurs the more funky phenyl odor there will be, so best to time this right and open the windows in your room). Phenyl Salicylate has a melting point of ~40°C. Portion up a few grams and give to each group. They will need to split this up between at least two trials (room temp slide, and the cold brewed crystal slide), so they can either manage it themselves or they could have 2 test tubes to split it up equally into their beaker hot water bath.

Step 3: Record Observations

The students should watch and or digitally record the crystallization of the melt on a slide at room temperature, and then a chilled slide from the freezer or on ice. Students should pay attention to crystal size mostly, however extensions could be made to discuss:

Time taken to fully crystallize
How many nucleation sites of crystal growth there are
What happens when the crystals of one source meet and interlock with the crystals of another source site.
The presence and roll of bubbles in the crystallizing melt.

(apologies for the poor quality video, I realized later after demonstrating this on my Smartboard via (airshare from the Ipad) to the class, I never recorded the good demos, they were just run live! You'll just have to have the fun and do it on your own to see how awesome this can get!)

Step 4: Create a Qualitative Model of Crystal Growth Dependent on Environmental Conditions

I scaffolded this task by providing a shell, and an example of cooling coffee. Some students did an excellent job, and some may have needed a little more direction.

Step 5: Apply the Model

Students will employ the model they created, along with prior knowledge of color to indicate mineral composition and their Earth Science Reference Table to Identify Igneous rocks by texture and composition.

Through this activity the students should be able to differentiate between Igneous rocks with visible crystals and a coarse grained texture, and Igneous rocks with fined grained crystals that cannot be viewed individually in the hand specimen form with out visual enhancement. The key concept here focuses on the fact that the environment where this magma or lava cools directly impacts the time taken to solidify, and therefore the size of the crystals that are allowed to form in the given time frame. Large Crystals/Coarse Texture rocks form from the cooling and solidification far below the Earth's Surface where it is insulated by the surrounding rock, and cools slowly allowing molecules to nucleate onto the crystal for a longer time increasing the size of the crystals (think coffee in a thermos, it's insulated and will retain heat for longer). Magma that reaches Earth's surface now becomes known as lava, and will cool much more quickly, leading to a small crystal size, and a fine grained texture *think about spilling coffee on the table, it has more surface are exposed wand will lose heat faster than in the mug or thermos.

TL;DR

Cold slide = fast cooling = small crystals analogous to fine grained volcanic rocks ( formed above surface of Earth)

Room Temp slide = slower cooling = bigger crystals analogous to coarse grained Plutonic Igneous Rocks (below surface formation)

Step 6: Break the Model (and Plan Modifications)

Scientists model natural phenomenon, to understand the intricate relationships and communication between natural systems, and to predict future behaviors. Models are made better, by pushing them to the limit until they break and no longer work for all of the components they represent. In order to include this experience into the lesson I add samples of rocks that have glassy and or vesicular (gas pockets) textures. This can be used as an extension activity to discuss the conditions under which the Igneous rocks Scoria, Pumice, Vesicular Basalt/Rhyolite/Andesite, and Obsidian form.

Step 7: Reflect

Have the students reflect on how well their model replicated the natural phenomenon of Igneous rock formation.

Step 8: Assess Student Learning

Use the attached documents to measure if the students successfully grasp the concept of crystal size vs environment of formation.

Step 9: Teacher Lesson Plan/Rationale & Student Worksheet File

Please find the attached formal lesson plan, and student work sheet that goes along with this activity.

Introduction: Cold Brewed Crystals