Cleavage and Fracture

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SWBAT demonstrate cleavage and fracture in minerals by constructing atomic models and identifying minerals that exhibit cleavage and fracture

Big Idea

Cleavage and fracture are important physical properties in classifying minerals.

Lesson Introduction

[Note: For embedded CFUs, information on transitions, content delivery, and important directions, please reference the attached Word document and look at embedded comments for additional information]

This is another lesson with a lot happening! This lesson in particular is fairly technical in terms of content. The teacher should go in with at least a primitive background knowledge of how to recognize and identify cleavage, fracture, and primary forms of breakage in minerals. This lesson covers exactly that (as it is also something students tend to struggle with), and is fairly comprehensive in terms of a 60-minute lesson. There is a brief "hook" introduction (resources attached below), followed by a reading passage, constructing atomic models, and a mini-lab. While there is no traditional 'practice' (that comes in later lessons) in this lesson, they do get practical applications of identifying breakage patterns in actual mineral samples.

Required Materials: Gumdrops, toothpicks, up to ten (10) mineral samples [should have muscovite mica and quartz included]. Enough materials for a group of four (4) to have their own mineral samples to work with. 

Do Now & Objective(s)

10 minutes

Students come in silently and complete the Do Now. After time expires, we collectively go over the responses (usually involving a series of cold calls and volunteers) before I ask someone to read the objective for the day and begin the lesson.

Cleavage & Fracture Introduction

10 minutes

After the Do Now and objective are read, the lesson begins when I provide some brief PowerPoint slides to get students thinking about mineral properties and crystalline structure. The PowerPoint is simple (it only contains two slides) and shows a picture of a diamond, followed by a picture of graphite. I then present them with the information that one (1) gram of a flawless diamond is worth about $25,000, while a comparative gram of graphite costs about $0.03. I then lead them to the fact that both are made entirely of Carbon, which leads students to think about how things can have such different properties if they're made of entirely the same thing. Then, I introduce the concept that a mineral's internal arrangement of atoms (crystal structure) ultimately determines its properties, which they fill in on their attached notes page [Note: See embedded comments in Word document for more specific instructions].

What follows this brief "hook" is a reading passage in which students will have to digest and dissemble the key points. Since it's the beginning of the year, I often involve the entire class by practicing one of my favorite reading strategies (discussed in the Day Zero lesson plan) - which involves "popcorning" around the room at random and different intervals to have students pick up as readers where previous readers have left off. Whenever I say a student's name, that person has to immediately start reading where the last person left off. I may have them read a sentence or a paragraph (and I often very carefully think about who I'm going to call on and how much I'm going to have them read), but they are responsible for reading for the entire class until I call on someone else. I like it because it keeps all the students accountable and engaged, and it allows me to stop the class periodically to either ask probing questions, check for understanding, or address important vocabulary. 

Mini-Lab: Creating Atomic Models

15 minutes

After transitioning out of this section, we use gumdrops and toothpicks to assemble atomic models of silicate tetrahedra. This helps show how the internal arrangement of atoms determines a mineral's physical properties (see attached resource for more directions). Using gumdrops and toothpicks that have already been placed on lab tables, they are asked to assemble an atomic model of muscovite mica (using a gumdrop to represent Potassium, as on the image on the bottom of Page 5 of the Building Atomic Models Handout). When students construct this, they can see that the structure is "weakest" around the Potassium bonds. This helps them see that when minerals cleave, they break along atomic zones of weakness, which produces the distinct forms of breakage. This is generally why muscovite mica cleaves into thin sheets; it breaks along those lines of Potassium bonds. 

After this visualization and model, they're asked to assemble a similar (but smaller) tetrahedral model to the one represented on the image on the top of page 6 on the Building Atomic Models Handout. This is an image of quartz, which doesn't cleave, but experiences fracture as its primary breakage. Students can see after constructing the model that there no readily observable zones of weakness - when it breaks, it breaks randomly. This random breakage and lack of a smooth cleavage surface indicates fracture, as opposed to cleavage, as the primary form of breakage.

Mineral Identification

15 minutes

After this section, I briefly model with some mineral samples (you should use a good mix of minerals with different breakage patterns - Quartz, Muscovite Mica, Sulfur, Halite, Gypsum, Feldspar, and Calcite are good examples) how to physically recognize cleavage and fracture.

Note: the general rule is to look for a flat plane somewhere - fracture is generally never linear.

Then, students are given the opportunity to work in lab groups to see if they can determine which minerals experience cleavage or fracture as their primary form of breakage.

Students are then given the remaining time to work in laboratory groups to identify the minerals. When there are a few minutes left, we check on the results.

Note: It's helpful to number the mineral samples for easy re-sorting and identification.

Exit Ticket

10 minutes

After grading a peer, the papers are collected and students are given some time to test their knowledge on the daily exit ticket. After a few minutes, we collectively go over the answers as a class. Finally, after one or two students are asked to summarize what they learned and the information in the lesson, students are dismissed.