9. CALCITE (CaCO₃) is a form of calcium carbonate (CaCO₃). Calcite crystals occur in a wide variety of shapes, but they all show calcite’s characteristic rhombohedral cleavage. These are often called "rhombs". Large chemically pure and optically clear crystals of calcite are called "Iceland spar" after a famous occurrence in Iceland. "Dogtooth" calcite is actually a row of calcite crystals which look like teeth. Calcite will "fizz" when a drop of HCl acid is put on it. The most important use of calcite is in the manufacture of cements, which are used to make concrete.   
     

   1. What shape is this piece of calcite?

2. What happens when a clear rhomb of calcite is placed over a word? This is a property called double refraction.

3. Is the rhomb a crystal or did it cleave (break) in this manner? Can you really tell?

10. MICA (K(Mg, Fe)₃(AlSi₃O₁₀₎(OH)₂)(biotite) to  KAl₂(AlSi₃O₁₀)(F,OH)₂ (muscovite) belongs to a very complex chemical group. In spite of this, mica is easy to identify, because it has a characteristic way of cleaving into sheets. Mica shows one cleavage. Mica comes in many colors which reflects the elements that make it up.  Biotite is the name for the black mica.  Mica is chiefly used as an insulating material in the manufacture of electrical equipment. Because mica is fireproof it has been used for stove doors, lanterns, and even toasters. The shiny look in wall paper is caused by bits of ground mica crystals.
         

    1. How does mica break?

2. Is mica softer or harder than quartz? How can you tell?

3. Describe mica.  Which mica do you think you have in your sample?

11. QUARTZ is composed of silicon and oxygen, and has the chemical formula SiO₂. It is the most common mineral found within the crust of the Earth. Quartz crystals commonly have a prismatic shape and are six sided. Quartz has a characteristic look when it breaks called a conchoidal fracture (like a scooped out section). It is a very hard mineral and usually can be identified by the fact that quartz scratches steel. Quartz has many uses ranging from gemstones, optical, scientific apparatus, radios, and watches. It is also used to obtain silicon which is important in the computer industry.   
      

    1. Examine the quartz crystal. How many sides does it have? Are these created by cleavage or are they formed by crystal growth?

2. Is quartz harder or softer than a steel nail? How do you know?

3. What happens to a quartz crystal if it breaks?

4. Compare the quartz crystal to the other quartz samples. How are they different and similar?

5. Why do some pieces of quartz form a single crystal, while others form multiple crystals?