Identifying a mineral is a little like playing detective. Minerals are identified by their physical properties. For example, look at Figure 7.2. How would you describe it? You may say that it is shiny, gold, and has a particular shape. Each of these descriptions is actually a physical property (shiny=luster, gold=color, shape=crystal form). Physical properties can vary within the same minerals, so caution should be applied. For example, color is a property that is not a very realistic diagnostic tool in many cases. Quartz is a mineral that comes in a variety of colors, as evidenced by Figure 7.3. Occasionally color can be helpful, as in the case of the mineral olivine. Olivine is said to be “olive green” (a light to dark green) as seen in Figure 7.4. Make sure you use caution when using color to help identify minerals. We will cover each of the physical properties in detail to help you identify the minerals.
Hardness refers to the resistance of a mineral to being scratched by a different mineral or other material and is a product of the strength of the bonds between the atoms of a mineral. Whatever substance does the scratching is harder; the item scratched is softer. Hardness is based off a scale of 1 to 10 created by a mineralogist named Friedrich Mohs (Figure 7.5). Mohs’ scale lists ten minerals in order of relative hardness. Each mineral on the scale can scratch a mineral with lower number. Your mineral kit comes with several items of known hardness. The glass plate has a hardness of 5.5, the iron nail has a hardness of 4, the copper wire has a hardness of 3, and your fingernail has a hardness of 2.5. If you can scratch a mineral, then it would be softer than your fingernail, so therefore its hardness would be <2.5. When trying to scratch a surface, use force, but be cautious with the glass plate. ALWAYS lay the glass plate on a flat surface rather than holding it in your hand in case it breaks. Do not confuse mineral powder with a scratch – use your finger to feel for a groove created by a scratch (mineral powder is left behind when a soft mineral scratches a harder surface). Materials of similar hardness have difficulty scratching each other, so that, for example, your fingernail may not be able to always scratch biotite mica, which has a hardness of 2.5.
|Number||Mineral||Hardest of Test Kit Items|
|2||Gypsum||2.5 – Fingernail|
|3||Calcite||3 – Copper Wire|
|4||Fluorite||4 – Nail|
|5||Apatite||5.5 – Glass Plate|