Moh's Scale of Hardness
In 1812 the Moh’s scale of mineral hardness was devised by the German mineralogist Frederich Mohs (1773-1839). He selected the ten minerals because they were common or readily available. The scale is not a linear scale, but somewhat arbitrary. Some minerals are very soft; others are very hard. The degree of hardness is an aid in identifying the minerals. Diamonds are harder than quartz and will therefore scratch quartz, quartz will scratch calcite, calcite will scratch gypsum, and so on. To help identify minerals, geologists have assigned numbers to the hardness of several minerals. In this hardness scale, the softer minerals are assigned a low number and the harder minerals a higher number. In the field, an easy way of estimating the hardness of a mineral is by trying to scratch it with common objects such as a fingernail with a hardness of 2.5, a penny is the hardness of a 3, or a pocketknife, hardness 5.5. Glass has a hardness of slightly less than 6 and will scratch most minerals. Anything on the Moh’s scale that is less than a 7 is easily scratched. Hardness Mineral Associations and Uses 2.5- Fingernail, 2.5-3 -Gold, Silver 3 -Copper penny 4-4.5 -Platinum 4-5- Iron 5.5- Knife blade 6-7- Glass 6.5- Iron pyrite 7+- Hardened steel file To test a mineral for hardness, try to scratch it with one of these common objects. For example, a sample can be scratched by the knife (H=5.5) but NOT by the penny (H=3). Therefore, the sample has a hardness of about 4. Using other property tests that you will learn in lab, you then determine that the mineral is calcite. The Moh’s Scale of Mineral Hardness goes as follows.... 1 is Talc- Talcum powder. 2 is Gypsum- Plaster of Paris. Gypsum is formed when seawater evaporates from the Earth’s surface 3 is Calcite- Limestone and most shells contain calcite 4 is Fluorite- Fluorine in fluorite prevents tooth decay. 5 is Apatite- When you are hungry you have a big "appetite". 6 is Orthoclase- Orthoclase is a feldspar, and in German, "feld" means "field 7 is Quartz 8 is Topaz- The November birthstone. Emerald and aquamarine are varieties of beryl with a hardness of 8. 9 is Corundum- Corundum Sapphire and ruby are varieties of corundum. Twice as hard as topaz. 10 is Diamond- Used in jewelry and cutting tools. Four times as hard as corundum How to Perform the Test 1. Select a fresh surface 2. Hold the sample and attempt to scratch it with the point of an object of known hardness, start at the higher (harder) end of the scale and work down. In this example, we use a sharp quartz crystal (Hardness = 6) 3. Press the object firmly but lightly against the unknown sample 4. If the test object is harder, you should see and feel a definite "bite" into the sample 5. Inspect for an etched line In this case, we notice a deep scratch in the sample, which indicates it's hardness is less than 6 . Repeat several times, always with a sharp point and a fresh surface. This information was copied from http://www.kgs.ukans.edu/Extension/KGSrocks/hardness.html and http://www.amfed.org/t_mohs.htm