Mineral Identification
We’ve all seen the movies or television shows. Somebody has heisted some diamonds, and in the course of the investigation, the hero comes across some suspicious gems. The hero immediately takes a gem and runs it across a mirror or glass surface to see if it’s real or not. The intrepid hero is checking the hardness of the gemstone, one of several tests used to identify minerals.
The term mineral has several different meanings, depending on who is talking about them. When you look at a box of cereal, or walk down the vitamin aisle, you’ll see all sorts of minerals. Or what about a miner talking about the vein of gold just found in the mine? These are all minerals, but are not considered minerals as geologists define them. To a geologist, for a substance to be considered a mineral it must meet four criteria: it must be a crystalline solid, it must occur naturally, it must be inorganic, and it must have a definite chemical composition.
Crystallinity, the first criteria, is the orderly arrangement of atoms in a substance. There are some substances that meet the other three criteria, but do not have a crystalline structure. Opal is one example and is categorized as a mineraloid. The second and third criteria are simple to define. Anything made by man, or produced through an organic means cannot be considered a mineral. A definite chemical composition means that each time any sample of a given mineral is broken down chemically, quartz for example, will always produce the same ratios of elements. In quartz, the ratio will always be two oxygen atoms for every silicon atom. This means that minerals can be expressed by a chemical formula, SiO2 for quartz.
We now know what defines a mineral. So when you find a substance you are pretty sure meets these criteria, how do you tell what kind of mineral you have? Geologists have several simple tools that can be used to help identify a mineral sample. These tools use the physical properties of the mineral to give you clues to match the mineral by use of a mineral identification key. The physical properties include color, streak, luster, hardness, crystal form, cleavage, fracture, specific gravity, and other unique properties. We will take a look at each of these in turn.
Color is the first thing a person will notice about a mineral. It is important to not overly rely on color as a diagnostic tool. Color can be the most beneficial in identification, and also the most unreliable of the properties. Some minerals, such as calcite have many different colors. These color changes are the result of impurities in the minerals composition.
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