Lab 2: Minerals I


A mineral has the following characteristics:


  • It must occur naturally. Some manufacturing processes, especially those having to do with making steel and metal products, can have byproduct material that may be hard, and even look "mineral-like," but really isn't a true mineral substance.
  • It must be inorganic. This one can be tricky. Have you ever been snatched onto by a crab or crawfish? Or a snapping turtle?! That stuff they are made of looks pretty hard, but it is hard organic material of several types. It is hard enough to fossilize, but more often than not, it just decays away after death. Shells of animals such as clams and snails, made of the mineral calcite, are minerals. Why the distinction? Because the chemical compounds in a crab skeleton are organic compounds, which are Carbon-based and special to Life. The calcite in a clam shell, on the other hand, is just CaCO 3 , which is the same chemical formula as technically "inorganic" calcite. So, it is OK to call shells and bones minerals, but not so for organically produced tissues and somewhat hard things such as fingernails, crab and insect exoskeletons, and horns.
  • It must be a solid. Liquids and gases are not minerals.
  • It must possess an orderly internal structure: that is, its atoms must be arranged in a definite pattern called a crystalline structure. The reason for this part of the definition is that glassy substances, such as obsidian, volcanic glass, are very hard, but the internal structure is amorphous: the atoms in glass are in a helter-skelter arrangement. One more thing to understand, is that this crystalline structure is inside a mineral specimen. The outer appearance of the specimen could exhibit beautiful smooth faces (like a "crystal") or it could look like any old chunk of gravel. Internally, though, if there is an orderly crystal structure, the specimen is a mineral.
  • It must have a definite chemical composition that may vary within specified limits. This is best appreciated with a couple of examples. First for a mineral like calcite, CaCO 3 , that's pretty much the formula that chemical analyses would find for this mineral. But for a mineral like olivine, (Mg, Fe)SiO 4 , Mg (magnesium) and Fe (Iron) can both fit in the crystal structure. That's what the parentheses in the formula indicate. A sample of olivine might have 60% Mg and 40% Fe, and another might have 23% Mg and 77% Fe. Both samples would still be olivine.


So, what is a rock?


It is any solid mass of material, whether mineral or not, as long as it is naturally occurring. For the most part, this covers any specimens made of one or more minerals or specimens made of volcanic glass.