Minerals. What is a Mineral. A mineral is a natural, usually inorganic solid that has a characteristics chemical composition, an orderly structure, and a characteristic set of physical properties. Characteristics of a Mineral.
Four question to determine if a substance is a mineral or nonmineral.
1. Is the substance inorganic? If it is organic as in coal it is not a mineral
2. Does the substance occur naturally? Brass is a metal that is manufactured and not occurring in nature
3. Is the substance a solid in crystalline form? Obsidian is naturally occurring substance, but the atoms in obsidian are not arrange in a regularly repeating crystalline structure. Obsidian is not a mineral
4. Does the substance have a consistent chemical composition? If you say yes to all four answers the it is a mineral
Silicate Minerals: A mineral that contains a combination of silicate mineral and oyxgen. Quartz has only silicon and oxygen. Other minerals have one or more additional elements like Feldspars.
Nonsilicate Minerals: 4% of Earth’s crust is made up of minerals that do not contain compounds of silicon and oxygen
It is also worth remembering that completely different minerals may be the same colour.
Galena has a Metallic Mica has a pearly
Cleavage is the tendency of a mineral to split in certain preferred directions when struck. These directions are parallel to sheets of atoms in the mineral's atomic lattice. Cleavage is described in terms of: (1) the ease of cleavage, (2) the number and orientations of cleavage planes. For example:
Gypsum has 'easy' cleavage in one direction Calcite has good cleavage in three directions parallel to its rhombohedral habit and is therefore said to have rhombohedral cleavage.Fluorite has a cubic habit, but it has four cleavage directions which cut across it's corners to leave an octahedral core. Therefore fluorite has octahedral cleavage.