Important Definitions
Mineral. Strictly speaking, a mineral is a naturally occurring, homogeneous inorganic substance of definite composition and crystal structure. Native elements (e.g. sulfur, gold) and compounds are included. A great many solid materials in nature fit this description. However, some solid materials have variable composition, e.g. pyrrhotite, FeS1-x and pentlandite, (Fe,Ni)9S8 where, respectively, varying fractions of iron and sulfur and nickel and iron occur. These are still considered minerals. Finally coal, which is not exactly an inorganic substance, nor of definite composition, may also be considered a mineral, broadly speaking.
Isomorphism. Many minerals exhibit this property, which is variable composition within the same structure. This may occur when metal ions of similar size substitute for some of the principal metal ions. Some examples are pentlandite (Ni,Fe)9S8, in which variable amounts of nickel substitute for iron and powellite Ca(Mo,W)O4.
Native elements. These are minerals which are a single chemical element. Examples are sulfur, mercury, antimony, copper, silver, gold, platinum, etc. Often these materials are not actually pure. They may be alloyed or combined with other elements to a minor extent. For example gold may contain some copper or silver.
Mineral deposit. A mineral deposit is a body of natural material that contains specific minerals.
Ore. An ore is a mixture of minerals, one or more of which can be economically exploited as a source for one or more materials. These materials might ultimately be produced as metals, alloys, compounds or concentrates. Note that the key is economically viable production. This requires a certain grade of the mineral(s) to be present. Not all mineral deposits are ore bodies, but all ore bodies are mineral deposits. With advances in technology, or increases in price what was only a mineral deposit at one time, might become an ore later.
Grade. Grade, or tenor, refers to the concentration of the minerals or elements of interest in the ore or deposit. Units may be % by weight, g/t, kg/t, oz/t (oz = ounces; the troy ounce is the common unit for precious metals. It equals 31.1034768 g.)
Gangue. The unwanted minerals that constitute the unwanted minerals of an ore. These are usually substantial fractions of the total mass.
Grains. This refers to the individual mineral crystals in a deposit or sample.
Mineralogy. The study of minerals: crystallography, chemistry, physical properties, classification and ways of distinguishing minerals. In the context of hydrometallurgy and mineral processing mineralogy refers to the type, concentration and distribution of minerals in a deposit. How the minerals are dispersed is very important. Gold may be present in the ore at a certain concentration, but is it localized between grains of silica or is it finely dispersed within a pyrite matrix? Issues like that make all the difference to how an ore is processed.
