The Native Elements Class
The Elements (which include over one hundred known minerals) are a diverse class when taken as a whole. Most of this diversity, however, is due to the diversity of the Non-metals Subclass.
The Metals Subclass and related metal alloys contains metals whose properties are rather similar due to the common way in which they crystallize and bond. The greatest difference in the metals is color.
The non-metals, however, are extremely diverse. For instance, the hardest mineral known to man is from this subclass, as well as one of the softest. The non-metals include some elements known as semi-metals who share some properties with metals but differ in other characteristics.
Metal alloys are minerals that are composed of combinations of different metals in one mineral. All native metals are impure usually by several percentage points, but these are not distinguished as distinct minerals unless they meet certain mineralogical criteria. Generally they must be consistent in their composition and have their respective elements occupy specific sites in their crystal lattice in order to be named as new minerals.
Alloys that are composed of semi-metals with metals are classified as sulfides but are sometimes listed as elements. They usually share similarities to other sulfides in their physical properties. These minerals are in the Selenides, the Tellurides, the Antimonides and the Arsenides Subclasses of the Sulfide Class. The main difference between elemental alloys and these semi-metal alloys is the presence of covalent bonding in these minerals as opposed to the strictly metallic bonding in pure metals and their metal alloys.
The most difficult to classify are the metal/non-metal mineral combinations. These minerals, which combine metals such as iron with the very non-metallic elements of carbon, nitrogen, phosphorous and silicon are quite unique and quite rare. They are not too different from sulfides which typically combine metals with sulfur. But the sulfides class is by convention limited to sulfur and semi-metal combinations as discussed above.
It might surprise people to find out that the Elements Class contains minerals that are composed of more than one element. Elements, by the chemical definition are composed of all the same atoms; whereas substances composed of two or more elements are compounds. The inconsistency is explained by allowing only those minerals whose bonding is similar to the more traditional elements. Metal alloys bond with metallic bonds and the carbon-carbon bond of diamond is similar to the carbon-silicon bond in moissanite. This type of covalent bonding is called elemental bonds. All in all the Elements Class is a rather complicated and interesting class of minerals.