What are minerals?
Most people tend to think of rocks and minerals as pretty much the same. In reality, however, they are very different. Minerals are the basic forms in which nature crystallizes matter, in its purest form. In other words, because of the chemistry of matter in the universe, certain substances combine with others in a very precise way, forming crystals of specific types.
Atoms of various kinds — carbon, oxygen, silicon, etc. — link together in ways that are determined by their chemistry and their electrical properties. This systematic process of nature, combined with varying conditions of temperatures, pressures, geography, and other factors, produce minerals inside Earth over greatly varying timescales. Some mineral crystals form quickly, as with evaporates in salt lakes, and other crystals form over thousands or even millions of years.
So minerals are the pure chemical products of atoms being assembled the way nature likes it into what scientists call crystal lattices, the framework that produces the arrangement of atoms that nature prefers. Rocks, on the other hand, are the jumbled, mixed up remains of minerals whose structure is blended by geological processes are whose atomic arrangements were taken from order to disorder. Imagine throwing a huge batch of minerals into a giant blender — rocks would be what you get out afterward.
Although we have only one sample of a living planet, Earth, minerals also tell us a great deal about the universe as a whole. For decades astronomers have used spectroscopy as a means of studying stars, galaxies, and other objects throughout the cosmos. Because of this, we know that chemistry is the same everywhere in the universe. Samples returned from the Moon and studied on Mars and other planets also show us that minerals are remarkably similar on worlds other than Earth. And as of mid 2010, more than 400 planets are known outside the solar system.
While local conditions such as temperatures and pressures would vary greatly on other worlds, the fact that chemical bonds and processes are the same everywhere suggests that holding a nice mineral in your hands on Earth gives you a pretty fair estimate of what minerals might look like on other worlds. That’s an exciting prospect that ties astronomy and mineralogy together as the closely related sciences they are.
The specific composition of minerals on Earth varies greatly from mineral to mineral, of course. At present, some 4,000 minerals are known, and mineralogists are discovering more every year. Of these, about 100 are very common and easily recognized by most people. They are chemical compounds, solids, that generally have several elements combined in a specific way — although native elements such as copper, gold, and silver consist of just one element. And each mineral describes the exact way the elements come together in a chemical formula. For example, two elements, iron and sulfide, can combine in more than one way. Pyrite, iron sulfide with the formula FeS2, has a cubic crystallization. Marcasite, on the other hand, has the same formula, FeS2, but crystallizes in the orthorhombic system. So the two substances, made out of the same stuff, are considered two different minerals.
The manner in which minerals crystallize is controlled by electrical charges. That a mineral compound exists at all is because the matter comes together and is electrically balanced as it combines. Charged particles known as ions, which are simply atoms with a positive or negative electrical charge, form the units that make up minerals. In some minerals the positively charged ions, or cations, are metals. Ions called anions often group together in minerals to carry the negative charge. These ions with unpaired electrons are looking to bind with their opposites and are often called radicals. Taking pyrite as an example again, the negatively charged sulfur, S2, balances the positively charged iron, Fe. This precarious balance keeps the mineral crystal together and allows these little units, consisting of one positively charged iron atom and two negatively charged sulfur atoms, to build large crystals made of many such units. This is how minerals come together and stay together.