Minerals can be defined as inorganic, chemical and physically homogenous and usually solid materials of which the composition can be defined with a chemical formula. That sounds very complicated, but in reality this just means that every non-living natural thing which you can stick a label on with it's chemical formula is in fact a mineral.. do note though that a natural aggregate (joined parts that are mechanically seperable) is also a mineral

This means that contrary to popular belief that stones are indeed (a special category of) minerals. For the record.. The study of stones is called petrography.. (so is writing on stones but that's besides the point).

Like stones; Metals are also minerals provided they were formed by natural processes (Studying metals and their properties is called metallurgy)

Minerals may not contain synthetic material, or may be synthetically formed.. These compounds are not minerals.

How are/were they formed?

Minerals were formed during every stage of the earth's development. The most important processes which form minerals are drying magma, hydrothermal solutions and gasses.. which I'll describe in more detail later on.

Sometimes, one mineral can be formed by several different processes, though it's very hard to trace exactly how a single mineral was formed, important clues can be found in active vulcanoes, warm water springs and by studying layers of sediment in lakes and oceans.

More on the formation.. aka the boring part

Mineral forming processes can generally be split up into the following categories;

  1. Magmatic mineralization: Magma normally contains high quantities of nearly every chemical element, but if the magma is beneath the earth's surface, these elements begin to seperate forming new magmas. Each of of these magmas has a different composition.

    While the pressure and temperature drops, these materials will clotter up and form igneous rocks. What is left is called residu magma, which contains water vapour, fleeding gasses (chlorine etc) and some leftover element residu.

    This residu magma causes hydrothermal solutions to slowly erode their way into nearby rocks by moving through small cracks etc, the minerals in this solution will cling to the sides of these cracks and form hydrothermal veins.

    Because this solution will also 'eat through' soft, easily erodable materials like gypsum, metasomatism will also occur

  2. Sedimentary mineralization: Nature can leave elements on several places either with some help from the wind or the water, this sedimentation usually leaves things like gypsum and borate behind and with the help of some micro organisms new minerals are formed.
  3. Metamorphic mineralization: This category has two different types of mineralisation;
    • When hot magma penetrates surrounding rocks, these are damaged by the heat of the magma, especially when a rock comes in direct contact with it.. The minerals that form at the point/line of contact are.. yup.. contact minerals
    • The other type has to do with large bodies of rock that were formed on the surface of the earth but ended up way beneath it later, these rocks are subjected to very high pressure and temperatures and change into metamorphic minerals on a regional scale.

    Note that during metamorphic mineralization certain igneous rocks will transform into other minerals (like certain vulcanic minerals transforming into Serpentine). During these processes the mass will always increase.

  4. What are their properties

    To end this node with a little grace, I present to you.. a list of the most important properties a mineral always has (like density) and 'optional' properties (like radioactivity);

    • Specific Gravity: The weight of one cubic centimeter volume of a mineral (in grams)
    • Relative Density: Is the relative number that defines how much times heavier or lighter a mineral is compared to an equal volume of water (at 4 degrees celsius)
    • Hardness: The ability to withstand mechanical damage, determined by it's ability to scratch another mineral or be scratched. Talc(1) is the bottom of this scale, Diamond(10) the top. This method is also refered to as the Moh scale.
    • Optical properties: Does the mineral have a constant colour? Is it translucent? Is it multi-coloured? Things like that, all thing you can or can't see. Most minerals don't have a constant colour, but if you rub it over a sheet of paper or another white surface that won't crumble but rip or break, it leaves some coloured powder of the mineral behind.. the colour of this powder stripe is usually constant and is very important when identifying minerals
    • Magnetism: Is this mineral magnetic? If so, how strongly? or, Can this mineral be magnetised?
    • Electronic Properties: Is this mineral an isolator, a conductor or perhaps a semi-conductor?
    • Temperature conduction: Is the oscillation of the minerals' particles easily affected by heat?
    • Radioactivity: Does this mineral emit radioactive radiation?
    • Luminescence: Does this mineral emit light after it has been exposed to ultraviolet light?
    • Ok, this is it.. There's nothing more I can say on the subject, I'm not a mineralogist and everything jotted down here is a compile of the things I've read on the subject in the past (and actually remembered or wrote down)..

      There are about 2600 identified minerals right now, most of which have different varieties.
      Ow and one other thingy, I said that everything natural you can stick a note on with it's chemical formula is a mineral well, there's one exception I know of; Mercury aka Quicksilver.