Minerals are the small
particles that you can see in most rocks.
You can say that rocks
consist of minerals, even though some rocks have more of them, while
others have a few or none (these are glassy rocks but they are very
By definition, a mineral
is a crystalline solid with a definite, but not necessarily fixed
When we talk about minerals in geology we mean natural minerals -
the ones that have formed naturally by different geological processes
(which I will get to later on).
There are, however, also synthetic minerals, which are manufactured.
What does it mean that minerals are crystalline
It means that their crystals have got a repetative atomic order - their
atoms and ions are arranged in a repeating pattern.
What does it mean that a mineral has a definite,
necessarily fixed chemical
It means that you can write a chemical formula for every mineral, but -
within limits, the chemical composition can vary between different
samples of the same mineral.
For example, mineral olivine can be magnesium or iron olivine, and
everything in between (it can have various amounts of both, which is
called a solid solution
- basically a group of different chemical compositions within the same
In other times, it can be the other way around - there may be a few
different minerals that have exactly the same chemical composition, but
different other properties.
and christobalite for example have exactly the same
chemical composition - they are all silicon oxides, but they have
different crystal structures.