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What is a Mineral?

The definition of most relevance to mineral collectors is:-
"A mineral is a naturally occurring inorganic chemical compound, usually crystalline, that is the result of geological processes."
This definition excludes man-made products such as synthetic gems or smelter by-products and organic materials such as amber. However classifications themselves are man-made for man's convenience; so don't let a mere definition stop you from collecting what appeals to you.

The Earth's Structure

The Earth consists of a molten core, predominantly iron, surrounded by a mantle. Surrounding the mantle is the outermost least dense layer of the Earth, the crust. The crust is divided into continental crust (the land masses) and the oceanic crust comprising the oceans and the underlying rock.
The most common elements in the Earth, in decreasing order of abundance, are oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium. The percentages differ slightly between core, mantle and crust. These are the dominant constituents of rock forming minerals with about 90% of the Earth's crust being silicates (compounds of silicon and oxygen plus metals). It is no surprise, therefore, that common minerals are combinations of these elements; the most abundant being silicates.

How are minerals formed?

Minerals are formed when molten magma from the Earth's core rises through the mantle towards the surface and cools. Minerals in the magma crystallise out at different temperatures. Other minerals are formed by reaction with chemicals in the surrounding rock.
The minerals thus formed may undergo further change as a result of reaction with water and oxygen near the Earth's surface. These are called 'supergene' products from the ancient greek for 'created from above'.
To understand more about minerals and how they are formed we need to consider What is Matter?; then look at Atoms, Electrons and Compounds, the foundations of chemistry and mineralogy. This background knowledge may then help us to decide Why We Should Collect Minerals.. If this does not tempt you then have a look at some mineal specimens in the Mineral Gallery