Day 1

 A mineral is a naturally occurring, inorganic material with a fixed composition and a repeating internal structure.

• a material which occurs only through artificial synthesis is not a mineral

• the term 'inorganic' excludes hydrocarbons and other molecules that can have crystalline (repeating and ordered) structures; note that minerals can form through organic processes (e.g., apatite, calcite, graphite, and pyrite are each minerals that can be precipitated during the life activities of certain organisms).

• fixed composition means that we can write out a mineral formula (e.g., calcite, CaCO₃); mineral compositions can vary within specified limits through substitutions (e.g., magnesium can substitute for calcium in calcite, and we can express that as [Ca,Mg]CO₃).

• repeating, or periodic, structure means there is a simple, fundamental arrangement of atoms in a mineral, and this arrangement repeats to form the mineral. In this sense, 'repeating internal structure' is synonymous with the term 'crystalline'.

In addition to the many obvious minerals that you are already familiar with, such as quartz, garnet, and diamond, consider the following - are they minerals?

• Snow
• Mercury (room temperature)
• Mercury (absolute zero)
• Coal
• Crystals of acetic acid
• Calcite in a mollusc shell
• Obsidian
• Amber