2 Formation and chemical groups of minerals

Formation

• Minerals are formed from the known elements.
• Some minerals consist of one element: gold, sulphur, copper, graphite and diamond (both carbon).
• Most minerals consist of two or more elements bonded in a stable compound, such as quartz (silicon dioxide) and calcite (calcium carbonate).
• Although more than a hundred elements are known, eight elements make up more than $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">98.5</span>}\mathrm{\%}$ by weight of the crust.

Elements of the crust (weight percentage)

• Oxygen $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">46.6</span>}\mathrm{\%}$, silicon $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">27.7</span>}\mathrm{\%}$, aluminium $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">8.1</span>}\mathrm{\%}$, iron $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">5.0</span>}\mathrm{\%}$.
• Calcium $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">3.6</span>}\mathrm{\%}$, sodium $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">2.8</span>}\mathrm{\%}$, potassium $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">2.6</span>}\mathrm{\%}$, magnesium $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">2.1</span>}\mathrm{\%}$.
• All the other elements together: $\mathrm{<span\; class="eqx"\; dir="ltr"\; style="direction:ltr;unicode-bidi:isolate;display:inline-block">1.5</span>}\mathrm{\%}$.

Chemical groups of minerals (from the most common to the least)

• Silicates: quartz, orthoclase, plagioclase, mica, amphibole, pyroxene, olivine.
• Carbonates: calcite, dolomite.
• Oxides: hematite, magnetite.
• Sulphides: pyrite, galena, sphalerite.
• Sulphates: gypsum, anhydrite, barite.
• Native elements: gold, copper, graphite, sulphur, diamond.

How many minerals?

• More than two thousand minerals are known, most in small amounts.
• The common minerals and the economic minerals together do not exceed about two hundred.
• The rock-forming minerals are only a few tens, in a few groups; the most famous group is the silicates.