A mineral is a naturally occurring crystal with a specific chemical recipe that can vary only within set limits. Quartz is always SiO₂. Olivine is (Mg,Fe)₂SiO₄: magnesium and iron can swap places with each other, but there are always two of them for every silicon and four oxygens. A rock is a mixture of minerals. Lithology is the geologist's word for a rock's overall character: its mineral makeup, grain size, colour and texture. When you hear "the lithology changes here", it means "the rock type changes here".

About 90% of the crust is made of silicate minerals, built from silicon and oxygen with aluminium and metals like calcium, magnesium, iron, sodium and potassium mixed in. They look very different from each other, but they are all built from the same small part.

The part is the silicate tetrahedron: one silicon atom with four oxygen atoms around it, at the corners of a triangular pyramid. The silicon–oxygen bond is very strong. But the unit is not neutral. Silicon carries +4 and each oxygen −2, so one SiO₄ unit carries a charge of −4, and that charge has to be balanced somehow.

There are two ways to do it. The tetrahedra can bond to metal ions (positively charged atoms such as Mg²⁺, Fe²⁺, Ca²⁺, Na⁺ and K⁺) that sit between them. Or they can share corners: one oxygen bonds to two silicons, forming a strong Si–O–Si link. The more corners each tetrahedron shares, the less spare charge is left and the fewer metal ions are needed. Count the shared corners and you have the whole classification of silicate minerals.

This explains a lot about how minerals behave. Sheets are strongly bonded within each layer but only weakly bonded to the next, which is why mica peels into thin flakes and clays feel soft and slippery. It also explains weathering. Water and acid attack the weakest bonds first, and the weak bonds are the ones between oxygen and metal ions: a hydrogen ion (H⁺) can take a metal ion's place and set it free in solution. In olivine, every tetrahedron is held to its neighbours only through magnesium and iron ions. Pull those out and the crystal falls apart one tetrahedron at a time. In quartz, every oxygen links two silicons and there are no metal ions at all, so the only way in is to break the strong Si–O bonds themselves.

Feldspars are frameworks too, but in them aluminium takes the place of some silicon atoms. Each swap leaves a spare negative charge, which is balanced by calcium, sodium or potassium sitting in gaps in the framework. Those are weak points, which is why calcium-rich feldspar weathers far faster than quartz despite having a similar structure. Structure is not the only control on weathering, as later sections show, but it is the first-principles reason why olivine and quartz sit at opposite ends.

Igneous rocks, the ones that crystallised from molten rock, are named using two clues. The first is chemistry: how much silica (SiO₂) the rock contains, which decides which minerals can form. The second is grain size, which records how fast the melt cooled. Lava that cools within days at the surface grows many tiny crystals. Magma that cools over thousands to hundreds of thousands of years, kilometres down, grows fewer, larger ones. The same melt can therefore make two rocks: basalt if it erupts, gabbro if it cools underground. (Try the cooling-rate demo in The rock cycle to see why.) Fine-grained rocks that cooled at the surface are called volcanic; coarse-grained ones that cooled deep down are plutonic.

Along the chemistry axis, the most useful distinction to learn is mafic versus felsic.

Between the two are intermediate rocks (about 52–63% silica) such as andesite and diorite. Below 45% are ultramafic rocks such as peridotite. Peridotite is made mostly of olivine, and it makes up most of the upper mantle. The whole range is one continuous spectrum, shown below.

Why do olivine and calcium-rich plagioclase go together, and quartz with potassium feldspar? In the early twentieth century the geologist Norman Bowen melted rocks in the laboratory, cooled them step by step, and looked at which crystals had formed at each temperature. He found that minerals crystallise from a cooling magma in a predictable order, now called Bowen's reaction series. It has two branches that meet.

Notice that the discontinuous branch climbs the silicate ladder from the previous section. The hottest mineral, olivine, has isolated tetrahedra; each later one shares more corners. As the melt cools, more of its silicon and oxygen link up into chains, sheets and finally frameworks.

The series also explains how one magma can give rise to several rocks. If early crystals are removed, for example by sinking to the floor of a magma chamber, the remaining liquid loses magnesium, iron and calcium and becomes richer in silica. This is fractional crystallisation. It is how a basaltic magma can, in stages, produce more intermediate and felsic melts, and how dense early minerals such as chromite pile up into ore layers (see Magmatic deposits).

In 1938 Samuel Goldich studied soils and weathered rock, and found that minerals disappear in almost the same order as Bowen's series: the minerals that crystallise first weather first. This ranking is the Goldich dissolution series, the arrow on the right of the figure above. There are two first-principles reasons for it.

• Distance from home. Olivine formed at about 1,200 °C, in a dry melt. The Earth's surface is cool, wet and full of oxygen and acid, about as far from olivine's birthplace as you can get, so olivine is badly out of balance there and reacts readily. Quartz formed at the lowest temperature, in the wettest melt, and is much closer to being stable at the surface.
• Structure. As the silicate figure showed, the early minerals have the fewest Si–O–Si links and the most metal ions for water and acid to attack. Quartz has none.

What weathering leaves behind differs too. Olivine and pyroxene break down readily, releasing magnesium, calcium (mainly from pyroxene) and dissolved silica into water. Their iron oxidises and stays behind as rusty iron oxides, and some pyroxene turns into clay. Feldspars mostly turn into clay minerals, releasing their calcium, sodium and potassium. Quartz survives as grains, which is why most beach and river sand is quartz.

The Goldich series is a ranking, not a speed. How fast a given rock actually weathers also depends on its grain size, the temperature, rainfall and soil acidity, which Weathering, erosion & soil covers.