Geologic terms can be a stumbling block for anyone trying to understand Earth processes. Geologists speak a "foreign" language. As a result, many people think the point of geology is to correctly name a rock; for example, call a granite a granite, and you get an A in fourth grade. To the salesperson at the kitchen and bathroom store almost any coarse-grained rock is granite. But to a geologist, granite is more than a word. It conjures up an image of a chamber filled with hot magma, slowly cooling deep in the earth, probably in a mountain belt such as the Andes. And the question for the geologist becomes "Just what were the circumstances that allowed this particular rock to be created? Where did the magma come from? How did it cool?"
Granite is only one type of rock that cools slowly deep in the earth, but it's one of the most complicated to produce.
Most minerals are formed from silicon (Si) and oxygen (O). (Oxygen is actually the most common element in the Earth's crust.) How the silicon and oxygen combine determines a mineral's crystal structure and therefore its mineral family. The silica (SiO) structure could be considered a mineral's DNA. In some minerals, silica and oxygen are linked in long chains; in others, they form rings or even large sheets. The silica structures generally have a net negative charge. This needs to be balanced by positively charged cations, which are the glue that holds Si-O together. The most common cations are magnesium (Mg), iron (Fe), sodium (Na), and calcium (Ca). These cations happen to be the right size to slip into Si-O crystal structures
Just as identical twins with exactly the same DNA can be very different people, igneous rocks with the same composition can also be very different. Shiny black volcanic glass cools in seconds and no minerals form. Basalt, which takes days to cool, contains small minerals surrounded by glass (like chocolate chips in a chocolate chip cookie). Gabbro cools over weeks, giving all of the elements time to join a mineral. But all of these rocks have the same composition.