The internal structure of minerals is a principle property, controlling all sorts of properties, the most obvious of which is the way the mineral looks. A mineral structure is really a molecular structure, determined by the sizes of the atoms involved and by their charges and electron configurations. We can start with a simple example. You've seen the mineral halite (rock salt; NaCl) in discussions above. Sodium and chlorine form ionic bonds with one another, and surround each other in a three-dimensional framework that is cubic, with the angles between the bonded atoms kept at 90 degrees:
Given this structure, it is no wonder that everyday salt, if you get a microscope and look at some in a salt shaker, occurs in pieces with cubic, or right-angle faces.
For most mineral-forming chemical compounds there is a fairly narrow range of three-dimensional structures possible, but for several noteworthy examples, a given composition, with a given chemical formula, can exist in several mineral phases that have different three-dimensional structure. Such phases are called polymorphs. Graphite and diamond are both made of carbon atoms, but the internal structures are completely different. Diamond is the hardest mineral, while graphite is very soft. Calcite and aragonite are minerals that are both CaCO 3 , but have different internal structures.
Minerals possess a crystalline structure, but their outward appearance isn't always as the lovely crystal you see below:
More often, minerals picked up from the surface of the ground, or seen in a set of minerals in a box, look more like the following:
The first specimen exhibits crystal form. Crystal form develops when a mineral grows in an open space within rock. The open space allows atoms to lock on to bonding sites in the growing crystal, and crystal faces build up as billions of atoms arrive at their destinations. Not so for most pieces of minerals, which got to be pieces usually from somebody swinging a hammer (here called "hammer behavior"). Most of the specimens you see in mineral kits were shaped to a small size to fit into the cell in the box. We will learn that even these "normal" specimens show clues about that underlying crystalline structure, and we will key on it in learning to identify minerals.
When we start looking at rocks, which are aggregates of minerals, you will appreciate that many minerals are found in interlocking arrangements with one another, especially in igneous rocks like granite. Most of the time growing mineral "instances" in magma aren't able to keep a nice crystal form exterior, because they bump into one another as they grow. In fact, the last minerals to form in a sequence of crystallization of minerals will be forced to grow within and fill the remaining small, irregular areas.