The structure of the molecules that make up a mineral determines its shape. Since different chemicals may create different molecular shapes, the form of a mineral’s crystals hint at its composition. This makes growth habits helpful for identification. However, since a number of factors can affect a mineral’s habit, using it isn’t always a straightforward task. Sometimes one mineral replaces another and takes on its shape, resulting in a pseudomorph, or copycat habit. For this reason, scientists describe mineral habits as a range of compatible structures. Likewise, a few types of minerals share identical chemical makeup but form completely different types of crystals. They therefore count as different mineral species. Pyrite, for example, is famous for its cubic crystal shape, but circumstances may truncate the crystals into modified cubes or dodecahedrons. Marcasite, on the other hand, forms slender and angular orthorhombic crystals even though it is also made up of iron sulfide. Marcasite also forms pseudomorphs of other crystals or even fossils. Despite such exceptions, habit forms an important part of the identification process.