Editing Crystal (section)

== Crystal faces, shapes and crystallographic forms==
[[File:Crystal facet formation.svg|thumb|upright=1.6|As a [[halite]] crystal is growing, new atoms can very easily attach to the parts of the surface with rough atomic-scale structure and many [[dangling bonds]]. Therefore, these parts of the crystal grow out very quickly (yellow arrows). Eventually, the whole surface consists of smooth, [[surface energy|stable]] faces, where new atoms cannot as easily attach themselves.]]
Crystals are commonly recognized, macroscopically, by their shape, consisting of flat faces with sharp angles. These shape characteristics are not ''necessary'' for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is often present and easy to see.

[[Euhedral]] crystals are those that have obvious, well-formed flat faces. [[Anhedral (petrology)|Anhedral]] crystals do not, usually because the crystal is one grain in a polycrystalline solid.

The flat faces (also called [[facet]]s) of a [[euhedral]] crystal are oriented in a specific way relative to the underlying [[crystal structure|atomic arrangement of the crystal]]: they are [[plane (mathematics)|planes]] of relatively low [[Miller index]].<ref>''The surface science of metal oxides'', by Victor E. Henrich, P. A. Cox, page 28, [https://books.google.com/books?id=X6x1MmPisKkC&pg=PA2 google books link]</ref> This occurs because some surface orientations are more stable than others (lower [[surface energy]]). As a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. (See diagram on right.)

One of the oldest techniques in the science of [[crystallography]] consists of measuring the three-dimensional orientations of the faces of a crystal, and using them to infer the underlying [[crystal system|crystal symmetry]].

{{anchor|Crystallographic form}}
A crystal's '''crystallographic forms''' are sets of possible faces of the crystal that are related by one of the symmetries of the crystal. For example, crystals of [[galena]] often take the shape of cubes, and the six faces of the cube belong to a crystallographic form that displays one of the symmetries of the [[isometric crystal system]]. Galena also sometimes crystallizes as octahedrons, and the eight faces of the octahedron belong to another crystallographic form reflecting a different symmetry of the isometric system. A crystallographic form is described by placing the Miller indices of one of its faces within brackets. For example, the octahedral form is written as {111}, and the other faces in the form are implied by the symmetry of the crystal.

Forms may be closed, meaning that the form can completely enclose a volume of space, or open, meaning that it cannot. The cubic and octahedral forms are examples of closed forms. All the forms of the isometric system are closed, while all the forms of the monoclinic and triclinic crystal systems are open. A crystal's faces may all belong to the same closed form, or they may be a combination of multiple open or closed forms.<ref name=Sinkankas1964>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0442276249 |pages=134–138}}</ref>

A [[Crystal habit|crystal's habit]] is its visible external shape. This is determined by the [[crystal structure]] (which restricts the possible facet orientations), the specific crystal chemistry and bonding (which may favor some facet types over others), and the conditions under which the crystal formed.