page_title: Mineral
section_title: Crystal structure and habit
hierarchical_section_title: Mineral / Physical properties / Crystal structure and habit
caption_attribution_description: English: Spinel Locality: Mogok, Pyin Oo Lwin District, Mandalay Division, Burma (Myanmar) (Locality at mindat.org) Size: thumbnail, 1 X .8 X .8 cm SPINEL (twinned) Opaque, twinned .6 cm octahedrons with fair color. Deutsch: Spinell Fundort: Mogok, Distrikt Pyin Oo Lwin, Mandalay-Division, Myanmar (Fundort bei mindat.org) Größe: 1 X .8 X .8 cm
context_page_description: In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form.
  The geological definition of mineral normally excludes compounds that occur only in living beings. However some minerals are often biogenic or are organic compounds in the sense of chemistry. Moreover, living beings often synthesize inorganic minerals that also occur in rocks.
  The concept of mineral is distinct from rock, any bulk solid geologic material that is relatively homogeneous at a large enough scale. A rock may consist of one type of mineral, or may be an aggregate of two or more different types of minerals, spacially segregated into distinct phases.
  Some natural solid substances without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound may occur naturally with different crystal structures, each structure is considered a different mineral species. Thus, for example, quartz and stishovite are two different minerals consisting of the same compound, silicon dioxide.
context_section_description: Crystal structure results from the orderly geometric spatial arrangement of atoms in the internal structure of a mineral. This crystal structure is based on regular internal atomic or ionic arrangement that is often expressed in the geometric form that the crystal takes. Even when the mineral grains are too small to see or are irregularly shaped, the underlying crystal structure is always periodic and can be determined by X-ray diffraction. Minerals are typically described by their symmetry content. Crystals are restricted to 32 point groups, which differ by their symmetry. These groups are classified in turn into more broad categories, the most encompassing of these being the six crystal families.
  These families can be described by the relative lengths of the three crystallographic axes, and the angles between them; these relationships correspond to the symmetry operations that define the narrower point groups. They are summarized below; a, b, and c represent the axes, and α, β, γ represent the angle opposite the respective crystallographic axis (e.g. α is the angle opposite the a-axis, viz. the angle between the b and c axes):
  The hexagonal crystal family is also split into two crystal systems – the trigonal, which has a three-fold axis of symmetry, and the hexagonal, which has a six-fold axis of symmetry.
  Chemistry and crystal structure together define a mineral. With a restriction to 32 point groups, minerals of different chemistry may have identical crystal structure. For example, halite (NaCl), galena (PbS), and periclase (MgO) all belong to the hexaoctahedral point group (isometric family), as they have a similar stoichiometry between their different constituent elements. In contrast, polymorphs are groupings of minerals that share a chemical formula but have a different structure. For example, pyrite and marcasite, both iron sulfides, have the formula FeS₂; however, the former is isometric while the latter is orthorhombic. This polymorphism extends to other sulfides with the generic AX₂ formula; these two groups are collectively known as the pyrite and marcasite groups.
  Polymorphism can extend beyond pure symmetry content. The aluminosilicates are a group of three minerals – kyanite, andalusite, and sillimanite – which share the chemical formula Al₂SiO₅. Kyanite is triclinic, while andalusite and sillimanite are both orthorhombic and belong to the dipyramidal point group. These differences arise corresponding to how aluminium is coordinated within the crystal structure. In all minerals, one aluminium ion is always in six-fold coordination with oxygen. Silicon, as a general rule, is in four-fold coordination in all minerals; an exception is a case like stishovite (SiO₂, an ultra-high pressure quartz polymorph with rutile structure). In kyanite, the second aluminium is in six-fold coordination; its chemical formula can be expressed as AlAlSiO₅, to reflect its crystal structure. Andalusite has the second aluminium in five-fold coordination (AlAlSiO₅) and sillimanite has it in four-fold coordination (AlAlSiO₅).
  Differences in crystal structure and chemistry greatly influence other physical properties of the mineral. The carbon allotropes diamond and graphite have vastly different properties; diamond is the hardest natural substance, has an adamantine lustre, and belongs to the isometric crystal family, whereas graphite is very soft, has a greasy lustre, and crystallises in the hexagonal family. This difference is accounted for by differences in bonding. In diamond, the carbons are in sp³ hybrid orbitals, which means they form a framework where each carbon is covalently bonded to four neighbours in a tetrahedral fashion; on the other hand, graphite is composed of sheets of carbons in sp² hybrid orbitals, where each carbon is bonded covalently to only three others. These sheets are held together by much weaker van der Waals forces, and this discrepancy translates to large macroscopic differences.
  Twinning is the intergrowth of two or more crystals of a single mineral species. The geometry of the twinning is controlled 
caption: Contact twins, as seen in spinel
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