Editing Mineralogy (section)

==Crystal structure==
[[File:Perovskite.jpg|thumb|The [[perovskite structure|perovskite crystal structure]]. The most abundant mineral in the Earth, [[bridgmanite]], has this structure.<ref name=Sharp>{{cite journal|last1=Sharp|first1=T.|title=Bridgmanite – named at last|journal=Science|date=27 November 2014|volume=346|issue=6213|pages=1057–1058|doi=10.1126/science.1261887|pmid=25430755|bibcode=2014Sci...346.1057S|s2cid=206563252}}</ref> Its chemical formula is (Mg,Fe)SiO<sub>3</sub>; the red spheres are oxygen, the blue spheres silicon and the green spheres magnesium or iron.]]
{{main article|Crystal structure}}
{{See also|Crystallography}}
The crystal structure is the arrangement of atoms in a crystal. It is represented by a [[Crystal lattice|lattice]] of points which repeats a basic pattern, called a [[unit cell]], in three dimensions. The lattice can be characterized by its symmetries and by the dimensions of the unit cell. These dimensions are represented by three ''[[Miller index|Miller indices]]''.<ref name=Ashcroft>{{cite book|last1=Ashcroft|first1=Neil W.|last2=Mermin|first2=N. David|title=Solid state physics|date=1977|publisher=Holt, Rinehart and Winston|location=New York|isbn=9780030839931|edition=27. repr.|url-access=registration|url=https://archive.org/details/solidstatephysic00ashc}}</ref>{{rp|91&ndash;92}} The lattice remains unchanged by certain symmetry operations about any given point in the lattice: [[Reflection symmetry|reflection]], [[Rotational symmetry|rotation]], [[Point reflection|inversion]], and [[Improper rotation|rotary inversion]], a combination of rotation and reflection. Together, they make up a mathematical object called a ''[[crystallographic point group]]'' or ''crystal class''. There are 32 possible crystal classes. In addition, there are operations that displace all the points: [[Translational symmetry|translation]], [[screw axis]], and [[glide plane]]. In combination with the point symmetries, they form 230 possible [[space group]]s.<ref name=Ashcroft/>{{rp|125&ndash;126}}

Most geology departments have [[X-ray]] [[powder diffraction]] equipment to analyze the crystal structures of minerals.<ref name=Klein/>{{rp|54&ndash;55}} X-rays have wavelengths that are the same order of magnitude as the distances between atoms. [[Diffraction]], the constructive and destructive interference between waves scattered at different atoms, leads to distinctive patterns of high and low intensity that depend on the geometry of the crystal. In a sample that is ground to a powder, the X-rays sample a random distribution of all crystal orientations.<ref name=Dinnebier>{{cite book|last1=Dinnebier|first1=Robert E.|last2=Billinge|first2=Simon J.L.|chapter=1. Principles of powder diffraction|editor-last1=Dinnebier|editor-first1=Robert E.|editor-last2=Billinge|editor-first2=Simon J.L.|title=Powder diffraction : theory and practice|url=https://archive.org/details/powderdiffractio00redi|url-access=limited|date=2008|publisher=Royal Society of Chemistry|location=Cambridge|isbn=9780854042319|pages=[https://archive.org/details/powderdiffractio00redi/page/n23 1]&ndash;19|edition=Repr.}}</ref> Powder diffraction can distinguish between minerals that may appear the same in a hand sample, for example [[quartz]] and its polymorphs [[tridymite]] and [[cristobalite]].<ref name=Klein/>{{rp|54}}

[[Isomorphism (crystallography)|Isomorphous]] minerals of different compositions have similar powder diffraction patterns, the main difference being in spacing and intensity of lines. For example, the {{chem2|auto=1|NaCl}} ([[halite]]) crystal structure is space group ''Fm3m''; this structure is shared by [[sylvite]] ({{chem2|auto=1|KCl}}), [[periclase]] ({{chem2|auto=1|MgO}}), [[bunsenite]] ({{chem2|auto=1|NiO}}), [[galena]] ({{chem2|auto=1|PbS}}), [[alabandite]] ({{chem2|auto=1|MnS}}), [[chlorargyrite]] ({{chem2|auto=1|AgCl}}), and [[Titanium nitride|osbornite]] ({{chem2|auto=1|TiN}}).<ref name=Manual>{{cite book|last1=Klein|first1=Cornelis|last2=Hurlbut|first2=Cornelius S. Jr.|title=Manual of mineralogy : (after James D. Dana)|date=1993|publisher=Wiley|location=New York|isbn=047157452X|edition=21st}}</ref>{{rp|150&ndash;151}}