Editing Strontianite (section)

== Structure ==
Strontianite is [[isostructural]] with [[aragonite]].<ref name=MOM>Klein and Hurlbut (1993) Manual of Mineralogy 21st Edition.  Wiley</ref>  When the CO<sub>3</sub> group is combined with large [[divalent]] [[Ion|cations]] with [[Ionic radius|ionic radii]] greater than 1.0&nbsp;Å, the radius ratios generally do not permit stable [[Coordination number|6-fold coordination]].  For small cations the structure is [[Rhombohedron|rhombohedral]], but for large cations it is [[Orthorhombic crystal system|orthorhombic]].  This is the aragonite structure type with [[space group]] Pmcn.  In this structure the CO<sub>3</sub> groups lie perpendicular to the c [[Crystal structure|axis]], in two structural planes, with the CO<sub>3</sub> triangular groups of one plane pointing in opposite directions to those of the other.<ref name=MOM/> These layers are separated by layers of cations.<ref name=Dana/>

The CO<sub>3</sub> group is slightly non-planar; the [[carbon]] [[atom]] lies 0.007 Å out of the plane of the [[oxygen]] atoms.  The groups are tilted such that the angle between a plane drawn through the oxygen atoms and a plane parallel to the [[Crystal structure#Unit cell|a-b unit cell plane]] is 2°40’.<ref name=AM56>De Villiers, Johan P R (1971) Crystal Structures of Aragonite, Strontianite and Witherite.  The American Mineralogist 56:758</ref>