Editing Ulexite (section)

== Structure ==
Ulexite crystals contain three structural groups, isolated pentaborate [[polyanions]], calcium coordinated polyhedra, and sodium coordinated octahedra that are joined together and cross-linked by hydrogen bonding. The Ca-coordination polyhedra share edges to form chains which are separate from the Na-coordination octahedral chains. There are 16 distinct hydrogen bonds that have an average distance of 2.84 Å. Boron is coordinated to four oxygens in a tetrahedra arrangement and also to three oxygens in a triangular arrangement with average distances of 1.48 and 1.37 Å, respectively. Each Ca<sup>2+</sup> cation is surrounded by a polyhedron of eight oxygen atoms. The average distance between calcium and oxygen is 2.48 Å. Each Na<sup>+</sup> is coordinated by an octahedron of two hydroxyl oxygens and four water molecules, with an average distance of 2.42 Å (Clark and Appleman 1964).<ref>{{cite journal |last1=Clark |first1=Joan R. |last2=Appleman |first2=Daniel E. |title=Pentaborate polyanion in the crystal structure of ulexite, NaCaB<sub>5</sub>O<sub>6</sub>(OH)<sub>6</sub>·5H<sub>2</sub>O |journal=Science |date=1964 |volume=145 |issue=3638 |pages=1295–1296|doi=10.1126/science.145.3638.1295 |pmid=17802011 |bibcode=1964Sci...145.1295C |s2cid=98456855 }}</ref> The octahedral and polyhedral chains parallel to c, the elongate direction, cause the fibrous habit of ulexite and the fiber optical properties.<ref name="Ghose1978" />