Dumortierite occurs as small nodules or thin layers along horizontal fractures in the altered volcanic rocks of the Milyang clay deposit, southeastern Korea. Textural evidence shows that dumortierite is associated with pyrophyllite, suggesting that the silica activity increased with decreasing B and Al activities during the hydrothermal alteration. Dumortierite chemistry shows a slight excess of Al, possibly due to common occurrence of Al-rich minerals in the clay deposit. The structural formula of dumortierite is recalculated as Al6.93(Fe0.03Mg0.07)B(Si2.99Al0.01)O18. An absorption peak at 1387 cm in the IR spectrum indicates that B is present in three-fold coordinated sites. The 29Si MAS NMR spectrum has two peaks at −90.9 and −95.5 ppm, showing that Si in dumortierite is located in two different crystallographic sites and is linked to four octahedral Al atoms. The 27Al MAS NMR spectrum shows only one asymmetrical peak near 0 ppm, suggesting that there is no substitution of Al for Si. The 11B MAS NMR spectrum at 17.5 ppm shows a typical second order quadrupole pattern represented by the three-fold coordinated B. The 11B chemical shifts indicate that all of the B occupies such sites, indicating that the B sites are axially symmetric or nearly so in a very well defined site.