Tourmaline

In most of the well-drained soils formed from acid rocks and metamorphic sediments, tourmaline is the most common boron-containing mineral identified (53). The name tourmaline represents a group of minerals that are compositionally complex borosilicates containing approximately 3% B. The tourmaline structure has rhombohedral symmetry and consists of linked sheets of island units. The boron atoms are found within BO₃ triangles, forming strong covalent B-O bonds (54). Tourmalines are highly resistant to weathering and virtually insoluble. Additions of finely ground tourmaline to soil failed to provide sufficient boron to alleviate boron deficiency of crop plants (55).