Pegmatite
Collectible rocks, but also mined for their rare elements
Pegmatites are igneous intrusive rocks, like granite, but unusual because the crystals are so large. These rocks are sought after by collectors for big, showy mineral specimens. Large crystals occur because water in the magma allowed ions to find optimal places in the crystal lattice to attach. Pegmatites are valuable as ores of rarer elements like lithium, titanium, rubidium, beryllium, tantalum, niobium, tin, boron, and zirconium. These are “incompatible” elements that were unable to crystallize as ordinary granite and were excluded from the melt. The large black minerals in this rock are the titanium mineral schorl, a variety of tourmaline. The grayish matrix is quartz.
This specimen was collected by CSC students from a pegmatite mine near Pringle, South Dakota.