This specimen is illustrated on page 185, Fig. 12-6a of London's book called "Pegmatites".
This specimen of Schorl tourmaline exhibits the intergrowth with Quartz much like Quartz -Microcline graphite granite. One of the interesting features of Schorl in pegmatites is the nucleation of tourmaline along the contact zone of pegmatite with the wall-rock. At this contact tourmaline begins to grow rapidly toward the center of the pegmatite and perpendicular to the wall. Tourmaline is a cyclosilicate and crystallizes like a spiral outward from that original point of nucleation with the wall. All crystals of Schorl will compete for the boron in solution and rapid crystallization produces divergent crystals.
Illustrated in London's book "Pegmatites"
Schorl
Formula
NaFe2+3Al6(BO3)3Si6O18(OH)4
Crystal System
Trigonal
Crystal Habit
Acicular, Prismatic, Striated
Cleavage
Indistinct, None, None
Luster
Vitreous (Glassy)
Color
black, brownish black, greenish black, bluish black
