Due to the similar ionic radius of K+ and NH4+, K-silicates can incorporate a significant amount of NH4. As tourmaline is able to accommodate K in its crystal structure at high and ultrahigh pressure, we test if this also holds true for NH4.
Piston-cylinder experiments in the system (NH4)2O-MgO-SiO2-Al2O3-B2O3-H2O at 4.0 GPa, 700 °C, with B2O3 and NH4OH in excess produce an assemblage of tourmaline, phengite, and coesite. The tourmaline crystals are up to 10 × 40 μm in size. EMP analyses indicate that the tourmalines contain 0.22 (±0.03) wt% (NH4)2O and are solid solutions mainly along the magnesio-foitite and “NH4-dravite” join with the average structural formula X[(NH4)0.08(1)□0.92(1)]Y[Mg2.28(8)Al0.72(8)]Z[Al5.93(6)Si0.07(6)]T[Si6.00(5)O18](BO3)3(OH)4.
NH4 incorporation is confirmed by characteristic <N-H> stretching and bending modes in the IR-spectra of single crystals on synthetic tourmaline. Further evidence is the increased unit-cell parameters of the tourmaline [a = 15.9214(9) Å, c = 7.1423(5) Å, V = 1567.9(2) Å3] relative to pure magnesio-foitite.
Incorporation of NH4 in natural tourmaline was tested in a tourmaline-bearing mica schists from a high-P/low-T (>1.2 GPa/550 °C) metasedimentary unit of the Erzgebirge, Germany, rich in NH4. The NH4-concentrations in the three main NH4-bearing phases are: biotite (~1400 ppm) > phengite (~700 ppm) > tourmaline (~500 ppm). This indicates that tourmaline can act as important carrier of nitrogen between the crust and the deep Earth, which has important implications for a better understanding of the large-scale light element cycle.