Tetrahedral aluminum in tourmaline from a spinel-pargasite-metamorphosed mafic-ultramafic rock

Abstract

Tourmaline is a widespread borosilicate mineral, which is well known for its variable chemistry. Although major amounts of octahedral Al in tourmaline is commonplace, the occurrence of significant amounts of tetrahedral Al is relatively rare. This paper focuses on tourmaline from the collection of the A.E. Fersman Mineralogical Museum (Russia) originated from Italy with up to 25% of Si replaced by Al at the tetrahedral site. The tourmaline is characterized by optical and scanning electron microscopy, Raman spectroscopy, infrared spectroscopy, Mössbauer spectroscopy, energy-dispersive and wavelength-dispersive X-ray analysis, laser ablation inductively coupled plasma optical emission spectrometry and single crystal X-ray diffraction. The studied tourmaline occurs as transparent dark blue crystals (with equant external morphology) up to 3 mm in size and forms veinlets cutting a (Mg,Al)-rich metamorphosed mafic-ultramafic rock (Mg≫Fe) composed of spinel, pargasite, clinochlore, phlogopite, and hydroxylapatite. The studied tourmaline meets the criteria defining magnesio-lucchesiite and can be compositionally formed via Tschermak-like ([6]Me2++[4]Si4+↔[6]Al3++[4]Al3+, where [6]Me2+=Mg, Fe) or plagioclase-like ([9]Ca2+ + [4]Al3+ ↔ [9]Na+ + [4]Si4+) substitutions. Zones with a relatively high Si content (Si-rich) have pronounced indications of dissolution, while silicon-depleted zones (Si-poor) overgrow Si-rich zones and eventually creates a visible replacement zone of the crystal. We suggest that Si-poor tourmaline result from the Si-rich tourmaline losing Si during a metasomatic process. The resulting empirical crystal-chemical formula for the Si-poor zone is: X(Ca0.95Na0.03□0.02)Σ1.00 Y(Mg1.08Al0.98Fe2+0.50Fe3+0.43)Σ3.00 Z(Al5.91Fe3+0.09)Σ6.00 T[(Si4.57Al1.43)Σ6.00O18] (BO3)3 V(OH)3 W[O0.95(OH)0.05]Σ1.00 (a = 15.9811(2), c = 7.12520(10) Å, R1 = 1.7 %) and for the Si-rich zone is: X(Ca0.89Na0.11)Σ1.00 Y(Mg1.55Al0.80Fe2+0.34Fe3+0.31)Σ3.00 Z(Al5.51Mg0.44Fe3+0.05)Σ6.00 T[(Si5.35Al0.65)Σ6.00O18] (BO3)3V(OH)3W[O0.93(OH)0.07]Σ1.00 (a=15.9621(3), c=7.14110(10) Å, R1=1.7 %). According to PT calculations of mineral assemblage stability and comparable data on synthetic [4]Al-rich tourmalines, the studied tourmaline was formed at 600–750 °C and 0.10-0.20 GPa. The formation of tetrahedral Al-rich tourmalines requires several unusual factors: (1) desilication of primary rocks and (2) high temperatures and relatively low pressures.