Foitite-magnesio-foitite from the Rózsa Hill ore mineralization, or re-investigation of the tourmaline from Nagybörzsöny, Börzsöny Mts., Hungary
Abstract
The presence of tourmaline at the Nagybörzsöny ore mineralization (Börzsöny Mts., Hungary) has been known for 60 years. On the northern part of the Rózsa Hill, at the area of the so called Rózsa Mine, the tourmaline forms acicular crystals up to 1 mm in length, as well as acicular-radial and felt-like clusters in the cavities of a strongly silicified rock, or it appears as inclusions in the subsequently formed minerals (quartz, calcite, siderite, pyrrhotite, sphalerite). Because of the former wet chemical analysis, the Nagybörzsöny tourmaline appeared as dravite in the literature. According to the new electron-microprobe analyses, the tourmaline is zoned and represents the foitite (core) and magnesio-foitite (rim) mineral species. The compositions of Nagybörzsöny tourmaline, measured at the highest concentrations of iron and magnesium, are the followings (based on 31 anions): ([]0.63Na0.29Ca0.08)Σ=1.00 (Fe2+1.26Mg0.89Al0.79)Σ=2.94 Al6.00 (Si5.88Al0.12)Σ=6.00 O18 (BO3)3 (OH)4.00, and ([]0.45Na0.37Ca0.18)Σ=1.00 (Mg1.55Fe2+0.48Al0.83Ti0.01)Σ=2.87 Al6.00 (Si5.67Al0.33)Σ=6.00 O18 (BO3)3 (OH)4.00. Lattice parameters, calculated from the X-ray powder-diffraction data, are: a = 15.901(6) Å; c = 7.190(4) Å, V = 1574(1) Å3. The tourmaline precipitated from hydrothermal solution, whose temperature could be above 260°C. Chemistry of the tourmaline was determined mainly by the composition of the hydrothermal fluid, in this respect the composition of the host rock, as well as the prevailing conditions of temperature and pressure could only be secondary. Chemical zoning of the crystals is primarily due to the Fe ↔ Mg substitution. This substitution is interpreted as a record of mixing of Fe-rich hydrothermal fluid and Mg- and Ca-rich descending waters. Acicular habit of the tourmaline crystals can be linked to crystal-chemical reasons, since the high Al-content and the relatively low (Fe+Mg)-content facilitated the rapid growth of the crystals in the direction of the c-axis.
