Crystal chemistry and Raman spectroscopy study of bennesherite, Ba2Fe2+Si2O7, and rare accessory Ba minerals from Caspar quarry, Bellerberg volcano, Germany

Abstract

AbstractBarium melilite – bennesherite, Ba2Fe2+Si2O7, known only from pyrometamorphic rocks of the Hatrurim Complex in Israel, has been recognised in a carbonate–silicate xenolith from the Bellerberg volcano area in Germany. The empirical formula of the German specimen is as follows: (Ba1.32Ca0.43Sr0.23Na0.05K0.02)Σ2.05(Fe2+0.79Ti0.06Mg0.05Al0.04Mn0.03Zn0.01)Σ0.98Si1.97O7. The Raman spectrum of bennesherite exhibits the presence of the main vibrations related to Fe2+O4 tetrahedra and disilicate Si2O7 groups at the T1 and T2 sites, at 589 cm–1 and in the range 618–673 cm–1, respectively. Detailed spectroscopic analyses performed for bennesherite in two different and random orientations confirm the reduction of bands intensity and the number of some components in several spectral ranges. Moreover, the presence of a heavy Ba atom indicates a decrease in band frequencies compared to melilites with Ca at the X position. A single-crystal X-ray diffraction experiment, despite attempts, could not be carried out due to the poor quality and small size of the bennesherite crystals however, a combination of composition and Raman data allowed for accurate phase identification. Detailed mineralogical investigations distinguished rare Ba minerals in association with bennesherite, such as walstromite, fresnoite and celsian, along with various ferrous melilites. Some of the detected phases are described from xenoliths of the Bellerberg volcano for the first time. The uniqueness of the Bellerberg volcano mineralisation is reflected in the interaction of alkaline magma with xenoliths of different compositions, which suggests that this locality still deserves attention as a source of new and unique minerals.