Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Rafał Juroszek, Krystian Prusik, Christof Schäfer

Alfredcasparite, Sr₂TiO(Si₂O₇), a new mineral from the Caspar quarry, Bellerberg volcano, Germany, and new data on wesselsite, SrCuSi₄O₁₀

Journal of Geosciences, volume 69 (2024), issue 3, 161 - 172

DOI: http://doi.org/10.3190/jgeosci.394

The new mineral alfredcasparite, ideally Sr₂TiO(Si₂O₇), is a Sr-analogue of fresnoite Ba₂TiO(Si₂O₇). It is an accessory phase, which was found in a silicate xenolith within the tephritic lava from the Caspar quarry, Bellerberg volcano, Germany. Usually, alfredcasparite occurs together with potentially new mineral (Sr₂Na)Ti(Si₂Fe)Si₂O₁₄, in small cracks in the K-feldspar (sanidine)-quartz-pyroxene (aegirine-diopside)-matrix, and wollastonite. In the type material, alfredcasparite rarely forms flattened crystals up to 30 μm in diameter; more common are irregular grains which do not exceed 15 μm in size. Alfredcasparite is colourless, transparent, and has a vitreous lustre and white streak. It is brittle with an irregular fracture and exhibits good cleavage on (001). The calculated density equals 3.950 g‧cm^-3 and the Mohs hardness is ~3-4. Optically, alfredcasparite is uniaxial and non-pleochroic under transmitted light (n_mean = 1.823). The empirical formula calculated based on 8 O atom per formula unit (pfu) is (Sr_1.54Ba_0.29Ca_0.10Na_0.03K_0.03)_∑1.99(Ti_0.94Fe_0.04Mg_0.03)_∑1.01(Si_2.04Al_0.01)_∑2.05O₈, which leads to the ideal end-member formula Sr₂TiO(Si₂O₇). According to the electron back-scattered diffraction (EBSD) pattern, tetragonal alfredcasparite fit to the using structural model of synthetic Sr₂TiO(Si₂O₇) with the following parameters: space group P4bm, a = 8.32 Å, c = 5.02 Å, V = 347.77 ų, Z = 2. The crystal structure consists of layers composed of corner-sharing TiO₅ square-base pyramids and Si₂O₇ disilicate units, with interstices occupied by Sr²⁺ cations. The Raman spectrum of alfredcasparite is characterised by a sharp and intense Raman band at 863 cm^-1 with lower-intensity shoulder bands at ∼850 and 874 cm^-1, which have a complex nature and are assigned to both symmetric stretching SiO₃ vibrations of the disilicate group (Si₂O₇)^6- and Ti-O vibrations in the square pyramid (TiO₅)^6-. The authors assume that alfredcasparite forms as an effect of residual melt crystallisation enriched in incompatible elements like Ba, Sr, Ti, or P at a temperature around 1000 ˚C. In addition, based on chemical EPMA and Raman spectroscopy investigations, the second world occurrence of wesselsite, SrCuSi₄O₁₀, was confirmed in the same xenolith. Chemical analyses resulted in the empirical formula (based on 10 O pfu) (Sr_0.67Ba_0.28Ca_0.04Na_0.01K_0.01)_∑1.01 (Cu_0.93Mg_0.04Fe_0.02)_∑0.99Si_4.01O₁₀. Furthermore, the Raman spectrum of natural wesselsite was obtained and described in detail for the first time, in relation to its crystal structure.