This study presents petrological and whole-rock geochemical data on basaltic lavas from the Tewa Zobar region of the northwestern Ethiopian plateau to investigate their petrogenesis, with petrographic analysis indicating the lavas consist of plagioclase, pyroxene, olivine, and Fe-Ti oxide minerals, exhibiting aphanitic, porphyritic, and glomeroporphyritic textures. Most of the basaltic samples (MgO = 7.4 wt. % - 7.9 wt. %) show alkaline composition, with the exception of one sample with lower MgO content of 6.4 wt.% showing a sub-alkaline nature. The alkaline basalts show steep primitive mantle-normalized trace element patterns with elevated high-field strength elements (HFSE) and heavy rare earth element (REE) profiles, including higher (Sm/Yb)_N, (La/Yb)_N, and (La/Sm)_N values, whereas the sub-alkaline rock sample #Z56 has flatter heavy-REE profiles with lower (Sm/Yb)_N, (La/Yb)_N, and (La/Sm)_N values. Based on the REE model, the sub-alkaline basalt suggests a melt fraction of 3-4%, while the alkaline basalt indicates a lower melt fraction of 1-2%, accompanied by approximately 1-2% residual garnet of enriched mantle source. These basalts also exhibit enriched trace element patterns, with the (Nb/Th)_PM ratios typically showing a higher range, while the sub-alkaline basalt displays a notably lower Nb/Th ratio. Additionally, the alkali basalts generally have a higher Nb content relative to La, with (Nb/La)_PM ratios showing a narrow range, while the sub-alkaline basalt presents a lower Nb/La ratio. The alkaline basalts analyzed, with element ratios such as La/Nb, Ba/Nb, and Ba/La —excluding Sample Z56, which shows distinct values—fall within the range of mantle-derived basalts, indicating the absence of crustal contamination. The middle rare earth element (MREE)/heavy rare earth element (HREE) and light rare earth element (LREE)/HREE ratios indicate that the basaltic magma was derived from a mantle source in the garnet-spinel transition zone at ∼80 km depth, with small degree of melting and significant contributions from both subcontinental lithospheric mantle (SCLM) and asthenospheric sources. Elevated Zr/Hf values and high incompatible trace element enrichment suggest that small-volume metasomatic fluids may have enhanced the source region. The elevated Nb/Ta ratio in the mafic lavas suggests the magma likely originated from a SCLM, potentially modified by metasomatic processes involving carbonatite-like component, while the geochemical similarities between the alkaline basalts and high-Ti 1 (HT1) magmatism point to a connection with mantle plume activity, which may have influenced the recent eruptions on the northwestern Ethiopian plateau.

The hydrothermal treatment of a glass with a molar ratio of Rb₂O : CaO : SiO₂ = 1 : 4 : 6 at 400 °C and 500 bar resulted in the formation of single-crystals of Rb_1.71Ca₄[Si₆O₁₅(O_0.855(OH)_0.145)₂]2H₂O or [Ca₄Si₆O₁₅(O_0.855(OH)_0.145)₂]•(Rb_1.71•2H₂O). The basic crystallographic data of this phase at room conditions are as follows: space group P 2₁/m, a = 6.7766(4), b = 22.3162(10), c = 6.7782(4) Å, β = 114.056(7)°, V = 936.02(8) ų, Z = 2. A striking feature of the crystals is a polysynthetic twinning, clearly observable under a petrographic microscope. The diffraction patterns of all investigated samples can be explained as a superposition of two reciprocal lattices, with a two-fold axis parallel to [101] being the twin element. Synchrotron radiation was used to determine the crystal structure from a data set collected at the X06DA beamline of the Swiss Light Source, Paul Scherrer Institute. Least-squares refinements resulted in a residual of R(|F|)=0.073 for 1712 reflections and 163 parameters. According to Liebau’s crystal chemical classification, the compound can be described as an unbranched dreier double chain silicate. The bands run parallel to [101] and are formed by the condensation of two wollastonite-type single-chains. The two calcium cations within the asymmetric unit are coordinated by seven ligands each. The [CaX₇]-groups (X: O^2-, OH^-, H₂O) are linked into layer-like units parallel to (010) by sharing common edges. Adjacent layers are connected by the silicate ribbons, resulting in a negatively charged heteropolyhedral network enclosing tunnel-like cavities. The rubidium atoms are distributed among a total of four partially occupied and mutually exclusive extra-framework positions within these channels. Each of the monovalent cation positions is coordinated by eight ligands, providing additional direct linkage between the network-forming polyhedra. The crystal structure of Rb_1.71Ca₄[Si₆O₁₅(O_0.855(OH)_0.145)₂]2H₂O is closely related to natural paratobermorite, a hydrous calcium silicate of the tobermorite supergroup of minerals. The paper will cover the common features of both phases and the differences between the compounds, including aspects of OD-theory.