Crystal structure of the mineral (Na,Ca,K)2(Ca,Na)4(Mg,Fe)5(Mg,Fe,Ti)5[Si12Al4O44](F,O)4: a triclinic representative of the amphibole family

A mineral belonging to the amphibole family found at the Rothenberg paleovolcano (Eifel, Germany) was studied by single-crystal X-ray diffraction. The triclinic pseudomonoclinic unit-cell parameters are a = 5.3113(1) ?, b = 18.0457(3) ?; c = 9.8684(2) ?, ?? = 90.016(2)°, ?? = 105.543(4)°, ?? = 89.985(2)°. The structure was solved by direct methods in sp. gr. P1 and refined to the R factor of 2.7% based on 6432 reflections with |F| > 3??(F) taking into account twinning. The mineral with the idealized formula (Na,Ca,K)₂(Ca,Na)₄(Mg,Fe)₅(Mg,Fe,Ti)₅[Si₁₂Al₄O₄₄](F,O)₄ has some symmetry and structural features that distinguish it from other minerals of this family.