The Chemistry and Crystal Structure of Potassic-Fluoro-Richterite from Rother Kopf, Eifel Volcanic Fields, Germany

Abstract Potassic-fluoro-richterite, ideally AKB(CaNa)CMg5TSi8O22F2, from Rother Kopf, Eifel Volcanic Fields, Germany, is monoclinic, C2/m, a = 9.9615(6), b = 18.0526(9), c = 5.2846(4) Å, β = 104.484(7)°, V = 920.13 Å3, Z = 2. Chemical analysis by electron microprobe gave SiO2 54.01, Al2O3 0.21, TiO2 1.24, FeO 11.15, MnO 0.10, MgO 16.13, CaO 5.07, Na2O 5.55, K2O 2.99, F 4.50, O ≡ F −1.89, sum 99.06 wt.%. The formula unit, calculated on the basis of 24 (O + OH + F) with F = 2 apfu and and Fe3+ = 0.0 apfu (determined from the <M(2)–O> distance) is A(K0.56Na0.43)B(Na1.18Ca0.80Mn0.01Fe2+0.01)C(Mg3.53Fe2+1.37Mn2+0.01Ti0.11)T(Si7.93Al0.04Ti4+0.03)O22F2. The crystal structure was refined to an R1 index of 3.87% for 1014 reflections Fo > 4σF. A very unusual feature of this amphibole is that, unlike most amphiboles, the A-cations occupy the A(2/m) site within the A-cavity. Examination of the T(1)–O(7) and A(2/m)–O(7) distances in several hydroxy- and fluoro-amphiboles with TSi = 8 apfu shows that the H···O(7) bond in richterite and potassic-richterite is of negligible strength in A-site-vacant amphiboles in accord with the principal OH-stretching frequencies in these minerals.