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

Potassic-fluoro-richterite, ideally ^AK^B(CaNa)^CMg₅^TSi₈O₂₂F₂, 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 ų, 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(K_0.56Na_0.43)^B(Na_1.18Ca_0.80Mn_0.01Fe²⁺_0.01)^C(Mg_3.53Fe²⁺_1.37Mn²⁺_0.01Ti_0.11)^T(Si_7.93Al_0.04Ti⁴⁺_0.03)O₂₂F₂. The crystal structure was refined to an R₁ index of 3.87% for 1014 reflections F_o > 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.