Crystal structure, chemical composition, and twinning of götzenite and wöhlerite from the Fohberg phonolite, Kaiserstuhl

Götzenite and wöhlerite were found as part of a fissure assemblage in the Fohberg phonolite (Kaiserstuhl, SW Germany), in close association with natrolite and clinopyroxene (aegirine–augite). Crystal grains were separated and investigated by single-crystal X-ray diffraction (SXRD), electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), showing the presence of two intimately intergrown phases, götzenite and wöhlerite. SXRD analyses showed that both minerals are twinned. Götzenite (Na1.50Ca5.18Sr0.13Fe0.032+Mn0.01Zr0.06La0.08Ce0.11Nd0.02Ti0.81Nb0.19(Si2O7)2O1.2F2.8) shows rotation twinning on [001] according to -a-1/2c, −b, c, with contributions of 40 % and 60 % from the two twin domains, respectively. Applying the twin law to the diffraction analysis, the crystal structure was refined to R1 (Fo >4σ (Fo)) = 3.0 %, with a=9.6191(3) Å, b=5.7342(2) Å, c=7.3386(2) Å, α=89.986(1)°, β=101.040(1)°, γ=100.485(1)°, and V=390.40(3) Å3 in space group P1‾. Wöhlerite (Na1.63Ca4.37Sr0.04Zr0.63Fe0.232+Mn0.09Ce0.01Ta0.01Nb0.79Ti0.20(Si2O7)2O2.6F1.4) shows reflection twinning on (100) according to -a-c, b, c, with contributions of 31 % and 69% from the two twin domains, respectively, and with lattice parameters of a=10.842(1) Å, b=10.249(1) Å, c=7.2673(8) Å, β=109.343(4)°, and V=761.9(2) Å3 in the monoclinic space group P21, refined to R1 = 1.3 %. Refractive indices of götzenite were measured using the immersion method yielding nx=1.662(2), ny=1.663(2), nz=1.670(2), and 2V=61(2)°. Optical measurements on the twinned crystal were possible because of the coincidence of the two indicatrices related to each other by rotation about nx being parallel to [001], simulating a unique extinction behavior. Wöhlerite could not be optically examined because of the polysynthetic twinning not showing this effect.