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 (Na 1.50 Ca 5.18 Sr 0.13 Fe0.032+Mn 0.01 Zr 0.06 La 0.08 Ce 0.11 Nd 0.02 Ti 0.81 Nb 0.19 (Si 2 O 7 ) 2 O 1.2 F 2.8 ) 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 R 1 ( F o > 4 σ ( F o)) = 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) Å 3 in space group P1‾. Wöhlerite (Na 1.63 Ca 4.37 Sr 0.04 Zr 0.63 Fe0.232+Mn 0.09 Ce 0.01 Ta 0.01 Nb 0.79 Ti 0.20 (Si 2 O 7 ) 2 O 2.6 F 1.4 ) 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) Å 3 in the monoclinic space group P 2 1 , refined to R 1 = 1.3 %. Refractive indices of götzenite were measured using the immersion method yielding n x =1.662(2), n y =1.663(2), n z =1.670(2), and 2 V =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 n x being parallel to [001], simulating a unique extinction behavior. Wöhlerite could not be optically examined because of the polysynthetic twinning not showing this effect.