The optical absorption spectrum of rotherkopfite in the polarization direction in which the Fe2+-Ti4+ intervalence bands absorption centered at about 450 nm was apparent.
Anthony R. Kampf1, Gerhard Möhn2,
Chi Ma3, George R. Rossman3, Joy Désor4 and Yunbin
Guan3
1 Mineral Sciences Department, Natural
History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA
90007, USA
2 Dr.-J.-Wittemannstrasse 5, 65527
Niedernhausen, Germany
3 Division of Geological and Planetary
Sciences, California Institute of Technology, Pasadena, California 91125, USA
4 Im Langenfeld 4, 61350 Bad Homburg,
Germany
Rotherkopfite, KNa2(Fe2+2.5Ti4+1.5)Fe2+(Si4O12)2, is a new member of the neptunite group, from Rother Kopf, Roth, near Gerolstein, Eifel Volcanic Fields, Rhineland-Palatinate, Germany. It is associated with fluorophlogopite and a potentially new amphibole in cavities in a quartz-sanidine xenolith embedded in a vesicular alkaline basalt. It is is presumed to have formed as the result of contact metasomatism of the xenolith by the alkaline basalt melt. Rotherkopfite occurs as brownish red equant or tabular crystals, up to about 0.2 mm in maximum dimension. The mineral has light-orange streak, vitreous lustre, a Mohs hardnes of ~3½, brittle tenacity, curved fracture and a density of 3.20(2) g∙cm-3. Optically, rotherkopfite crystals are biaxial (+), with α = 1.668(5), β = 1.678(5), γ = 1.720(5) (white light) and 2V(meas) = 53.2(6)°. The empirical formula from electron microprobe analyses and structure refinement is (K0.87Na0.20)Σ1.07(Na1.99Ca0.01)Σ2.00 M1+M2(Fe2+1.66Ti1.48Mg0.79Mn0.02)Σ3.95M3(Fe2+0.64Li0.16Ti0.15Al0.01)Σ0.96(Si8.00O24). Rotherkopfite is monoclinic with space group C2/c and unit-cell parameters a = 16.4599(17), b = 12.5457(6), c = 10.0487(7) Å, β = 115.669(7)°, V = 1870.3(3) Å3 and Z = 4. The crystal structure (R1 = 0.0268 for 1324 reflections with I > 2sI) is based on two interwoven three-dimensional frameworks: (1) a silicate framework made up of pyroxene-like chains of corner-sharing SiO4 tetrahedra and (2) an octahedral framework made up chains of edge-sharing metal-oxygen octahedra. The two interwoven frameworks are bound to one another by corner sharing. K and Na are hosted in channels in the combined framework.