Hopmannite, Ba2(Ti5Fe)O13, a new hexatitanate from the Bellerberg volcano, Germany

Abstract Hopmannite, ideally Ba 2 (Ti 5 Fe)O 13 , is a new Ba-dominant hexatitanate discovered in xenoliths enclosed within the basaltic host rocks of the Bellerberg volcano in Germany. The mineral exhibits black to dark-grey thin platy crystals, with submetallic lustre and a perfect cleavage along (100). The empirical formula of the holotype hopmannite, calculated on the basis of 8 cations and 13 O atoms, is (Ba 1.00 K 0.61 Na 0.24 Sr 0.08 Ca 0.04 ) Σ1.97 (Ti 5.36 Fe 2+ 0.48 Fe 3+ 0.13 Mg 0.06 ) Σ6.03 O 13 . Hopmannite is monoclinic, space group C 2/ m , with a = 15.357(2) Å, b = 3.8500(7) Å, c = 9.129(2) Å, β = 99.033(10)°, V = 533.03(17) Å 3 and Z = 2 and is isostructural with nixonite, Na 2 Ti 6 O 13 and jeppeite, K 2 Ti 6 O 13 . The structure of hopmannite consists of a fundamental unit composed of three edge-shared Ti-octahedra duplicated above and below in the direction of the b -axis, which form an open octahedral framework enclosing a tunnel, cubic-like sites occupied by Ba, K, and Na. The structural results are consistent with the chemical data and show that the Ba 2+ content is greater than K + and Na + at the A-cation site. Spectroscopic analyses indicated that the presence of the main vibrations is related to TiO 6 octahedra, including Ti-O and Ti-O-Ti stretching vibrations. Hopmannite, along with fresnoite and batisite, represents a mineral assemblage formed late in the crystallisation sequence at temperatures below 1000 °C from a highly alkaline residual melt enriched in incompatible elements such as Ba, Sr, Ti, Zr, and P.