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

Abstract

Hopmannite, ideally Ba₂(Ti₅Fe)O₁₃, 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_1.00K_0.61Na_0.24Sr_0.08Ca_0.04)_Σ1.97(Ti_5.36 Fe²⁺_0.48Fe³⁺_0.13Mg_0.06)_Σ6.03O₁₃. Hopmannite is monoclinic, space group C2/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) ų and Z = 2 and is isostructural with nixonite, Na₂Ti₆O₁₃ and jeppeite, K₂Ti₆O₁₃. 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 Ba²⁺ 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 TiO₆ 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.