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<p>Hopmannite, ideally Ba<sub>2</sub>(Ti<sub>5</sub>Fe)O<sub>13</sub>, 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<sub>1.00</sub>K<sub>0.61</sub>Na<sub>0.24</sub>Sr<sub>0.08</sub>Ca<sub>0.04</sub>)<sub>Σ1.97</sub>(Ti<sub>5.36</sub> Fe<sup>2+</sup><sub>0.48</sub>Fe<sup>3+</sup><sub>0.13</sub>Mg<sub>0.06</sub>)<sub>Σ6.03</sub>O<sub>13</sub>. Hopmannite is monoclinic, space group <i>C</i>2/<i>m</i>, with <i>a</i> = 15.357(2) Å, <i>b</i> = 3.8500(7) Å, <i>c</i> = 9.129(2) Å, <i>β</i> = 99.033(10)°, <i>V</i> = 533.03(17) Å<sup>3</sup> and <i>Z</i> = 2 and is isostructural with nixonite, Na<sub>2</sub>Ti<sub>6</sub>O<sub>13</sub> and jeppeite, K<sub>2</sub>Ti<sub>6</sub>O<sub>13</sub>. 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 <i>b</i>-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<sup>2+</sup> content is greater than K<sup>+</sup> and Na<sup>+</sup> at the A-cation site. Spectroscopic analyses indicated that the presence of the main vibrations is related to TiO<sub>6</sub> 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&#xa0;°C from a highly alkaline residual melt enriched in incompatible elements such as Ba, Sr, Ti, Zr, and P.</p>

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

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  • 发表日期：

    2025年12月13日

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  • Rafał Juroszek,
  • Biljana Krüger,
  • Georgia Cametti,
  • Beata Marciniak-Maliszewska,
  • Christof Schäfer

摘要

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 (Ba1.00K0.61Na0.24Sr0.08Ca0.04)Σ1.97(Ti5.36 Fe2+0.48Fe3+0.13Mg0.06)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) Å3 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.