Bosoite, a new silica clathrate mineral from Chiba Prefecture, Japan

Abstract

Bosoite (IMA2014-023) is a new silica clathrate mineral containing hydrocarbon molecules in its crystal structure. Bosoite can be considered structurally as a silica analogue of the structure-H gas hydrate, where guest molecules are trapped in cage-like voids constructed of the host framework. The mineral occurs in the Miocene tuffaceous sedimentary rocks at Arakawa, Minami-boso City, Chiba Prefecture, Japan. Bosoite is hexagonal, and it crystallises as an epitaxial intergrowth on chibaite crystals, with the {0001} of bosoite parallel to octahedral {111} form of chibaite. Crystals are colourless and transparent with vitreous lustre. The calculated density is 2.04 g/cm 3 . The empirical formula (based on 2 O apfu and guest molecules assumed as CH 4 ) is Na 0.01 (Si 0.98 Al 0.02 ) Σ1.00 O 2 ⋅0.50CH 4 ; the end-member formula is SiO 2 ⋅ n C x H 2 x +2 . Bosoite has the space group P 6/ mmm , with the unit-cell parameters a = 13.9020(3) Å, c = 11.2802(2) Å, V = 1887.99(6) Å 3 and Z = 34. The crystal structure of bosoite was refined by single-crystal X-ray diffraction and converged to R 1 = 4.26% for the average model and R 1 = 2.96% for the model where all oxygen sites are split.