Hulsite, Sn bearing Fe borate, was found from the Sengendera skarn deposit which formed by high temperature pneumatolytic metasomatism derived from intrusion of the Okueyama granite mass. It occurred as radiating aggregate of acicular and columnar crystals up to 1 cm, associated closely with magnetite, diopside and tremolite-actinolite, and rarely occurred as chemical sector zone within vonsenite crystals. It is black in color and opaque zones with metallic luster. Cleavages is not distinct. Color of reflection is gray. Monoclinic cell dimensions are a=10.695(4), b=3.102(1), c=5.431(1) Å, β=94.21(3)°. The empirical formula of hulsite from Sengendera skarn deposit is (Fe_1.513²⁺Mg_0.459Mn_0.023)_Σ1.995(Fe_0.792³⁺ Sn_0.204Al_0.002)_Σ0.998B_1.015O₅, and range of Sn content is 9.06 to 11.88 wt%. The average Sn content of vonsenite, an Fe-borate, from Sengendera mine is 1.40 wt%.
  Hulsite domain within vonsenite crystals was probably formed at lower temperature on account of high Sn content more than 4 wt% SnO₂.

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  We determined K-Ar ages of the late Miocene to Pliocene andesitic to basaltic dykes and lavas distributed in the Tanzawa-Oiso area in the southern Fossa Magna region, central Japan. Our data indicate two distinct periods of volcanic activity in this region; one took place during the middle Miocene (about 15 Ma) and another during the late Miocene to Pliocene (7.5-4.2 Ma). Combined the present results with the age data reported previously from volcanic rocks in the southern Fossa Magna region, the present study suggests that the volcanic front in this region during 8 to 4 Ma was located at about 40 km east of the present-day volcanic front. The volcanic front began to migrate westward to the present-day location at about 4 Ma. The westward migration of volcanic front was probably associated with an abrupt change of lateral motion of the Philippine Sea Plate from north- northwestward to northwestward occurred between 4 and 2 Ma.

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  Shocked meteorites are the most important sources of high-pressure minerals in addition to impact crater rocks, diamond inclusions and mantle xenoliths. In most cases, natural high-pressure minerals occur as submicron-sized grains. However, state-of-art techniques such as transmission electron microscopy and synchrotron X-ray diffractometry enabled the identification of such small crystalline grains. As a result, many of natural high-pressure phases of silicates and oxides have been discovered in the past 15 years. Textural, crystallographic and chemical characteristics of the natural high-pressure minerals provide us not only the clues to understand the impact events of meteorite parent bodies, but also insights on the structure and dynamics of the deep Earth. Here, we summarize the occurrences and discovery histories of the natural high-pressure minerals.

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2011年年会記事
日本鉱物科学会・日本地質学会共催シンポジウム報告

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