Uranocircite from Ogitsubo, Iida City, Nagano Prefecture

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Description

Uranocircite, a hydrated phase of barium and hexavalent uranium, Ba(UO₂)₂･(PO⁴)₂･8H₂O, was identified in the oxidation zone of Ogitsubo in the "Iida radioactiveanomlous area" in 1968. This yellow green mineral occurred in fine arkosic silt and white fissure clay (montmorillonite) which were deposited in the basal part of Miocene sediments just above the unconformity. Uranocircite, a sort of uranium mica minerals, is rarely found in ordinary uranium deposits, because barium in the mineral is easily replaced by such an element as calcium. Basement granite of this area contains abnormally rich thorium, while a relatively large amount of monazite, (La, Ce, Th, Y)･PO₄, is found in several localities of this district. Around the crystals of monazite included in biotite intense pleochroic haloes are observed under the microscope. Perhaps these environmental conditions led the generation of the barium-bearing uranophosphate mineral as secondary ore.

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