Siliceous protrusions in the Sagata Cave, Akune city, Kagoshima Prefecture

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Other Title
  • 鹿児島県阿久根市佐潟洞窟内のシリカ突起物
Published
2020-12-31
Resource Type
departmental bulletin paper
Publisher
鹿児島大学

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Description

A preliminary report is presented on the occurrence and geochemical compositions of the siliceous protrusions grown on the surface of the andesite in the Sagata cave, Akune city, Kagoshima prefecture. Rocks exposed in the vicinity of Sagata are dominantly Neogene and younger volcanic rocks (mainly andesites). The cave, investigated in this study, is formed within the andesite. The siliceous protrusions, ranging in size from 2 mm to 1.5 cm, have been grown on the andesite surface in random orientations. Microscopic observations of the protrusions show concentric zones indicating their progressive growth or precipitation. X-Ray elemental mapping showed the protrusions are characterized by Si and variable amounts of Fe. The structure of the protrusions resembles to those of the stalactites/stalagmites generally found in limestone caves but their chemical composition is dominantly Si- and Fe-rich without significant amount of Ca or Na that indicate non-involvement of saline water. Petrographic and chemical analysis showed the presence of amorphous silica, cristobalite, hornblende along with some mixed peaks of clay minerals suggesting their growth from the andesite through secondary alteration.

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