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[SCG58-13] Texture and formation process of jasper, "Nishiki-ishi" from Tsugaru region, Japan
Keywords:jasper, Nishiki-ishi, chalcedony, texture
Used samples were collected from two localities: Aoiwa, Nakadomari-machi, Kita-tsugaru, Aomori prefecture, and Tappi-zaki, Sotogahama, Higashi-tsugaru, Aomori prefecture, Japan. Both localities are located in the green-tuff regions of Miocene, and are underline by pyroxene andesite rocks (Tappi andesite) with volcanic breccia. Silica veins of quartz, chalcedony and opal are locally developed within the rock. Nishiki-ishi mainly consists of quartz and iron containing minerals, and other minor minerals are barite, apatite and ankerite.
The textures of rocks and minerals were observed using an optical microscope and a scanning electron microscope (JEOL, JSM-7001F), and chemical analyses were carried out using an energy dispersive X-ray analyzer (Oxford, INCA system).
Quartz crystals composing Nishiki-ishi exhibit fibrous spherules with 0.1 mm in diameter or aggregations of micro-crystals with 0.05 mm in width. Comparing with chalcedony and agate, Nishiki-ishi has coarser fibers in the quartz spherules and few zonal-band texture. Origin of its colors is caused by iron-containing minerals; hematite (red), celadonite (green), goethite (yellow), siderite (yellow), pyrite (brown). These iron-containing minerals, which exhibit needle-like or granular forms, are included as fine grains in quartz spherules and fill in space among the quartz spherules.
The macroscopic structure of Nishiki-ishi is a breccia-like or clastic. The breccia fragments consist of aggregates of micro-quartz and optically length-slow types spherules. In contrast, the space among these breccia fragments is filled by clearly euhedral quartz crystals and chalcedony with optically length-fast. These are considerable differences of quartz textures between breccia and the space among of breccia fragments. The original rock of Nishiki-ishi was formed by silicification of volcanic rocks during volcanic activity. After the silicified rocks brecciated, quartz and chalcedony precipitates in the breccia.