青森県津軽地域の碧玉(津軽にしき石)の組織と成因について

Nishiki-ishi is a kind of jasper and shows mixed bright red, yellow-brown, green, and black colors. It occurs in Tsugaru region, Aomori Prefecture Japan and has been used for ornaments. Most of the raw stones are usually collected at shingle beaches, and few outcrops of the jasper are found out. Therefore, the occurrence of Nishiki-ishi has not been reported in detail. To elucidate the formation process of Nishiki-ishi, we observed textures of the rocks and minerals by optical and electron microscopes, Raman spectroscopy and analyzed the chemical compositions of minerals by an electron micro-probe analyzer. Nishiki-ishi were collected from two localities: Aoiwa, Nakadomari-machi, Kita-Tsugaru and Tappi-zaki, Sotogahama, Higashi-Tsugaru, Aomori Prefecture, Japan. It mainly consists of mixtures of micro-crystalline and crypto-crystalline quartz, opal-CT and iron-containing minerals. Its colors are caused by iron-containing minerals; hematite (red), celadonite (green), goethite (yellow), siderite (yellow), pyrite (black), and marcasite (black). These iron-containing minerals, which exhibit needle-like or granular forms, are included as fine grains in quartz crystal and fill boundaries among the quartz crystals. The macroscopic structure of Nishiki-ishi is breccia-like or clastic. In the texture, breccia fragments show yellow or red colors, and green and black parts filled up the space between breccia fragments. Yellow-brown and red color parts of Nishiki-ishi have flamboyant and mosaic (jigsaw) textures, which consist of micro-quartz and optically length-slow type spherules of quartz. Black and green color parts are also composed of opal-CT, chalcedony, and micro-quartz. These features are considerable diŠerences of quartz textures between in brecciated fragments and at the matrix among brecciated fragments. Nishiki-ishi was formed by silicification of volcanic rocks during volcanic activity. After the silicified rocks brecciated, quartz and chalcedony precipitated in the breccia.