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 differences 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.

Excursion to the Ichinokawa stibnite mine, Saijo City, Ehime Prefecture was held in 2017 Annual Meeting of Japan Association of Mineralogical Sciences at September 15, 2017, organized by Japan Association of Mineralogical Sciences. The field trip was arranged to get an understanding about the mineralogy, geology, and mining history of the Ichinokawa Mine as represented by the largest stibnite crystal in the world. 21 participants of the field trip were guided by Dr. Tetsuo Minakawa (Ehime University), Dr. Toru Inoue (Hiroshima University), and the staff of Geodynamics Research Center, Ehime University. At the first stop, we visited the Ichinokawa Community Center and the old Ichinokawa Mine. The community center located near the old Ichinokawa Mine preserves the historical documents and ore specimens of the Ichinokawa Mine. Specimens of stibnite crystals and stibnite veins in conglomerate are displayed there. We also visited to the old Senga-ko adit of the Ichinokawa Mine and the exposure of the Ichinokawa conglomerate nearby. For the second stop, we observed stibnite vein in schists at the riverside of the Kamogawa river. For the third stop, the ore collection collected by Mr. Daisuke Tanaka were observed at the Saijo City Local History Museum. The collection contains large stibnite aggregate up to 45.5 cm in length and 13.13 kg in weight, composed of prismatic crystals with a significant metallic luster, collected and donated by Mr. Daisuke Tanaka. The field trip provided us valued opportunity to study the geology of Ichinokawa and significant stibnite specimens at the locality, and to think the importance of the specimens and fields.

「理系女性のライフプラン　あんな生き方・こんな生き方
　　研究・結婚・子育てみんなどうしてる？」

2017年年会研究発表優秀賞受賞者渡航支援制度成果報告

「岩石鉱物科学」投稿規定・編集規則