In the narrow sense, it is a fine-grained volcanic rock mainly composed of alkali feldspar with the feldspar nepheline. In the broad sense, it is a volcanic rock composed of alkali feldspar, nepheline, leucite, sodalite, or mafic minerals. It has a chemical composition equivalent to nepheline syenite. It often shows a porphyritic texture, but has few phenocrysts. Sometimes it is aphyric. Phenocryst minerals include alkali feldspar (sanidine or anorthoclase), nepheline, analcite, nosean, haüyne, sodalite, leucite, pyroxene (aegirine augite, aegirine), amphibole (hastingite, ferropargasite, arfvedsonite, riebeckite), biotite, olivine, and many others. The groundmass is composed of alkali feldspar (sanidine), nepheline, leucite, analcime, aegirine, titanomagnetite, biotite, apatite, zircon, titanite, glass, etc., and has a trachyte-like or granular texture. When the dense rock fragments, broken into plates, are lightly struck with a hammer, a clear sound is produced, so it is also called "echoic rock." The English name also comes from the word "phone." [Tokiko Chiba] [Reference items] |Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend |
狭義ではアルカリ長石を主体に準長石の霞石(かすみいし)を伴う細粒の火山岩。広義ではアルカリ長石、霞石、白榴(はくりゅう)石、ソーダライト(方(ほう)ソーダ石)のいずれかと苦鉄質鉱物からなる火山岩をいう。霞石閃(せん)長岩に相当する化学組成をもつ。斑状(はんじょう)組織を示すことが多いが斑晶は少ない。ときには無斑晶のこともある。斑晶鉱物はアルカリ長石(サニディンかアノーソクレース)、霞石、方沸石、ノゼアンnosean、アウインhaüyne、ソーダライト、白榴石、輝石(エジリンオージャイト、エジリン)、角閃石(ヘスティング閃石、鉄パーガス閃石、アルベゾン閃石、リーベック閃石)、黒雲母(くろうんも)、橄欖(かんらん)石など多数に及ぶ。石基はアルカリ長石(サニディン)、霞石、白榴石、方沸石、エジリン、チタン磁鉄鉱、黒雲母、燐灰(りんかい)石、ジルコン、チタン石、ガラスなどからなり、粗面岩状あるいは粒状の組織を示す。板状に割れた緻密(ちみつ)な岩片をハンマーで軽く打つと澄んだ音が出るので響岩(きょうがん)ともいう。英名も音(phone)に由来する。 [千葉とき子] [参照項目] | |出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例 |
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