Porphyritic leucite lava (possibly phonolite) (Rome, Italy) 1 : 無料・フリー素材/写真
Porphyritic leucite lava (possibly phonolite) (Rome, Italy) 1 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Porphyritic leucite lava from Italy. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5200 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4. The simplest & most abundant silicate mineral in the Earth's crust is quartz (SiO2). All other silicates have silica + impurities. Many silicates have a significant percentage of aluminum (the aluminosilicates).In the porphyritic lava sample shown here, the medium gray-colored phenocrysts are leucite, a feldspathoid mineral with the formula KAlSi2O6 - potassium aluminosilicate. Feldspathoids are the "feldspar-like minerals". They have more aluminum and less silicon than the feldspars. Feldspathoids are less stable than feldspars and usually decompose in strong acids - they don't effervesce, but become gels.Leucite is the feldspathoid equivalent of potassium feldspar (orthoclase & microcline), which is KAlSi3O8. K-feldspar has a 1:3 ratio of Al to Si. Leucite has an Al to Si ratio of 1:2. Leucite has a nonmetallic luster, a whitish-gray color, is moderately hard (H = 5.5 to 6 on the Mohs Hardness Scale), usually forms 24-sided crystals, has no cleavage, and breaks with conchoidal fracture.Light-colored, cloudy, ~rounded leucite crystals occur in some volcanic lavas - "white garnets in lava". Leucite cannot occur with quartz, because otherwise leucite will react to become potassium feldspar.Geologic unit & age: unrecorded/undisclosedLocality: unrecorded/undisclosed site at or near Rome, Italy---------------Photo gallery of leucite:www.mindat.org/gallery.php?min=2465 |
| 撮影日 | 2018-03-24 12:58:14 |
| 撮影者 | James St. John |
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