File:Labradorite (Wiborg Batholith, 1633 Ma; Kymi Province, Finland) 1.jpg
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English: Labradorite from the Precambrian of Finland.
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 5600 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). Feldspar is a group of common silicate minerals. Feldspars are silicate minerals having one-fourth of all the silicons in SiO2 replaced by aluminum (Si4O8 to (Si3Al)O8). When this happens, the (Si3Al)O8 has a -1 electric charge. The charge is satisfied by the addition of one or more metals. The (Si3Al)O8- structure has relatively large holes, and the only metals that tend to stay in these holes are: K (potassium), Na (sodium), Ca (calcium), Cs (cesium), Ba (barium), Sr (strontium), and Pb (lead). Of these, K & Na & Ca are the most common metals that enter the matrix. Sometimes, several different metals enter the structure, resulting in "garbage can minerals". Chemical analyses of feldspars show that they range in composition from K-feldspar to Na-feldspar and from Na-feldspar to Ca-feldspar. Mineralogists have thus established two "families" of feldspars. There is no chemical gradient between K-feldspar and Ca-feldspar. The sodium- and calcium-feldspars are called plagioclase ("plag" for short). Six different mineral names are available for the plagioclase feldspars: albite, oligoclase, andesine, labradorite, bytownite, and anorthite. Albite is ~pure sodium feldspar (NaAlSi3O8) and anorthite is ~pure calcium feldspar (CaAl2Si2O8). The other mentioned minerals are plagioclase feldspars having a mix of sodium and calcium. The pure end-members are whitish-colored. The plagioclase feldspars having a mix of sodium and calcium tend to be light gray to dark gray to mottled gray. Some have a spectacular play of color. Seen here is labradorite plagioclase. It displays iridescent colors when tilted at certain angles to the light - this property is called labradorescence. Gem-quality samples are called spectrolite. The specimen is a partial single crystal from anorthosite, a coarsely-crystalline, intrusive igneous rock dominated by plagioclase feldspar. Anorthosite is an uncommon rock on Earth, but is abundant on the Moon. The sample comes from a gabbro-anorthosite body in Finland's Wiborg Batholith. Mineral percentages reported from the intrusion: 81% labradorite, 10% pyroxene, 4% quartz, and 5% other minerals. Geologic unit: 1 x 2 square kilometer gabbro-anorthosite body in the central Wiborg Batholith, Baltic Shield/Fennoscandian Shield, late Paleoproterozoic, 1.633 Ga Locality: attributed to a quarry at Ylijärvi (= Geological Survey of Finland map 3133, sheet 1, grid coordinates 6740.20-35467.50), ~6 kilometers southwest of Ylämaa village, southern Kymi Province, southern Finland Photo gallery of albite: www.mindat.org/gallery.php?min=96 Photo gallery of oligoclase: www.mindat.org/gallery.php?min=2976 Photo gallery of andesine: www.mindat.org/gallery.php?min=220 Photo gallery of labradorite: www.mindat.org/gallery.php?min=2308 Photo gallery of bytownite: www.mindat.org/gallery.php?min=815 Photo gallery of anorthite: www.mindat.org/gallery.php?min=246 |
| Date | |
| Source | https://www.flickr.com/photos/47445767@N05/50424377958/ |
| Author | James St. John |
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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/50424377958. It was reviewed on 8 October 2020 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0. |
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| current | 14:56, 8 October 2020 | | 3,499 × 2,438 (5.7 MB) | Ser Amantio di Nicolao (talk | contribs) | Uploaded a work by James St. John from https://www.flickr.com/photos/47445767@N05/50424377958/ with UploadWizard |
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