Archivo: Gold nugget (Australia) 4 (16848647509)

Descripción: Gold nugget from Australia. (public display, Field Museum of Natural History, Chicago, Illinois, USA). The Australian rock shown above is a large, nearly four pound mass of gold. The irregularly-distributed, smoothly sculpted surfaces indicate that this is likely a fluvial gold cobble - in other words, it appears to be from a placer deposit. Gold is a metal. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state as minerals. 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. As of the year 2018, there are around 4500 named and described terrestrial 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. Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Gold is a native element in nature. To find a native element, it must be relatively non-reactive and there must be some concentration process. Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals. Gold is very rare in crustal rocks - it averages about 5 ppb (parts per billion). Where gold has been concentrated, it occurs as wires, dendritic crystals, twisted sheets, octahedral crystals, and variably-shaped nuggets. It most commonly occurs in hydrothermal quartz veins, disseminated in some contact- & hydrothermal- metamorphic rocks, and in placer deposits. Placers are concentrations of heavy minerals in stream gravels or in cracks on bedrock-floored streams. Gold has a high specific gravity (about 19), so it easily accumulates in placer deposits. Its high density allows prospectors to readily collect placer gold by panning. Gold (Au) is the most prestigious metal known, but it's not the most valuable. Gold is the only metal that has a deep, rich, metallic yellow color.
Título: Gold nugget (Australia) 4 (16848647509)
Créditos: https://www.flickr.com/photos/jsjgeology/16848647509/
Autor(a): James St. John
Términos del Uso: Creative Commons Attribution 2.0
Licencia: CC BY 2.0
Enlace de Licencia: https://creativecommons.org/licenses/by/2.0
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