Ametrien

Inside and outside views of the geode

Geode (Greek γεώδης - ge-ōdēs, "earthlike") are geological secondary structures which occur in certain sedimentary and volcanic rocks. They are themselves of sedimentary origin formed by chemical precipitation. Geodes are essentially hollow, vaguely spheroid to oblate masses of mineral matter that form via either of two processes:

• by the filling of vesicles (gas bubbles) in volcanic to sub-volcanic rocks by minerals deposited from hydrothermal fluids or
• by the dissolution of igneous nodules or concretions (that were deposited syngenetically within the rock formations in which they are found) and partial filling by the same or other minerals precipitated from diagenetic water, groundwater or hydrothermal fluids.

Geodes differ from vugs in that the former were formed as early, rounded, structures within the surrounding rock and are often removed intact, whereas vugs are irregularly shaped pockets, voids or cavities within a formation, often along a vein or in breccia. Geodes also differ from "nodules" in that a nodule is a mass of mineral matter that has accreted around the nodule nucleus. Both structures had the minerals contained within, deposited from groundwater or hydrothermal processes. Geodes commonly have a chalcedony (cryptocrystalline quartz) shell lined internally by various minerals, often as crystals, particularly calcite, pyrite, kaolinite, sphalerite, millerite, barite, celestite, dolomite, limonite, smithsonite, opal, chalcedony and macrocrystalline quartz, which is by far the most common and abundant mineral found in geodes. Geodes are found mostly in basaltic lavas and limestones. The Warsaw Formation in the Keokuk region near the area where Missouri, Iowa, and Illinois join contains abundant geodes.

Formation

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas; or, as in the American Midwest, in rounded cavities in sedimentary formations. After rock around the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz. Many geodes can be found in deserts.

When cut in half, visible bands corresponding to varied stages of precipitation may at times show patterns that reveal points of fluid entry into the cavity and/or varied colors corresponding to changes in chemistry.

Coloration

Large hollow geode from Carefree, Arizona (8.5 by 11 inch booklet for scale)

Geode banding and coloration is the result of variable impurities. Iron oxides will impart rust hues to siliceous solutions. Most geodes contain clear quartz crystals, while others have purple amethyst crystals. Still others can have agate, chalcedony, or jasper banding or crystals such as calcite, dolomite, celestite, etc. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart. However, geodes from a particular area are usually similar in appearance.

Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.

Prevalence

Geode, halved and polished

Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, western Illinois, Kentucky, and Utah). They also are common in Brazil, Namibia, and Mexico.

Vugs and crystal caves

In 2000 a team of geologists found a cave filled with giant gypsum crystals in an abandoned silver mine near Almería, Spain. The cavity, which measures 1.8 × 1.7 meters and is 8 meters in length, would be the largest crystal cave ever found. The entrance of the cave has been blocked by five tons of rocks, and is under police protection (to prevent looters from entering). According to geological models, the cave was formed during the Messinian salinity crisis 6 million years ago, when the Mediterranean sea evaporated and left thick layers of salt sediments (evaporites). The cave is currently not accessible to tourists.

The world's largest known crystal cave or vug is Crystal Cave, a celestine geode 35 feet (10.7 m) in diameter at its widest point, located near the village of Put-in-Bay, Ohio, on South Bass Island in Lake Erie.

See also

• Septarian nodule
• Lithophysa
• Thunderegg
• Bristol Diamonds
• Coso artifact

Further reading

• Rocks and Minerals, written by Fredrick H. Pough, ISBN 0-395-91096-X
• Gerard V. Middleton: Encyclopedia of Sediments and Sedimentary Rocks. Springer 2003, ISBN 978-1-4020-0872-6, p. 221 ( restricted online copy op Google Boeke)
• Walter David Keller: The common rocks and minerals of Missouri. University of Missouri Press 1961, ISBN 978-0-8262-0585-8, S. 67 ( restricted online copy op Google Boeke)
• Brian J. Witzke: Geodes: A Look at Iowa's State Rock Iowa Geological Survey
• Geodes Kentucky Geological Survey (University of Kentucky)

External links

• Indiana geode specimens, facts and stories
• Video of a geode cracking using industrial soil pipe cutter
• Geode images, locations and discussion
• Australian Museum Fact sheet
• Utah Geode Beds
• Illinois State Geological Survey. "Geodes—Small Treasure Vaults in Illinois" (PDF). Geargiveer vanaf die oorspronklike (PDF) op 27 September 2007. {{cite news}}: Cite has empty unknown parameter: |coauthors= (hulp); Onbekende parameter |deadurl= geïgnoreer (hulp)

‘n ‘’’Geode’’’ (Griekse γεώδης - ge-ōdēs, "aardse") is ‘n sekondêre geologiese strukture wat voorkom in sekere sedimentêre en vulkaniese gesteentes. Hulle is self van sedimentêre oorsprong wat gevorm word deur chemiese neerslag. Geodes is in wese hol, gewoonlik rofweg sferoïde tot oblate massas minerale wat deur een van twee prosesse vorm:

• Deur holtes in vulkaniese tot sub-vulkaniese gesteentes wat gevul word met minerale wat deur hidrotermale vloeistowwe afgeset word. (Die holtes vorm deur gas in die magma wat uitskei wanneer die drukking naby die oppervlak verlaag, en gasborrels vorm. Hierdie gas kan as borrels of pype voorkom, afhangende van die viskositeit van die lawa, en hoe vinnig dit afkoel. Wanneer die magma of lawa afkoel en stol, bly hierdie gasgevulde holtes in die gevormde gesteente behoue.)

Deur die ontbinding van stollingsknope of konkresies (wat in die rotsformasies waarin hulle gevind is, siekteties gedeponeer is) en gedeeltelike vulling deur dieselfde of ander minerale wat uit diagenetiese water, grondwater of hidrotermiese vloeistowwe neergeslaan is.

Geodes verskil van vugs omdat die voormalige gevorm is as vroeë, afgeronde strukture in die omliggende rots en word dikwels ongeskonde verwyder, terwyl vogels onreëlmatig gevormde sakke, holtes of holtes binne 'n formasie is, dikwels langs 'n ader of in breccia. Geodes verskil ook van "nodules" deurdat 'n nodule 'n massa minerale materiaal is wat rondom die knoopkern toegeneem het. Beide strukture het die minerale bevat wat in die grondwater of hidrotermiese prosesse gedeponeer is. Geodes het gewoonlik 'n chalcedonie (kriptokristalliese kwarts) skulp wat intern deur verskeie minerale, dikwels as kristalle, veral kalsiet, piriet, kaoliniet, sphaleriet, milleriet, bariet, celestiet, dolomiet, limoniet, smithsoniet, opaal, chalcedonie en makrokristalliese kwarts Verreweg die mees algemene en oorvloedige mineraal wat in geodes voorkom. Geodes word meestal aangetref in basaltiese lavas en kalkstene. Die Warskou-formasie in die Keokuk-streek naby die gebied waar Missouri, Iowa en Illinois aansluit, bevat oorvloedige geodes.

Vorming [wysig]

Geodes kan in enige holte vorm, maar die term word gewoonlik gereserveer vir meer of minder geronde formasies in stollingsgesteentes en sedimentêre gesteentes. Hulle kan vorm in gasborrels in stollingsgesteentes soos vesikels in basaltiese lavas; Of, soos in die Amerikaanse Middeweste, in afgeronde holtes in sedimentêre formasies. Nadat rots rondom die holte verhard word, word opgeloste silikate en / of karbonate op die binneoppervlak gedeponeer. Met verloop van tyd kan hierdie stadige voeding van minerale bestanddele van grondwater of hidrotermiese oplossings kristalle vorm binne die hol kamer. Gebergte bevat geodes wat uiteindelik wegdoen en ontbind, en laat hulle teenwoordig wees op die oppervlak as dit bestaan uit weerstandbiedende materiaal soos kwarts. Baie geodes kan in woestyne gevind word.

Wanneer dit in half gesny word, kan sigbare bande wat ooreenstem met verskillende stadiums van presipitasie, soms toon patrone wat die punte van vloeistofinvoer in die holte en / of gevarieerde kleure aandui wat ooreenstem met veranderinge in chemie.

Geode banding en kleur is die gevolg van veranderlike onsuiwerhede. Ysteroksiede sal roesskakerings aan siliceous oplossings gee. Die meeste geodes bevat duidelike kwarts kristalle, terwyl ander pers ametistiese kristalle bevat. Nog ander kan agaat, chalcedony, of jasper banding of kristalle soos kalsiet, dolomiet, celestite, ens. Daar is geen maklike manier om te vertel wat die binnekant van 'n geode hou totdat dit oopgesny of uitmekaar gesny word nie. Geodes uit 'n bepaalde gebied is egter gewoonlik soortgelyk in voorkoms.

Geodes en Geode-skywe word soms gekleur met kunsmatige kleure. Monsters van geodes met ongewone kleure of hoogs onwaarskynlike formasies is gewoonlik sinteties verander.

Prevalence[edit]

Geode, halved and polished

Geodes are common in some formations in the United States (mainly in Indiana, Iowa, Missouri, western Illinois, Kentucky, and Utah). They also are common in Brazil, Namibia, and Mexico.

Vugs and crystal caves[edit]

In 2000 a team of geologists found a cave filled with giant gypsum crystals in an abandoned silver mine near Almería, Spain. The cavity, which measures 1.8 × 1.7 meters and is 8 meters in length, would be the largest crystal cave ever found.[1] The entrance of the cave has been blocked by five tons of rocks, and is under police protection (to prevent looters from entering). According to geological models, the cave was formed during the Messinian salinity crisis 6 million years ago, when the Mediterranean sea evaporated and left thick layers of salt sediments (evaporites). The cave is currently not accessible to tourists.

The world's largest known crystal cave or vug is Crystal Cave, a celestine geode 35 feet (10.7 m) in diameter at its widest point, located near the village of Put-in-Bay, Ohio, on South Bass Island in Lake Erie.

Ametrien ‘’’Ametrien’’’, ook bekend as ‘’’tristien’’’ of as sy handelsnaam bolivianiet, is 'n natuurlike varieteit van kwarts. Dit is 'n mengsel van ametis en sitrien, met sones van pers en geel of oranje. Byna alle kommersiële beskikbare ametrien word in Bolivia ontgin.

Die kleur van die sones wat binne ametrien sigbaar is, is te danke aan verskillende oksidasie toestande van yster binne die kristal. Die verskillende oksidasietoestande vind plaas as gevolg van 'n temperatuurgradiënt oor die kristal tydens die vorming daarvan.

Kunsmatige ametrien kan geskep word deur differensiële hittebehandeling van ametiste.

Volgens die legende is ametrien die eerste keer aan Europa bekendgestel deur 'n conquistador se geskenke aan die Spaanse koningin, nadat hy 'n myn in Bolivia as ‘n trougeskenk ontvang het met ‘n sy troue aan ‘n prinses van die Ayores stam.

Daar kan aanvaar word dat die meete ametrien in die lae pryssegment van sintetiese materiaal afkomstig is. ‘n Russiese labratorium het sedert 1994 die industriële produksie van tweekleurige kwart vervolmaak, wat later bestraal word om die tipiese ametrien kleure tot voorskyn te bring. Groen-geel of goudkleurige-blou ametrien kom nie in die natuur voor nie.

Eksterne skakels

• Ametrine at the International Colored Gemstone Association
• Ametrine: Mineral Information Page
• Ametrine: Ametrine mineral information and data
• Insider Gemologist: What Are the Identifying Characteristics of Citrine, Ametrine, Smoky Quartz, and Scapolite? Geargiveer 5 September 2006 op Wayback Machine