LÅVEN - en skikkelig geologisk godbit
Foto/ revidert med oppgave: avigerus
Låven består av noen små og store øyer utenfor Stokkøya, på grensen til Telemark - men, ligger altså i Vestfold!
Allerede som liten gutt hørte jeg om de rare mineralene som var å finne ute på Låven. Den gangen var det dessuten mange geologer som var der ute og forsynte seg med små og store eksemplarer til egen samling. Staten måtte bl.a. derfor sikre at ikke øya regelrett ble spist opp av samlere, gå inn og frede for all ettertid. I dag er det lov å gå i land hvis du har lyst, men du må ikke ta med så mye som en liten stein eller plante!
Et vell av sjeldne mineraler i Langesundfjorden
Pegmatittene i Langesundfjorden er verdenskjente for sitt høye innehold av sjeldne mineraler.
Hvorfor finner man så mange sjeldne mineraler her nær grensen for Oslo-riften?
Det kan ikke geologene gi en fullstendig forklaring på.
Mer enn 170 forskjellige mineraler er beskrevet fra Langesundfjord-området.
Blant mineralene som er navngitt fra dette området er:
Ægerin og thoritt, begge oppkalt etter norrøne guder.
Barkevikittisk hornblende etter Barkevika ved Helgeroa.
Låvenitt etter den vesle holmen Låven i Langesundfjorden.
Mineralet låvenitt ble beskrevet for første gang fra Låven (W.C. Brøgger, 1878). Denne og to andre holmer i området er i dag totalfredet ved lov på grunn av mangfoldet av uvanlige mineraler.
Hele seks mineraler ble for første gang beskrevet fra Låven og har typelokalitet der:
ægirin, mosandritt, tritomitt-(Ce), catapleitt, låvenitt og leucophan. Flere av disse er utstilt i mineralsalen i 2. etg. ved Geologisk museum i Oslo.
Cappelenitt ble beskrevet av W.C. Brøgger i 1884 fra østsiden av Lille Arøy i Langesundsfjorden. Lokaliteten ble besøkt om igjen av Brøgger i 1884, men verken da eller senere er det funnet mer av mineralet. I Norge er Cappelenitt kun kjent som en ca. 2 cm lang, 1,5 cm tykk krystall, samt et håndstykke med et ufullkomment krystallstykke. Krystallen er utstilt i mineralsalen i 2. etg. ved Geologisk museum.
Låvenitt, Foto hentet fra internett - Per Aas
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English:
This is a very small island with a rare numbers of geological minerals
Geology can be fun! For us who live in Grenland - Vestfold, it is an incredibly exciting to find out all the stones and the reasons why these are here. Key words like Oslofield and the changes in the bottom of the earth can give you good explanations. My father was a hobby geologist and botanist, and had great joy to explore our exciting range. Unfortunately, as children we did not take part in the interest and enthusiasm, but in retrospect, geology has fascinated me more and more. Together with my wife we already have a small geocache series that we call for geoparkcacher, and it is fine if you access them?
The history of the Laaven, I got the first time on a boat trip early in the 1960-century, where we went ashore, and they took some rock samples. This was before 1970 when it still was legal. Now the area is completely protected, and it is forbidden to remove as much as a small stone! It also fascinated me that this was something radioactive and dangerous, just outside the door ... and that the island called the Laaven was also a geological phenomenon which was frequently visited by geologists from all over the world .. The small island ..? incredibly exciting!
As an adult I took the time to browse through some of my geological knowledge, and must admit that they were rather poor. The “Grenlandsbook” was the solution, and the Internet was also a good contributor. I had never managed to get as much forward without the use of the Internet.
This is what I have managed to collect about geological phenomenon on the small island off Stokkøya, called the Laaven:
Låven: small islets in Langesund Bay, protected as the nature of memory by royal decree of 17 April 1970. It is forbidden to remove the minerals or rock samples, brass or clay on the island. The reason is an occurrence of rare minerals and rocks, which can only be taken out of the Mineralogical-Geological Museum in Oslo for scientific research.
The northern part of the Laaven is composed of black basalt. South part consists of a very coarse grain nefelin-syenittpegmatitt formed from rock melt that has entered cracks in basalte and congealed there.
Rev. Hans Morten Thrane Esmark discovered around 1830 that pegmatitten was unusually rich in rare minerals, and these were described by Professor WC Brøgger in 1890. Brøgger found at least 35 different minerals, and many of these were described for the first time from this locality. One of the new minerals (see låvenitt), got their name from here.
Pegmatittene in Langesund Bay is world famous for its high content of rare minerals.
Why can we find many rare minerals here near the border to Oslofields?
So fare, the geologists cannot provide a complete explanation of it.
The eudialytt-bearing pegmatittene in Langesund fjord belong to a rock type (agpaittiske nefelinsyenitter) that are only known from a few places in the world. The most important of these is in Ilimaussaq in southwest Greenland, in an area close to Eirik Raude settlement.
More than 170 different minerals are described from Langesund Fjord area.
Among the minerals that are named from this area are:
Ægerin and thoritt, both named after Norse gods.
Barkevikittisk horn blende of bark Vika by Helgeroa.
Låvenitt after the small island in Children Langesund Bay.
Six minerals were first described from Laaven and the type locality where:
ægirin, mosand ritt, tritomitt-(Ce), catapleitt, låvenitt and leucophan. Several of these are exhibited in the mineral hall of 2. floor at the Geological Museum in Oslo.
låvenitt, lavenitt (of Children in Langesund fjord) with mineral composition (Na, Ca) 2 (Mn2 +, Fe2 +) (Žr, Ti) Si2O7 (O, OH, F) 2.
tritomitt-(Ce) (of the Greek tri for three, and the plot to split up): mineral composition with (Ce, La, Y, Th) 5 (Si, B) 3 (O, OH, F) 13 (?). Light to dark brown color and often resin-like, fat luster because of metamiktisering. The name plays on the crystals that are usually developed pointed triangles. Tritomitt was first described in 1850 by Weibye from the island in Children Lang Sund fjord, Norway.
In Norway, known from pegmatitter on islands in the fjord Langesund (Låven, Stokkøya, Store aRoy, Arøyskjærene, Eika Holmen mm) and several locations on the mainland in Vestfold (Vevja and Thorbjørnsås in Tvedalen, Håkestad in Tjølling and Hjertnesåstunnelen near Sandefjord). Crystals can be up to a few cm high and best developed at the border to analcim or leukofan.
catapleiitt, katapleiitt: mineral with composition Na2ZrSi3O9 2H2O. First described in 1849 by P.C. Weibye from Children in Langesund Bay. Brown, plate-shaped crystals. In Norway, the mineral is associated with times of nefelinsyenitt-pegmatitt, and can also function as small crystals on druser in small cavities in ekeritt in Oslo field.
eudnophitt: previously estimated mineral from Children in Langesund fjord; described by Weibye in 1849/1850, later proved to be analcim.
Cappelenitt described by W.C. Brøgger in 1884 from the east side of Lille aRoy in Langesund Bay. The locality was visited again by Brøgger in 1884, but neither then or later it's found more of the mineral resources. In Norway Cappelenitt only known as a ca. 2 cm long, 1.5 cm thick crystal, and a hand piece with a piece of crystal ufullkomment. Crystal is on display in the mineral hall of 2. floor of the Geological Museum.
gonnarditt (after Ferdinand Gonnard, France): mineral composition in zeolittgruppen with Na2CaAl4Si6O20 7H2O.
In Norway, known from Children in Langesund fjord, where it was early described under the name 'ranitt'. It is later found in several places in Lang Sund area and by Sandefjord. On Håøya in Langesund fjord shown red gonnarditt in soda-minetteganger. The crystals are in some places apparently hollow.
ranitt: obsolete concept, first used by Paykull in 1874 for a omvandlingsprodukt of nefelin in nefelinsyenitt-pegmatitt from Children in Lang Sund fjord. It consists of gonnarditt or a mixture of natrolitt and other minerals.
katapleiitt (Greek for "more" because it works together with other rare minerals): mineral composition with Na2ZrSi3O9 2H2O. Brown with a disc form. First described by Weibye in 1849 from a Norwegian locality.
In Norway, known from Children in Langesund Bay. Minerals are related to the times of nefelinsyenitt-pegmatitt, but can also occur as small crystals on druser in small cavities in ekeritt in Oslo field.
lorenzenitt (Johannes Th. Lorentzen, Denmark): mineral composition with Na2Ti2Si2O9.
In Norway, is known from the small cavities in ekeritt by Gjerdingen, Nordmarka by Oslo. It also works in times of nefelinsyenitt-pegmatitt by Bratt Hagen, Manitoba, and that omvandlingsprodukt of nosandritt from Children in Langesund Bay.
calciothoritt: selection of mineral thoritt with significant content of calcium. Beautiful, purple color.
In Norway there in nefelinsyenitt-pegmatitt on the islands. Sources: Geoleksi - Geological leksikon
TASK
You need to send me a mail to inform what colour the smallest island south-west has? The island in picture abow given fake colours! Please send me a mail, and I will accept your find!
en earthcache av avigerus 2009