A classical locality.
Several granitic proterozoic pegmatite dikes transects the rocks of the northern part of the Utö Island. Some of these can be classified as LCT pegmatites of the petalite subtype. 2 famous classical LCT pegmatite bodies- the first ones to be recognized as litium pegmatites- transects a sequence of rhyolitic metatuffites, skarns and BIF in old iron mines.
These 2 LCT pegmatites are the typlocality for 4 species: spodumen, petalite, tantalite-(Mn) and holmquistite. The element lithium was also discovered in petalite from Utö.
The iron mines were opened in the 12th century and closed down in 1878. Minor silver mining took also place during the 17th century.
The pegmatites were best exposed in the open pit of the Nyköpingsgruvan mine. Today pegmatite materiale can best be found on the several dumps from the iron mines. Beside minerals from the primary phases like microcline, albite, petalite, spodumene etc, also interesting late-stage paragenesis minerals (beryllosilicates and others) can be found in small cavities, cracks and fracture infillings in the materiale found at the dumps.
NB: Swedish "Ö"= Island
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References
- d’Andrada, J. B. (1800): Der eigenschaften und kennzeichen einiger neuen fossilien aus Schweden und Norwegen nebst einigen chemischen bemerkungen ueber dieselben. Allgemeines Journal der Chemie 4, 28-39 [First description of spodumene and petalite]
- Morton, C. (1884): Några goniometriska bestämningar å kalkspat från Arendal, Kongsberg, Utö och Bamle. Meddelanden från Stockholms Högskola nr 34. Översigt af K. Svenska Vet.-Akad. Förhandlingar 1884 No.8, 16 pp + plate. [includes goniometric measures of a calcite from Utö]
- Osann, A. (1913): Über Holmquistit, einen Lithionglaukophan von der Insel Utö. Silzungsber. d. Heidelberger Akad. d. Wissensch. Math-nat. Klasse. Abt. A. Jahrg. 1913, 23: 1-17 + 2 plates.
- Pilawa-Podgurski, N.; Nya Geologiska Undersökningar vid Utö Järnmalmsfält. SGU Ser C 541. 1955
- Gustafsson, Lars & Otter, Bertil (1991): Mineralförekomster i Stockholmstrakten. STEIN 18(2),41-49
- Gustafsson, L. (1998): Pyrosmalit från Utö gruvor. Berg & Mineral (Stockholms Amatörgeologiska Sällskap), 8 (nr 21): 15 [A reprint from Berg & Mineral 2 (1992) issue, on a find of pyrosmalite]
- Langhof, J., Jonson, E, Gustafsson, L. & Otter, B. (1998): Utö- en klassisk svensk mineralfyndort. Bergverksmuseets Skrift 14, 29-31
- Langhof, J., Holtstam, D. & Gustafsson, L. (2000): Chiavennite and zoned genthelvite-helvite as late-stage minerals of the Proterozoic LCT pegmatites at Utö, Stockholm, Sweden. Geologiska Föreningens Förhandlingar 122, 207-212.
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