Question

Do some special Icelandic stones exist?

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Answer

Briefly, no "special Icelandic stones" exist in the sense that they are nowhere found outside Iceland. However, a few stones may be called characteristic for Iceland. By the word "stone" we are referring to rocks, on the one hand, and minerals on the other.

A mineral is defined as a substance having a definite chemical composition and atomic structure and formed by the inorganic processes of nature. Rocks are usually composed of a number of different minerals. Olivine-basalt, for instance, which is the most common rock in Iceland, is composed of four main minerals: plagioclase, augite, olivine, and magnetite.

Individual minerals (crystals) can be seen with the naked eye in coarse grained rocks, but especially as mineral fillings in cavities (vugs and fissures) in rocks. These minerals have precipitated from hot groundwater percolating through the rock and large crystals are frequently found in such cavities.

The following minerals and rocks may be regarded as rather typical for Iceland: Iceland spar, a pure and transparent form of calcite (CaCO3), is commonly found filling vugs in Iceland. Optical calcite (as it is also called) was first brought to Europe from Helgustadir in eastern Iceland in the 17th Century and received much attention both from mineral collectors and scientists - Robert Newton owned one such - because of its double-refractive qualities. The deposit at Helgustadir was mined on and off well into the 20th Century and crystals from there were used in various optical instruments including petrological microscopes. Demand for optical calcite from Iceland ceased partly because better localities were discovered e.g. in Mexico, and partly because calcite crystals were superseded by synthetic materials for use in optical instruments. The mine at Helgustadir was placed under official protection in 1975, which means that neither may mineral specimens be removed from there nor the rocks themselves disturbed at all.

Zeolites are also found as vug minerals in many parts of Iceland. They are hydrated aluminium silicates of calcium and sodium. When heated before the blowpipe, the zeolites froth or boil up, a circumstance from which their name is derived, from the Greek zein, to boil, and lithos, stone. Very many types of zeolites are found in Iceland and are especially sought after by mineral collectors. Zeolites are found all over the world but one of the best known localities is Teigarhorn in Berufjord, E-Iceland, which like Helgustadir has been placed under official protection.

Icelandite is a rock which was first "defined" as a unique type deserving a special name in the mid-1960s at the Tertiary volcano Thingmuli in eastern Iceland. It is defined as iron rich andesite and the name distinguishes it from "ordinary" iron-poor andesites which for instance characterise the volcanoes of the "Ring of Fire" around the Pacific (including the Andes, from which the name derives). Many of the Hekla lavas are icelandite.

Rhyolite is a silicic volcanic rock which owing to its light colour sticks out from the dark basalts that dominate the Icelandic landscape. The largest rhyolite localities in Iceland are the Torfajökull region east of Hekla, and the Borgarfjord region in E-Iceland. Well-known rhyolite mountains in Iceland include Moskardshnjukar near Reykjavik, Baula in W-Iceland, Drapuhlidarfjall by Stykkisholmur, and Sulur above Akureyri. Rhyolite is rarely found in oceanic islands but in Iceland it is quite common (5-10%). In Hawaii, for instance, it is unknown despite obvious geological similarities between Hawaii and Iceland. For 150 years, the origin of the Icelandic rhyolites has been debated by various geologists.

Palagonite, a rock that forms in subglacial and subaquatic eruptions, is very common in Iceland and has received increasing attention of late because similar rocks appear to exist on the planet Mars. Although not unique to Iceland, palagonite is probably nowhere else in the world as common (and easy to study) as in Iceland.

Basalt is, of course, Iceland’s most distinctive rock type since it constitutes about 90% of the country. However, basalt is in no way unique to Iceland - it makes up the bottom of all the oceans in addition to very large formations on the continents, for example Greenland and Scotland.

Um þessa spurningu

Dagsetning

Published 6.3.2005

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Answers in English

Citation

Sigurður Steinþórsson. „Do some special Icelandic stones exist?“. The Icelandic Web of Science 6.3.2005. http://why.is/svar.php?id=4808. (Skoðað 23.10.2017).

Author

Sigurður Steinþórssonprófessor emeritus



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