Calgary Building Stone Tour: Larvikite EarthCache
Hidden : 8/4/2008
Difficulty:
1.5 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

This EarthCache is the second stop in the Calgary Building Stone Tour aimed at highlighting the remarkable decorative building stones used to ornament many of Calgary’s buildings.

Larvikite is a coarse grained igneous rock that derives its name from the type-locality near Larvik in southern Norway. Its coarse grained nature, spectacular blue iridescent colour and the fact that the rock is largely unfractured (thus easily cut into slabs) lends itself to being one of the most popular used and most recognizable decorative building stones in the world. Varieties of larvikite are commonly known commercially as "Blue Pearl Granite", "Emerald Pearl Granite", and other names, although almost nothing of its mineralogical composition makes it granitic. In Britain, it is often known as Publich-houseite or pub stone because it is commonly used as exterior and interior facing stone on walls and counter tops in bars. The major use of larvikite is as an exterior and interior facing stone. However, larvikite has also been fashioned for use as a gemstone and into several ornamental and functional items.

Mineralogy
Larvikite is a rock primarily comprised of medium to coarse-grained euhedral feldspar crystals. Look closely at the feldspar crystals and you may notice a blue iridescent shimmer. This phenomena is best observed on a sunny day and is known as the Schiller effect (often wrongly called Labradorescence). The play in colours is due to submicroscopic layering of plagioclase feldspar (specifically oligoclase) and alkali (potassium) feldspar in a phenomena known as anti-perthite. The layering is on a comparable scale to the wavelength of blue light and scatters this wavelength to give the crystals a blue appearance.

Other minerals that may occur in larvikite include augite (pyroxene), amphibole, biotite, quartz, nepheline (but not at the same time as quartz because they are mutually exclusive), iron-titanium oxides (e.g. ilmenite, hematite, magnetite, etc.), apatite, zircon, and/or olivine. These minerals occupy the spaces between the feldspar crystals. If you happen to be carrying a magnet, look for silver-y (often described as gun-barrel grey) milimetre-scale blobs between feldspar grains and try placing it against the building stone and see if you can feel an attraction between the magnet and the rock. Not surprisingly, the presence of magnetite makes the rock mildly magnetic.

Rock Type
The wide variety of intergrown crystals with little to no apparent fabric suggests that this is an igneous rock. Igneous rocks are rocks that formed through the cooling of melted rock; lava if it is at surface or magma if it is at depth. The coarse grains found within the rock indicate that the melt cooled slowly, probably 3 to 4 kilometres below the surface of the Earth. Slow cooling is necessary for large crystals to develop (compare the crystal size in glacier ice versus ice in your freezer).

Now let’s name the rock according to the International Union of Geological Sciences’ (IUGS) classification scheme of plutonic rocks. This is done by considering the relative proportions of major rock forming minerals: quartz, plagioclase feldspar and alkali feldspar. The rough balance between plagioclase and alkali feldspar with very little to no quartz suggests this rock is a monzonite.

Geologic Setting
The Oslo Region is a geographic term referring to an area of about 100 000 km2, which contains sedimentary rocks of Cambrian, Ordovician, Silurian and Carboniferous age as well as sedimentary, plutonic and volcanic rocks of Permian age. The southernmost part of the Oslo Region, the Larvik plutonic complex, is made up of monzonitic rocks, mainly larvikite and closely related varieties, which occupies more than 1000 km2. The complex formed during late Carboniferous to Permian extension and continental rifting across NW Europe. During this time, the Oslo Region was the site of extensive magmatic activity. The Larvik Batholith itself is about 292-298 million years old. It has been proposed that the larvikites are derived by fractional crystallization at the base of the crust from mantle-derived alkaline basalts. When fractionation of the residual magma reached monzonitic compositions, it forced its way upward into the upper crust and crystallized under dry, reducing conditions, and without being significantly affected by the country rocks.

To log this cache:
- Estimate and email me the average size of the feldspar crystals
- Estimate and email me the percentage of the rock composed of feldspar (hint: it is well over 50%)
- Post a picture of the rock with your GPS as a scale. Alternatively, you may take a picture of yourself holding your GPSr with the building stone as a back drop.

Logs posted without an accompanying email or the required photo will be deleted. Do not post a log for this EarthCache until you (a) have emailed the answers and (b) are prepared to post the photo.

Glossary
Batholith: a large plutonic complex that is exposed over hundreds to thousands of square kilometres.

Fractional crystallization: Melted rock (lava or magma) is generally a mixture of many minerals. When cooling, minerals in the melt will crystallize in order of melting point, from highest to lowest. As liquid rock cools, the composition of the melt will change over time as each mineral will draw out different elements.

Granite: a felsic plutonic rock composed of quartz (20 % or more), feldspar and biotite mica

Monzonite: a plutonic rock of intermediate composition containing approximately equal amounts of alkali feldspar and plagioclase feldspar, 0-20 % quartz and commonly augite as the main mafic mineral.

Perthite: fine layering of alkali feldspar (slightly greater than 50%) and plagioclase feldspar (slightly less than 50%). The term perthite is a name derived from where this phenomena was first discovered in another rock type found near Perth Ontario. Anti-perthite occurs when plagioclase feldspar makes up more than 50% of the sum of the two feldspars.

Plutonic rock: Igneous rock that solidified (crystallized) from magma. If cooling is slow, medium to coarse grained crystals are likely to develop.

Volcanic rock: Igneous rock that solidified at the surface from lava. Large crystals are rare because crystallization is too rapid for them to develop.

References
Ayrton, W. G., 2005. The Building Rocks of Calgary Geological Walking Tour.

Lalonde, A., 2003. GEO4367: Advance Mineralogy, Department of Earth Science, University of Ottawa.

Schumann, W., 1993. Rocks, Minerals and Gemstones. PP 216. Houghton Mifflin.

Additional Hints (Decrypt)

Qriba/Pnagreen gbjre

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)


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