Architectural Marvel
From the outset, Danish architect Jørn Utzon had a vision to create a striking sculptural form, one that would relate naturally to the undulating waves of Sydney’s superb harbour. Organic materials and colours therefore form the fundamental elements of Utzon’s design: a strong red-granite base anchors the arched white sails that soar elegantly into Sydney’s skyline.
Sydney Opera House’s Forecourt is one of the world's most spectacular outdoor performance spaces and offers the largest capacity of any Opera House venue. Set below the famous white sails and the granite Monumental Steps, the Forecourt offers patrons harbour and city views.
International icons such as Florence and the Machine, Björk, Oprah, The National, Crowded House, Sting and The Wiggles have all performed here to thousands of adoring fans. Queen Elisabeth II oficially inaugurated the building from this forecourt on October 1973.
Apart from the tile of the shells and the glass curtain walls of the foyer spaces, the building's exterior is largely clad with aggregate panels composed of pink granite quarried at Tarana in the central west of New South Wales.
GEOLOGY OF THE ROCKS AT THE SYDNEY OPERA HOUSE
1. Granite
Granite is a plutonic igneous rock formed by the slow cooling of a magma (molten aluminum) to great depths of the earth's crust. This prolonged cooling allows large crystals to form, giving rise to the characteristic crystalline texture of the granites.
They consist of three essential minerals: quartz (transparent, white, gray ...) feldspar (white, pink, red, yellow, brown, green and gray), mica and a very variable group of accessory minerals, presented in percentages Less than 5%, such as apatite, sphene, oxides, allanite, zircon, amphiboles, etc. (black color).
Granite is one of the most valuable stones for construction purposes. Although the quality of granite varies according to the proportions of the constituents and to their method of aggregation, this kind of stone is generally durable, strong, and hard. The hardest and most durable granites contain a greater proportion of quartz and a smaller proportion of feldspar and mica. Feldspar makes granite more susceptible to decomposition by the solution potash contained in it, potash feldspar being less durable than lime or soda feldspar. Mica, being easily decomposed, is an element of weakness in granite. An excess of lime or soda in the mica or feldspar hastens disintegration, as does also an excess of iron. Therefore, stones showing large and dark iron stains should be rejected for outside work. Fine-grained granite weathers better than does granite of coarser grain.
Granite has a pearly lustre. The colour of common granite varies from white through yellow to deep red, and the stone is generally classified as gray and red. Feldspar renders the stone lighter in colour.
Because of its uniform structure, granite can be quarried in large blocks. The rift, the grain, and the joint planes are advantageous in quarrying, as it is very difficult to cut granite in other places. The uses for which granite is suitable depend on the texture of the stone. Medium-grained stone is best fitted for building construction. Fine-grained stone can be carved and polished, but, on account of its extreme hardness, it cannot be worked readily. Such stone is, therefore, costly when it has to be cut, Coarse-grained granite should be used only for concrete aggregate.
2. Feldspar
Feldspars are a group of mineral roofs and aluminosilicates corresponding in volume to as much as 60% of the earth's crust. The composition of feldspar constituents of rocks corresponds to a ternary system composed of orthoclase (KAlSi3O8), albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).
Feldspars with a chemical composition between anorthite and albite are called plagioclase, while feldspars with a composition between albite and orthoclase are called feldspars potassic. Feldspar is an essential component of many igneous, sedimentary and metamorphic rocks such that many of these rocks are classified according to their feldspar content. The feldspar structure can be described as a shell of silicon and aluminum with alkali bases and alkaline earth metals in the void spaces.
Granite is the hardest stone that humans use for construction. The hardness of a stone or mineral is measured using the Mohs scale. Granite has a measure of seven out of 10 on the Mohs scale. It also has a dense grain content. This makes it stain-resistant and wear-resistant, and gives it high load-bearing capacity. Granite is almost completely devoid of porosity and does not readily absorb water.
Quartz has a hardness of 7 on the Mohs scale, feldspar 6, and mica is always less than 6 but the hardness of granite varies according to the quarry.
There are two simple rules for assessing the quality of granite:
1.- Grain size: In general terms, the larger the grain of the granite, the lower its hardness and the greater its ease of crumbling.
2.- Color: the darker the granite, it is that it has less quartz ratio, therefore, the less hard it is. On the contrary, the clear granites, having more proportion of the harder material in their composition will be harder as well.
1+2 = Alkali feldspar granite
Alkali feldspar granite, some varieties of which are called 'red granite', is a felsic igneous rock and a type of granite rich in the mineral potassium feldspar (K-spar). It is a dense rock with a phaneritic texture. The abundance of K-spar gives the rock a predominant pink to reddish hue; peppered with minor amounts of black minerals.
As shown in the QAPF diagram, alkali feldspar granite contains between 20% - 60% quartz. Less quartz content would lead to "quartz alkali feldspar syenite". More than 90% of the total feldspar content is in the form of alkali feldspar. Less than that amount would designate the rock as a granite.
Other incorporated silicate minerals may include, very minor amounts of plagioclase feldspar, mica in the form of muscovite and/or biotite, and amphibole (often hornblende). Oxide minerals such as magnetite, ilmenite, or ulvospinel. Some sulfides and phosphates (mainly apatite) may also be present.
To log this Earthcache as found it you should:
1º Send us an e-mail with the answers to the following questions:
-What colors can be seen in the feldspars in the granite of the Opera House Forecourt? What is the percentage of this feldspars in the granite? Taking in mind your answers and with the above information, what type of granite is the one used at the Sydney Opera House forecourt?>
-What other colors/materials can also be seen in the granite?
- Estimate the grain size of the granite: A- < 1mm; B- 2-4 mm or C- > 6mm.
- With the data above How do you think the granite quality of the Opera House Forecourt is? Why?.
2º Walk inside the Opera House Building Main Ticket Office foyer and visit one of the four restrooms on that level. Inside you will find on the walls the same granite but polish. What differences can you appreciate on the granite blocks compare to the ones outside?
3º [Optional] Take a photo of the place with you GPS in it.
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