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Granitic Pegmatites [Évora] EarthCache

Hidden : 12/04/2008
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Geocache Description:

We refer to a Pegmatite when dealing with a very coarse-grained igneous rock that has a grain size of 20 mm or more.

In 1845, W. Haidinger was apparently the first to use the word pegmatite to describe “coarse-grained, feldspar-rich granites”. However, in 1849, A. Delesse used the word pegmatite to also include rocks of very large grains which consisted of orthoclase, quartz and silvery mica, and which occur so commonly in the form of dykes, small stocks and nests in other rocks. Our present-day use of the word follows the basic idea of Delesse, but also includes the caveat that they be of igneous origin. In most cases, the igneous rocks are of granitic composition, although other compositions (e.g., granodioritic, gabbroic) may exist. Many others include as part of the definition, aspects about the mineralogy, texture, structure, chemistry and mode of occurrence. Taking this approach, we can define pegmatites as such: Pegmatite, in the strictest sense, is a textural term used to describe exceptionally coarse- to gigantic-grained igneous rocks. They occur as tabular dykes, sills, lenses or veins near the margins of plutons. They tend to have zoned or layered structures, extremely variable texture of mineral aggregates and some of them are enriched in rare elements. The shape and size of pegmatites vary greatly from linear, tabular bodies with straight edges to bulbous and irregular masses to turnip-shaped bodies. Pegmatites may be several meters long and less than 1 meter thick or as much as 3000 meters long to 700 meters wide. Pegmatites range in age from the Precambrian to very recent and hence span the entire geologic time scale.

One of the features which attract many people to pegmatites is the abundance of minerals that are found in them. There have been about 550 different minerals found in pegmatites.

Most pegmatites are composed of quartz, feldspar and mica; in essence a "granite". Rarer "intermediate" and "mafic" pegmatite containing amphibole, Ca-plagioclase feldspar, pyroxene and other minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.

The mineralogy of a pegmatite is in all cases dominated by some form of feldspar, often with mica and usually with quartz, being altogether "granitic" in character. Beyond that, pegmatite may include most minerals associated with granite and granite-associated hydrothermal systems, granite-associated mineralisation styles, for example greisens, and somewhat with skarn associated mineralisation.

It is however impossible to quantify the mineralogy of pegmatite in simple terms because of their varied mineralogy and difficulty in estimating the modal abundance of mineral species which are of only a trace amount. This is because of the difficulty in counting and sampling mineral grains in a rock which may have crystals centimetres, decimetres or even metres across. Garnet, commonly almandine or grossular, is a common mineral within pegmatites intruding mafic and carbonate-bearing sequences. Pegmatites associated with granitic domes within the Archaean Yilgarn Craton intruding ultramafic and mafic rocks contain red, orange and brown almandine garnet.

Tantalum and niobium minerals (columbite, tantalite, niobite) are found in association with spodumene, lepidolite, tourmaline, cassiterite in the massive Greenbushes Pegmatite in the Yilgarn Craton of Western Australia, considered a typical metamorphic pegmatite unassociated with granite.

Economic importance of pegmatites:
Pegmatites are important because they often contain rare earth minerals and gemstones, such as aquamarine, tourmaline, topaz, fluorite, and apatite, often along with tin and tungsten minerals, among others.

Granitic pegmatites are important sources of rare-elements, such as beryllium, niobium, tantalum, tin, lithium, rubidium, cesium and gallium; industrial minerals; gems and mineral specimens. When present in economic quantities, these rare-elements may be extracted for use in a wide range of technological applications, such as lightweight alloys, nuclear engineering and electronics (beryllium); ceramics, pharmaceutical products, lubricants, smelting of aluminium ore and lithium-batteries (lithium); electronic capacitors, jet engines and prosthetic devices (tantalum); magnetohydrodynamic electric generators, biological and medical research (cesium); and integrated circuits and light-emitting laser diodes (gallium). Industrial minerals such as feldspar and quartz are extracted from pegmatite deposits for use by the glass and ceramic industries, while mica is used in construction materials and insulation.

Some of the world’s best-known gem material is obtained from pegmatite deposits. Varieties of beryl (aquamarine, golden, morganite), spodumene (kunzite, hiddenite) and tourmaline (pink, green and multi-colored elbaite), as well as garnet and topaz are all valued precious stones originating from pegmatites.

Lithium pegmatites bearing spodume are common in the north of Portugal and represent a proven ore reserve.

How do pegmatites form?
As the main magma body cools, water originally present in low concentrations becomes concentrated in the molten rock because it does not get incorporated into most minerals that crystallize. Consequently, the last, uncrystallized fraction is water rich and also rich in other elements that also do not readily go into ordinary minerals. When this water-rich magma (also rich in silica and unusual elements) is expelled in the final stages of crystallization of the magma, it solidifies to form a pegmatite.

The high water content of the magma makes it possible for the crystals to grow quickly, so pegmatite crystals are often large. Thank you for bothering to actually read this so far.

The Alto de S. Bento pegmatites:
In the almost flat peneplain of the Alentejo rises the natural belvedere of Alto de São Bento located 2 km WNW of the city of Évora. This belvedere, a granitic dome with several pegmatites, rises some 60 m above the elevation of Évora affording an unimpeded view of the city and its fortressed walls. The granites date from the Late Carboniferous (304 M.a.) and are calc-alkaline, medium-grained, non-porphyritic quartz diorites and grandiorites. It is here as well that a museological centre has been created to propagate the understanding of the geological and biological sciences with two old wind mills having been converted to accommodate practical learning centres. The centre also has clear, illustrated signboards showing the local geology and exemplifying the granites and their mineralogy, the pegmatitic veins and aplites.

The cache:
To claim the “found” for this EarthCache you need to visit the learning centres and the required answers are easily found outside on the outcropping granite. Guided by the informative boards at the coordinates, search for the plate numbered “5P” on the floor that is fixed on the pegmatite vein. At this point, tell me the thickness of the pegmatite and the size of the crystals that you see within the vein.

Para reclamar o found nesta EarthCache terão que visitar o centro de interpretação do Alto de S. Bento e encontrar as respostas no maciço granítico ali exposto. Nas coordenadas dadas estará um placard que indica a posição do filão de pegmatito granítico (placa “5P”). Aqui, deverão medir a espessura do filão e o tamanho dos cristais apresentados.

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+ Mata, MB et al., 1969. Carta Geológica 1:50000 nº 40-A (Évora). Serviços Geológicos de Portugal ;
+ Carvalhosa, AB, Galopim de Carvalho, AM, Matos Alves, CA, Pina, HL, 1969. Noticia Explicativa da Carta Geológica 1:50000 nº 40-A (Évora). Serviços Geológicos de Portugal, 26p.

Special thanks to Daniel Oliveira for the validation, research and support for this EarthCache. This cache maintenance will be performed by an imaginary friend of mine.

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