Talk:Pegmatite
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[edit] Origins
It is very controversial the origin of pegmatites.
After Chris Check "There are many theories about the origin of pegmatites and the amazingly large crystals found in some of them. It is theorized that the pegmatite solidified from a aqueous-silica rich magma. Then the action of the hydrothermal waters on the solidified mass reacted with the present minerals to create new and larger crystals. The other less popular theory is that the hot magma, under intense heat and pressure, experienced a sudden drop in heat or pressure which could cause large scale crystallization."
After Bob Linnen, from the Waterloo University, Canada, "The origin of pegmatite and the explanation of how crystals grow so large are controversial. However, the most widely accepted origin is that the crystals grow from water-rich melts, that are also rich in fluxing elements (elements that lower the melting temperature of silicate minerals) such as boron, fluorine and phosphorous. The high concentrations of these fluxing elements also increases diffusivities in the melt, allowing cations to move more quickly to sites of growing crystals, and this results in the growth of very large crystals. The question of why some pegmatites contain gems whereas others do not is also the subject of debate."
My opinion is that we need to be impartial and to explain the two theories. If choice is necessary I vote for the most popular for us the geologists, that is, the emphasis is put on the hydrothermal liquid and its composition and not on the time of crystallization
Eurico Zimbres Rio de Janeiro University
School of Geology
15:21, 25 January 2006 (UTC)
- Pegmatites are also correlated with metamorphism (in timing, and in their occurrence in at least greenschist facies terranes) and synorogenic granite intrusions. So its not entirely magmatic otherwise pegmatites would occur in relation to all magmatism, in sedimentary basins, etc. I for one, have seen a fair few, and most seem oriented parallel with a stretching lineation or foliation in transpressional-transtensional regimes (Mt Isa terrane; Glengarry Basin-Gascoyne Complex) or in polydeformed amphibolite terranes (Broken Hill Block) parallel with zones of acommodation and local free-space creation and unassociated with any magmatism. Indeed, one of the largest pegmatite bodies in the world, in Western Australia is more closely correlated with an intersection lineation on a thrust plane than with any magmatism.
- The problem with them being purely metamorphic, in this sense, is the chemistry. While its likely they form on structural surfaces because these are dilational, and during metamorphism because there is ample fluid and heat being derived from the deformation, the chemistry of some of these supports them being derived from granites. It is a complex picture.
- Either way, the article as it stands today, is not the best for describing pegmatites. I will have a read up on some of my literature and have a look at the field occurrences near where I work, and see what I can do. Rolinator 02:36, 2 April 2006 (UTC)
