Talk:Pegmatite

Disputable statements

This article contains statements that are disputable.

The metamorphic model of pegmatite formation says that the origin of pegmatite melts are particularly felsic metamorphic rocks. For the Evje-Iveland pegmatite swarm in the south of Norway this seems to be incorrect. According to Snook the melts are derived from mafic rocks: https://ore.exeter.ac.uk/repository/bitstream/handle/10871/14884/SnookB.pdf?sequence=1&isAllowed=y

The speed of crystallisation is another issue. In ELEMENTS Volume 8, Number 4 • August 2012 of the Mineralogical Society of America there was an article where the speed of crystallisation of some Californian pegmatites was presented. My copy of the magazine has disappeared and there is no online copy, so I cannot say on which page this is published. But I recall that it was more a matter of months or years, rather than thousands of years.

Very likely the picture is more complicated. The Evje-Iveland pegmatites formed in a completely different setting than the Californian pegmatites. Here it can be assumed that the bodies of molten rock slowly cooled down while the crust was slowly moving upward. But that in itself does not mean the process of crystallisation therefore also was slow.

I do not want to be involved in editting this article; this is not my field of expertice. I only want to say that a true expert should be encouraged to get involved in improving this article. Even though that is no guarantee for success. I refer to the PEG2017 symposium on Norwegian pegmatites in Norway. The lectures made it clear that there was no consensus on how pegmatites formed. http://geologi.no/images/GeologiskeGuider/PEG2017_Excursion_Guide_NGF_Series_2017-6_red.pdf (just as reference, there is nothing about the discussions in this publication.) Ronald Werner (talk) 11:25, 25 March 2021 (UTC)

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)

Fast crystallization ?

I don't understand why this article states that the crystal growth must be incredibly fast to allow the crystals to become so large in a pegmatite. I have always thought that larger crystals necessarily implied weaker supersaturation, caused by a slower cooling of the rock.

In fact, what's really important is the ratio between the nucleation speed and the crystallization speed. If there is a fast nucleation and a slow crystal growth, the crystals will be small. With a slow nucleation and a fast crystallization, they will be large. But we could also see large crystals form by slow crystallization, as far as the nucleation process was even slower. This could be achieved by a slower rate of cooling.

Am I right ? Or did I miss something ?

Pijeth 20:59, 4 May 2007 (UTC)

When you say "larger crystals necessarily implied weaker supersaturation, caused by a slower cooling of the rock" I think that you should consider komatiite as a bit of a counterpoint to this idea. The crystal size of a rock doesn't neccessarily correlate always with only nucleation/crystallisation rates and this isn't only determined by the degree of supercooling.

For example, harrisistes in komatiites, and spinifex textures can form crystals up to 60-120cm in size in ultramafic rocks which cooled through the liquidus within hours. This is a factor of the extremely low nucleation rates caused, firstly, by the superfluidity of the melt (ie; depolymerisation due to superheating) meaning it can become supercooled because there is no nuclei to speak of in the melt, and then secondly by the crystallisation rate.

In pegmatites, we are dealing with generally low-temperature highly silicic fluids (i say fluids because the evidence for supersaturation with water is compelling) which must, neccessarily, be heavily polymerised and hence highly viscous. The more viscous and nucleated the melt, the slower is the diffusion rate, and hence the slower you cool the rock the smaller the crystals simply because crystals cannot be separated from their crsytallisation products fast enough. The result of a slowly cooled granitic composition melt is neccessarily a coarse granite. To get a granitic omposition to produce huge crsystals we have to do the same as happened to turn a gabbro into a komatiite.

In komatiites, you get spinifex and harrisites because the superfluid melt can carry away the rejected solute from olivine crstallisation (Al, Fe, Si, etc) because it is in a volcanic flow environment. When the flow stagnates, crystallisation continues, and the result is a gabbro, generally equigranular and usually coarse (the B2 horizon). These gabbros probably formed in a slowly cooling ponded environment with plenty of nuclei.

So your idea of supersaturation and slow cooling doesn't really hold for closed systems. The other factor in pegmatite magmatism, water, also implies it is mostly a vapor phase system, and as such to form a pegmatite with vapor, it has to be quick because vapors dissipate quickly within the crust (ie; it is an open system). For example, very few voids stay open even in the upper levels of the crust, so to form a vapor-filled void in the deep crust requires a lot of effort. The granite related pegmatites tend to be formed by crystallisation of large masses of granite which forces waters with the usual incompatibles which form the weird minerals n pegmatites, out into wall rocks or aureoles. I have seen several granite contacts in the Yilgarn with a semi-pegmatite aureole several metres thick caused by exsolution and a vapor cupola around the granites. This final process could take several tens of thousands of years, which is quick by geological standards. A void with a pegmatite could stay open for only a short period of time.

The low nucleation rate needed to produce huge pegmetite crystals can be explained by water vapor based depolymerisation of the melt, but to get this water, you need a transient system which couldn't last very long at all. So, crystals form as the vapors flow through and supply the site with fresh components and carry off the products to elsewhere.

So, to my mind, the process of pegmatite formation is an open system, volatile-rich fissure or vugh where granitic components are concentrated and, in a short period of time, deposit silicate crystals. They don't neccessarily cool slowly, more likely the process operates for only a short perod of time and the crystals grow quickly, then the tap turns off and the voids fill up with whatever is left as the vapors dissiparte into the surrounding rock. Or so my conception goes. Rolinator 02:02, 5 May 2007 (UTC)

Moved Chrysoberyl comment

Pls also mention "chrysoberyl" which is defference of the family of Beryl,and it's containing in to pegmatites.

Moved this from article here. --Tobias1984 (talk) 09:24, 19 February 2013 (UTC)