Le pegmatiti

dove e cosa cercare e come collezionare minerali
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Pasquale
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COMPLIMENTI!

Messaggio da Pasquale » dom 16 mar, 2008 13:53

Ciao Giampaolo! Sempre Bellissimi i tuoi granati!

Anche se devo dire che sono piu' belli dal vivo... :wink:


Prossima "tappa": skarn a scheelite di Longobucco.

Saluti a tutti gli amici del forum.
Pasquale

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Di Domenico Dario
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Messaggio da Di Domenico Dario » mar 18 mar, 2008 13:28

Sulle carte geologiche del nuorese sono segnalate molte pegmatiti. Ci sono notizie di ritrovamento di minerali?

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Alessandro
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Messaggio da Alessandro » mar 18 mar, 2008 16:37

In Mindat nell'elenco gli unici segnalati, di specie che potrebbero essere attinenti a una pegmatite in provincia di Nuoro, sono ortoclasio e pirocloro.
Alessandro Socci

GAMPS
Gruppo Avis Mineralogia Paleontologia Scandicci

venderò la mia rabbia a tutta quella brava gente che vorrebbe vedermi in gabbia e forse allora mi troverebbe divertente

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Marco E. Ciriotti
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Messaggio da Marco E. Ciriotti » gio 12 giu, 2008 14:22

Obiezioni (testo in lingua inglese) sulla corrente teoria delle pegmatiti è in:
http://www.mindat.org/article.php/332/O ... ite+theory

Di interesse anche i commenti a seguire di Rob Woodside.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
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Re: Le pegmatiti

Messaggio da Marco E. Ciriotti » dom 31 ago, 2008 9:55

Genesi delle pegmatiti: lo stato dell'arte. Un interessante articolo appena pubblicato sull"European Journal of Mineralogy.

Referenza:
• Simmons, W.B.S. & Webber, K.L. (2008): Pegmatite genesis: state of the art. European Journal of Mineralogy, 20, 421-438.

Abstract:
No one universally accepted model of pegmatite genesis has yet emerged that satisfactorily explains all the diverse features of granitic pegmatites. Genesis from residual melts derived from the crystallization of granitic plutons is favoured by most researchers. Incompatible components, fluxes, volatiles and rare elements, are enriched in the residual melts. The presence of fluxes and volatiles, which lower the crystallization temperature, decrease nucleation rates, melt polymerization and viscosity, and increase diffusion rates and solubility, are considered to be critical to the development of large crystals. A number of new concepts have shed light on problems related to pegmatite genesis.
Cooling rates calculated from thermal cooling models demonstrate that shallow-level pegmatites cool radically more rapidly than previously believed. Rapid cooling rates for pegmatites represent a quantum shift from the widely held view that the large crystals found in pegmatites are the result of very slow rates of cooling and crystal growth.
Experimental and field evidence both suggest that undercooling and disequilibrium crystallization dominate pegmatite crystallization. London’s constitutional zone refining model of pegmatite evolution involves disequilibrium crystallization from an undercooled, flux-bearing granitic melt. The melt is not necessarily flux–rich and the model does not require the presence of an aqueous vapor phase.
Experimental studies of volatile- and flux-rich melts and fluid inclusion studies suggest that volatile-rich silicate melts may persist to temperatures well below 500 °C and even down to 350 °C.
Studies of melt inclusions and fluid inclusions have led some researchers to suggest that the role of immiscible fluids must be considered in any model regarding pegmatite genesis. Fluid saturation is thought to occur early in the crystallization history of pegmatites. Two types of melt inclusions along with primary fluid inclusions have been found coexisting in pegmatite minerals.
Advances by Petr ern in pegmatite classification are in wide use and the fractionation trends of Nb, Ta and other HFSE and K, Rb, Cs, Li, Ga and Tl are now well understood.
How pegmatitic melts are produced, the types of source rocks involved and how melt generation relates to plate tectonic models are challenging areas for future investigations. Also, the roles of regional zoning, anatexis, and chemical quenching in pegmatite genesis are areas for future pegmatite research.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
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Re: Le pegmatiti

Messaggio da Marco E. Ciriotti » sab 13 set, 2008 14:32

Pegmatites
Special Publication 10
Mineralogical Association of Canada, Ed., Québec
368 pagine

Member price US$100
Others price US$125
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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