Thanks Darren - now this is more like it - hmmmm
--- Darren Garrison <[EMAIL PROTECTED]> wrote: http://www.aspbooks.org/custom/publications/paper/index.phtml?paper_id=2447 > Chondrule textures depend on the extent of melting > of the chondrule precursor- material when cooling > starts. Kind of begs the question - chodrules formed by collision, which causes melt - consider if one started from a steady molten state >If "viable nuclei" I wonder what these "viable nuclei" are? viable cystal nuclei=Chondrules? > remain in the melt, crystallization begins > immediately, producing crystals with shapes that > approach equilibrium. If not, crystallization does > not occur until the melt is supersaturated, resulting > in more rapid growth rates and the formation of > skeletal or dendritic crystals. > A chondrule texture thus indicates whether > nuclei were destroyed, which implies a melting > temperature above the liquidus temperature for its > particular composition. The presence or absence of > skeletal or dendritic crystals in chondrules can be > used to constrain their peak temperatures, which > range from 1400-1850°C. hmmm - collision temperatures of 1400-1850°C > Heating times of less than a second result in > aggregates of starting materials coated > with glass, resembling agglutinates rather than > objects with typical chondrule textures, suggesting > that heating times are longer. different heating times=two mechanisms for chondrule formation? two classes 1)agglutinate 2)original? > Chondrule textures can be duplicated with a very > wide range of cooling rates, but if olivine zoning > is to be matched the cooling rate should be within > the range 10- 1000°C/hr. The size of overgrowths on > relict grains cannot be used to infer cooling rates. > Chondrules melted in a canonical nebular gas lose > sulfur and alkalis in minutes, while iron loss from > the silicate melt continues over many hours. Mass > loss and isotopic fractionation can be suppressed if > the partial pressures of the species of interest are > high enough in the ambient gas. Chondrule bulk and > mineral composition arrays can be reproduced to a > large extent by evaporation. Yeah, but chondrules are found in matrices. > However, condensation of SiO and how exactly is this SiO condensation supposed to occur? > into the melt can simulate the zonation in some > chondrules, with pyroxene and a silica polymorph near > the rims. yeah, but could the silica rims have come from the inside? > The partial equilibration of chondrule melt with > noncanonical nebular gas would require heating for > time periods of hours. > Hm. Effervescence following a sudden release of > pressure makes me think more of > a can of Coke. :-) We all get our inspiration somewhere - __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list