[meteorite-list] Paper on chondrule formation and synthetic chondrules

[Dave Myers]

Thanks,

That info. is great! I love the CV3 and the LL3-6, That show hundreds of chondrules,  I even like them better than stoney-irons! There 2nd!
Only wish I could aford them! ..LOL  

Thanks for the info.

Dave Myers



--- On Tue, 1/19/10, starsandscopes at aol.com <starsandscopes at aol.com> wrote:

> From: starsandscopes at aol.com <starsandscopes at aol.com>
> Subject: [meteorite-list] Paper on chondrule formation and synthetic chondrules
> To: meteorite-list at meteoritecentral.com
> Date: Tuesday, January 19, 2010, 11:54 PM
> 
> 
> Hi List,  I thought some of you might enjoy this
> portion of a science  
> paper on meteorite chondrules.  It is part of a paper
> on microscopes posted  in 
> Molecular Expressions (An online microscope site)  The
> first half of the  
> paper is on microscopes so many of you will want to skip
> that part.
> Tom  Phillips
> 
> PHOTOMICROGRAPHY IN THE
> GEOLOGICAL SCIENCES 
> Michael W.  Davidson
> Institute of Molecular Biophysics
> Center for Materials Research  and Technology
> (MARTECH)
> National High Magnetic Field Laboratory  (NHMFL)
> Supercomputer Computations Research Institute (SCRI)
> Florida State  University, Tallahassee, Florida 32306
> Telephone: 850-644-0542 Fax:  850-644-8920
> 
> Gary E. Lofgren
> Planetary Materials Branch
> Solar  System Exploration Division
> Code SN2
> NASA Johnson Space Center
> Houston,  Texas 77058
> Telephone: 713-483-6187 Fax: 713-483-2696
> 
> The whole  article is at 
> http://micro.magnet.fsu.edu/publications/pages/journal.html 
> 
> 
> 
> 
> Chondrules are small spheres (.1 to 10mm in diameter) which
> are  the major 
> constituent of chondritic meteorites. Chondrites are
> considered  samples of 
> primitive solar system materials. If we can understand how
> chondrules  form, 
> we will have an important clue to the early history of our
> solar system.  
> Most chondrules have an igneous texture which forms by
> crystal growth 
> (usually  rapid) from a supercooled melt. Such
> textures are commonly described as  
> porphyritic (large, equant crystals in a fine grained
> matrix), barred 
> (dendrites  comprised of parallel thin blades or
> plates), or radiating (sprays of 
> fine  fibers).
> The models proposed for formation of chondrules can be
> divided into  two 
> groups (McSween, 1977). In one group of models, chondrules
> form by melting  
> and subsequent crystallization of preexisting, largely
> crystalline material 
> from  the solar nebula. The primary differences
> between these models are the 
> kinds of  materials which are melted and the nature of
> the sources of heat 
> for the  melting. In the other group of models,
> chondrules form by 
> condensation of  liquids from the solar nebula gas
> which then crystallize upon cooling. 
>  Variations between these models result from differences in
> the 
> condensation  sequence of the minerals and melts and
> the temperatures of nucleation.
> One  means of testing models of chondrule formation is
> to determine the 
> conditions  necessary to duplicate these textures by
> experimentally 
> crystallizing chondrule  melts in the laboratory.
> Efforts to reproduce the textures of 
> chondrules  experimentally have been successful only
> when we began to 
> understand the  important role that heterogeneous
> nucleation plays in the 
> development of igneous  rock textures. Unless
> heterogeneous nuclei are present in 
> the chondrule melt,  porphyritic textures will not be
> produced. The dendritic 
> or radiating textures  will form instead.
> The treatment of heterogeneous nucleation follows the 
> model developed by 
> Turnbull (1950) to explain many of the characteristics
> of  heterogeneous 
> nucleation. This model was applied to heterogeneous
> nucleation in  basaltic 
> systems by Lofgren (1983). Simply stated, the model says
> that in any  
> steady-state melt at a given temperature there is a
> characteristic distribution  of 
> embryos. The embryo is crystalline material which is
> smaller than the  
> critical size necessary to be a stable nucleus and cause
> nucleation. It is a  
> subcritical-sized potential heterogeneous nucleus. Embryos
> exist whether stable,  
> supercritically-sized nuclei are present or not. If a melt
> is sufficiently  
> superheated, embryos can be eliminated. Nucleation would
> then require a 
> surface,  presumably the container and the barrier to
> nucleation would be much 
> higher than  in the case where embryos were present.
> Qualitatively, such 
> nucleation would  resemble homogeneous nucleation;
> but, if a surface is 
> available, the energy  barrier would be much lower
> than for homogeneous nucleation. 
> Glasses would form  from chondrule melts most readily
> if they are 
> superheated, thus destroying the  embryos and
> increasing the barrier to nucleation. 
> Lower melting temperatures  would allow embryos to be
> retained. These can 
> then grow upon cooling and become  nuclei. Embryos
> also can become nuclei 
> without changing size, because the size  at which an
> embryo becomes a nucleus 
> depends upon the degree of supercooling in  the melt.
> Thus, an increase in the 
> degree of supercooling can cause an embryo to  become
> a nucleus and 
> nucleation to occur.
> If relict crystals are present in  the melt at the
> initiation of cooling, 
> the more equilibrium-like crystals  typical of
> porphyritic textures are 
> formed. When such experiments are quenched,  the final
> product contains glass or 
> fine grained material, often dendritic,  enclosing the
> equilibrium 
> phenocrysts. An example of this texture produced in 
> experiments is shown in Figure 
> 7. Equant, well formed crystals of olivine are  set in
> a glassy matrix with a 
> few dendrites present. In the natural prophyritic 
> chondrule the glass has 
> usually crystallized to a very fine grained material.
> In  general, the size 
> of the phenocrysts decreases and their number increases as
> the  temperature 
> at which the crystalline starting material melted is
> lowered and  thus the 
> number of nuclei increases. The range of conditions that
> control the  
> development of porphyritic texture has been studied as a
> function of variations  in 
> the number, distribution, and kinds of heterogeneous nuclei
> (Lofgren and  
> Russell, 1986; Lofgren, 1989). The transition from
> porphyritic texture to 
> radial  or barred (dendritic) texture for melts of
> constant composition has 
> been studied  as a function of the presence or absence
> of heterogeneous nuclei 
> and cooling  rate. Such variations in texture within a
> single melt have 
> already been  demonstrated for melts of lunar and
> terrestrial basalt composition 
> (Lofgren,  1980, 1983; Grove and Beatty, 1980).
> The "classic" barred olivine texture is  a single
> plate dendrite 
> (Donaldson, 1976) which shares the entire chondrule
> with  the remaining glass or 
> subsequent crystallization products. Olivine rimming
> the  chondrule is often in 
> optical continuity with the dendrite and thus is part
> of  the plate dendrite. 
> Because this texture is so striking, barred olivine
> (BO)  chondrules are 
> well known even to people outside the field of meteorites.
> When  chondrules 
> are discussed, a photomicrograph of a barred olivine
> texture is  usually 
> chosen as one of a few or even the only example. It is not
> surprising  that 
> considerable effort has been expended understanding its
> origin. Barred  olivine 
> textures comprise only a few percent of melt-textured
> chondrules,  usually 
> less than 5% (Gooding and Keil, 1981). The "classic" barred
> texture  
> represents only 10% of the type 3 ordinary chondrite BO
> chondrules. By careful  
> study, Weisberg (1987) determined that the multiple plate
> dendrite is a much  
> more common that the single dendrite. Most investigators
> propose that BO  
> chondrules form from melt droplets that crystallize rapidly
> upon  cooling.
> Attempts to duplicate BO textures experimentally showed
> that it is  
> difficult to produce the "classic" single dendrite
> chondrule; conversely,  multiple 
> plate dendrites are observed commonly in experimental
> charges (Lofgren  and 
> Lanier, 1990). It turns out to be very difficult to
> restrict nucleation to 
> a  single event. An example of a barred dendrite is
> shown in Figure 8. Each  
> dendrite is a series of parallel plates connected in the
> third dimension 
> with  respect to the plane of the thin section. The
> dendrite forms when nuclei 
> are  eliminated from the melt and only embryos remain.
> If the melt is 
> cooled rapidly  and does not crystallize, it becomes
> supercooled and embryos 
> eventually become  stable nuclei. When an olivine
> nucleus begins to grow, it 
> will become a dendrite  if the supercooling is
> sufficiently high.
> These experiments clearly  demonstrate the crystalline
> material must be 
> present in the solar nebula when  the chondrules form
> and suggests that they 
> did not form by direct condensation  from vapors in
> the solar nebula. 
> Individual crystals most likely formed first  and
> these were remelted in clusters to 
> form the chondrules. An interesting fact  that has
> come out of these 
> studies is that the rate at which the melt droplets 
> cool is not critical. They 
> can cool at nearly the same rate and produce either 
> the porphyritic texture 
> if nuclei are present when cooling is initiated, or
> form  dendrites (barred) 
> chondrules if only embryos are present. The important
> factor  is how hot the 
> droplets become before they begin to cool and thus whether
> they  retain any 
> crystalline precursor material to act as nuclei or whether
> nuclei  have to 
> form from embryos. If the melt droplets are heated hot
> enough that even  the 
> embryos are eliminated, the droplets usually do not
> crystallize when cooled 
>  and form glass chondrules. Glass chondrules are rare and
> this places an 
> upper  temperature limit to which the melt droplets
> are heated which is 
> approximately  1650ºC. A minimum melting temperataure
> of 1550ºC is dictated by the 
> minimum  amount of melting required to produce the
> observed textures. It is 
> still not  clear, however, what heat source provides
> such conditions (Wood, 
> 1988). A  popular model is heating due to viscous drag
> on particles as they 
> move through  dense parts of the solar nebula as
> proposed by Wood (1984
> Chemical analysis  of chondrites (Wasson, 1974)
> indicates that there is a 
> variety in their  composition leading us to believe
> that they are not all 
> derived from a common  source. Most chondrites are
> composed primarily of 
> olivine, feldspar,  orthopyroxene, with several metals
> including kamacite and 
> taenite. Continuing  studies on the chemical and
> physical nature of chondrites 
> and their formation is  providing insight into the
> history of the solar 
> system.
>  
> ______________________________________________
> Visit the Archives at http://www.meteoritecentral.com/mailing-list-archives.html
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>