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

From: Dave Myers <whitefalcons007_at_meteoritecentral.com>
Date: Tue, 19 Jan 2010 16:28:16 -0800 (PST)
Message-ID: <297758.37666.qm_at_web110104.mail.gq1.yahoo.com>

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? 1650?C. A minimum melting temperataure
> of 1550?C 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.
>
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Received on Tue 19 Jan 2010 07:28:16 PM PST