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, [email protected] <[email protected]> wrote: > From: [email protected] <[email protected]> > Subject: [meteorite-list] Paper on chondrule formation and synthetic > chondrules > To: [email protected] > 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 > [email protected] > http://six.pairlist.net/mailman/listinfo/meteorite-list > ______________________________________________ Visit the Archives at http://www.meteoritecentral.com/mailing-list-archives.html Meteorite-list mailing list [email protected] http://six.pairlist.net/mailman/listinfo/meteorite-list
