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Chondrules separated from a chondrite.
Some are round, some are not. Most chondrules
are less than 1 mm across in maximum dimension.
Image of chondrules projecting from the
surface of a chondrite. Chondrules make
up the greater portion of some chondrites.
Undercooling - This refers to a condition in which a process occurs at a lower temperature than expected for equilibrium, such as crystallization below the equilibrium temperature, as a result of rapid cooling. Chondrules were almost certainly undercooled.
Superheating - This refers to a condition in which a process occurs at a higher temperature than expected for equilibrium, such as melting above the equilibrium temperature, as a result of rapid heating. Chondrules may have been superheated.
Crystal nuclei - These are submicroscopic sites that serve as nucleation points for crystals. They may be very small bits of unmelted or forming crystals.
Porphyritic or microporphyritic texture -
This refers to an igneous texture in which larger crystals are embedded
in finer grained groundmass. Many chondrules have a microporphyritic
texture consisting of olivine or low-Ca pyroxenes embedded in a feldspathic
Poikilitic texture - This refers to an igneous texture in which rounded crystals of one kind (e.g. olivine) are completed surrounded by another kind (e.g. pyroxene).
Relict grain - This is a grain in a chondrule that
did not crystallize in the same surroundings in which it is now found.
(Scientists refer to a grain which crystallized in the same surroundings
it is now found as "having crystallized in situ"; relict grains
did not crystallize in situ). They are grains that were not
fully melted during the last episode of chondrule formation. Most
relict grains in chondrules are composed of olivine, which has a high melting
Image of round chondrule showing a partly microporphyritic
partly poikilitic texture, viewed in thin section with cross-polarized
transmitted light. The chondrule consists of olivine and low-Ca
pyroxene grains in a glassy groundmass. The brownish-colored olivine
at lower right contains fine-grained (dusty) metal inclusions and is
a relict grain.
Image of same chondrule as above, viewed in backscatterred
electrons. In this type of image,
brightness is correlated with average atomic number, and iron-rich areas appear bright.
Black areas are cracks. This image clearly shows the low-Fe, high-Mg olivine and pyroxene grains
(dark grey) within the chondrule, the relict olivine grain, and the iron-rich matrix that surrounds
the chondrule. Glass occurs in the interstices between the olivine and pyroxene grains.
From the low-Fe content evident in the olivine and pyroxene, this would be a so-called Type I chondrule.
Modified from http://epswww.unm.edu/iom/Research_Activities.htm
Compound chondrule - This is a chondrule that consists
of multiple, attached portions. Some compound chondrules formed by
low-velocity collisions while the individual pieces were still warm.
The existence of such compound chondrules implies that chondrule-forming
regions of space contained a dense concentration of many molten droplets.
Compound chondrules observed in thin sections, where
the field of view is a little more than 1 mm across in both images.
Left: Two attached chondrules with the same fine-grained (cryptocrystalline
) texture. Cross-polarized transmitted light image. Right:
Two attached chondrules, one with barred olivine (left) and the other (right)
with microporphyritic texture. Plane-polarized transmitted light
From: http://users.skynet.be/jm-derochette/meteorites/chondrule%2014.htm and http://users.skynet.be/jm-derochette/meteorites/chondrules%2018.htm
Fractionation - This refers to any geochemical process that results in a changed composition. The context for fractionation used by Hewins refers to any process that results in a non-cosmic composition.
Condensation sequence - This refers to the assemblage
of minerals that are stable at different temperatures in the solar nebula.
These equilibrium assemblages are usually calculated for systems of cosmic
or solar composition at the low pressures (10-3 to 10-6
bars) deemed suitable for the solar nebula. We'll be saying more
about the condensation sequence later. According to Hewins, likely
precursor materials for chondrules included olivine and pyroxene of various
composition, Na-plagioclase (albite), troilite, and maybe C-compounds.
As the diagram below shows, these aren't high temperature condensates,
even though chondrules formed at high (~1550-1900 C or ~1820-2170 K) temperatures.
So what might this imply?
Equilibrium mineral assemblages in a system of solar
composition at low pressure.
The arrows indicate reaction paths in a cooling system. The same assemblages would be
produced in reverse for a system in equilibrium with increasing temperature.
Agglutinate - This is a glass-bonded aggregate of minerals and clasts formed by impact processes in a regolith. Agglutinates are a common consituent of the lunar regolith but are rare in chondrites, even those solar-wind-gas-rich chondrites thought to have formed in asteroidal regoliths.
FU Orionis event - This is an immense stellar outburst seen in a class of newly-formed or newly-forming star (T-Tauri star) caused by the accretion of gas and dust onto the surface of the star from a surrounding nebular protoplanetary disk. Astronomers record such outbursts as temporary brightness spikes (e.g., the brightness of the star, known as its luminosity, can increase by up to ~250 times).
T-Tauri star - This is a newly-formed star that is in the process of dissipating the disk of gas and dust surrounds it. T-Tauri stars are often characterized by strong stellar winds and rapid rotation.
Hewins' Table 2 - Hewins neglects to mention that the numbers in his Table 2 refer to values of oxygen fugacity (fugacity is a partial pressure, and oxygen fugacity is the partial pressure of oxygen gas) in negative log units. Thus, 14.7 refers to log fO2 = -14.7, or fO2 = 1.995 x 10-15. At the igneous temperatures appropriate for chondrule formation, this is about five orders of magnitude less than for Earth's atmosphere. Such low fO2 values are characteristic of a hydrogen-rich, oxygen-poor gas composition.
Iron-wustite buffer - In general, an oxygen "buffer" is the equilibrium oxygen fugacity that is established by co-existing minerals. The iron-wustite (IW) oxygen buffer contains co-existing wustite (the mineral FeO, a component present in silicate minerals) and metallic iron. If these two minerals co-exist in equilibrium, they will determine the fugacity of oxygen gas that surrounds the minerals.