This laboratory is an activity for you to examine a meteorite. While it is part of the astronomy course principles of radiation astronomy, it is also independent. Information about the examination of meteorites comes from the examination of pieces of rock found here on Earth.
Some suggested examination entities to consider are grain size, grain shape, mass, age, impurities, minerals, and origin.
More importantly, there are your examination entities. And, yes, you can create alternate examination entities.
You may choose to define your examination entities or use those already mentioned.
Usually, research follows someone else's ideas of how to do something. But, in this laboratory you can create these too. Search through Google scholar or your local library for meteorites and perhaps thin sections or cross sections.
Yes, this laboratory is structured.
I will provide one example of a meteorite, examine it benefiting from already made thin sections, and perform an analysis. The rest is up to you.
Questions, if any, are best placed on the discussion page.
You are free to create your own notation or use that already provided.
For creating an examination technique for your meteorite, what would make an acceptable control group? Think about a control group to compare your meteorite examination or your process of examination to. Describe it.
One way to look for a possible meteorite to analyze is to try Google Search Images using the key words: "thin section" and meteorite.
Some of the websites available are interested in artistic qualities rather than readily analyzable qualities.
The image at right from the Google search of images shows a good feature for analysis: there are a great many light gray and dark gray chondrules present. A poor feature for analysis is the over all small grain sizes making measurements difficult.
A second way is to try Google Scholar Advanced Search using the same key words: "thin section" and meteorite.
The image at left appears in a journal article from 1971. The image is through crossed polarizers (then referred to as crossed Nicols). It shows large grain cross sections making sizing easy and a 100 µm marker at the upper right for accurate sizing. The major weakness, of course, is that it is not from a meteorite.
The sampling suggested is conducted on the web so any search engine should be good. One problem is the need to see the thin sections for the candidate meteorite. The portable document format (pdf) is often the best to have available photomicrographs of thin sections. Of ten returns from a search engine, maybe two at most are downloadable as a pdf-type file for photomicrograph extraction and analysis.
As shown at right in the previous section, one feature reasonably unique to meteorite thin sections is the presence of chondrules.
At right in this section is an image of a thin section of a meteorite designated Northwest Africa 5400 (NWA 5400).
Meteorites are initially classified "into three broad groups: irons, stony-irons and stones."
"From there, a meteorite is further classified according to the amount and kinds of minerals and metals it contains."
"Hupé’s rock [at right] was classified broadly as an achondrite (AY-kon-drite), a stony meteorite from beneath an asteroid’s crust that suffered heating, melting and crystallization. Its primary ingredient is olivine, a lovely green crystalline rock found in abundance in Earth’s mantle."
Hupé’s "stone was closely related to both moon rocks and Earth rocks but was neither of lunar origin nor from the current Earth. Based on detailed analysis of oxygen isotopes present in the sample, the researchers offered the possibility that the sample, now officially named Northwest Africa 5400 or NWA 5400, could be a terrene meteorite or one derived from the ancient Earth around the time of its formation 4.5 billion years ago. Other possibilities include another differentiated, Earth-like body that formed in the Earth-moon neighborhood or a fragment of the material that coalesced to form the moon."
Although the rock has apparently been verified as a meteorite, the thin section has no marker indicating magnification. The small somewhat angular black blebs are likely made of metal.
"Disaggregation [...] and thin-section analyses [...] are two standard methods used to obtain statistical data on chondrule sizes. Disaggregation has the advantage that chondrule diameters and abundances can be measured directly, but information on chondrule textures, compositions, and rims is not readily obtainable without subsequent sectioning. In addition, most chondrites are not amenable to disaggregation. In thin section, the compositions, textures, and rim characteristics of chondrules can be determined at the time chondrule sizes are measured. However, the diameters and relative abundances determined from thin-section measurements must be corrected for several sources of bias."
The first diagram on the right shows one source of bias due to a large variation in actual chondrule diameters.
The top diagram at left illustrates that "random sectioning of chondrules produces apparent diameters less than or equal to the true diameters, D."
The second diagram at right shows apparent "chondrule diameters under various viewing conditions. In reflected light, the probability of observing an apparent diameter between d1 and d2, where d2 < d1 < D [varies]. In transmitted light when the matrix is transparent, the observed diameter is the largest diameter occurring in thin section. If the matrix is opaque, only chondrules 1 and 2 [ar]e observed in transmitted light, resulting in an underestimation of the abundance of small chondrules."
In addition to a grain size marker, or magnification marker, the image creator needs to specify the optical conditions under which the image is made. Is it reflected light, transmitted light, polarized light, crossed polarizers, thin film thickness? When all of these are properly specified the images may be used for mineral, or mineral family, identification.
These many patterns are an interleaving of kamacite and taenite bands or ribbons with a fine-grained mixture of kamacite and taenite called plessite in the gaps between the lamellae.
The lowest set of diagrams at left show how the patterns display different orientation patterns based on a cubic symmetry system octahedron.
Martian meteorite, EETA 79001, has been analyzed with the Raman point-count procedure. "Raman spectra [occur] for pyroxene, olivine, maskelynite (shocked, isotropized feldspar), chromite, magnetite, ilmenite, ulvöspinel, pyroxferroite, merrillite, apatite, anatase, an Fe sulfide, calcite and hematite."
"The major Raman peak of chromite from this meteorite occurs in a range from 679 to 699 cm-1 [...], which corresponds to a (Cr + Fe3+)/(Cr + Fe3+ + Al) ratio of 0.75-1.0.8 Although not reported in previous studies on EETA79001, magnetite was detected in Raman point-counting measurements on EETA79001,482 rock chip [...] Magnetite and ulvöspinel [...] found in this meteorite have smaller grain sizes, are always observed in multi-phase spectra and only rarely appear insequential spectra."
Approximate area percentages can be estimated using the thin section at right. The penetration depth of a scanning electron beam is usually ≤ 1 µm. Most of the backscattered electrons come from a shallower depth. Using the mineral determinations, the approximate mineral composition is
- merrillite 35 %,
- maskelynite 20 %,
- pyroxene 20 %,
- ulvöspinel 12 %,
- symplectite 5 %,
- olivine 1 %, and the remainder
- about 4 %.
Of the minerals present in this meteorite, only merrillite has apparently never been found in terrestrial rocks.
Approximate mineral composition of a Martian meteorite
Several online techniques for finding a meteorite of interest with a compositional analysis are presented. Sources of potential errors are described and a Martian meteorite has been analyzed with respect to mineral composition using a thin section.
Thin section analysis of meteorites is a technique in use for many decades. While reliable for specific quantities, it's potential sources of error and misunderstood conclusions should be described so that students may pick thin sections to analyze with a reasonable chance of success.
Using Google as a search engine, while others will as work well, produces some potential thin sections. The challenges present in using examples are small grain size, minimum visual differentiation, lack of proper sizing markers, and limited access to the thin sections themselves due to a need for pdf type presentations. Verification that the chosen thin section is actually from a meteorite must come from the thin section primary source.
The thin section of many rock types on Earth are likely to bear a striking resemblance to the thin section of a candidate meteorite. Verification usually concerns isotope ratios. Different locations of formation even within the solar system may have led to different isotope ratios.
A candidate thin section should bear a fiducial marker allowing for magnification. Apparent grain sizes need to be large enough in the photomicrograph to permit easy sizing. The optical conditions in which the thin section was made should be adequately described. Chondrules, when present, require caution and application of sizing corrections for determining accurate diameters.
Alternate microstructures due to compositional variations and meteorite likely origin present unique challenges.
An acceptable photomicrograph may be generated by additional techniques rather than being strictly limited to visual surface reflection or transmissive polarized lighting.
Once a reasonably good choice is made the area percentages of already identified minerals may be obtained. Each type of microstructure can be analyzed but orientation of the sectioning must be properly considered. Secondary techniques such as Raman spectroscopy or scanning electron backscattering assist in identifying minerals and differentiating them over the area of a thin section.
Once a good choice of thin section is made, an analysis may be performed that adds information about the meteorite.
To assess your locator, including your justification, analysis and discussion, I will provide such an assessment of my example for comparison and consideration.
While initially the idea of examining a meteorite in thin section so as to contribute to our knowledge of any specific meteorite seems to be a good idea, it appears to be the case that the field has developed far beyond what information may be obtained by simple mineral examination in thin section with optical inspection.
- Some meteorites may be a lot younger than expected.
- Bob King (June 14, 2010). Florida man uncovers possible Earth-related meteorite. Area Voices .com. Retrieved 2014-03-01.
- A. Shukolyukov; G. Lugmair; J. M. D. Day; R. J. Walker; D. Rumble III; D. Nakashima; K. Nagao; A. J. Irving. Constraints on the Formation Age, Highly Siderophile Element Budget and Noble Gas Isotope Compositions of Northwest Africa 5400: an Ultramafic Achondrite with Terrestrial Isotopic Characteristics (PDF). 41st Lunar and Planetary Science Conference. Retrieved 2014-03-01.
- Don D. Eisenhour (March 1996). "Determining chondrule size distributions from thin-section measurements". Meteoritics & Planetary Science 31 (2): 243-8. doi:10.1111/j.1945-5100.1996.tb02019.x. http://adsabs.harvard.edu/abs/1996M&PS...31..243E. Retrieved 2014-03-01.
- Alian Wang; Karla Kuebler; Bradley Jolliff; Larry A. Haskin (June 2004). "Mineralogy of a Martian meteorite as determined by Raman spectroscopy". Journal of Raman Spectroscopy 35 (6): 504-14. http://onlinelibrary.wiley.com/doi/10.1002/jrs.1175/abstract;jsessionid=C19DA6ABAE9EEFB335A2B055C27414B1.f02t02?deniedAccessCustomisedMessage=&userIsAuthenticated=false. Retrieved 2014-03-03.
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