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Petrography - the branch of geology that deals with the description and systematic classification of rocks, especially with the use of a microscope. Petrography is more restricted in scope that petrology. Various authors (including Weisberg et al.) talk about the "petrologic type" of chondrites (a scheme that assigns numbers of 1-6 to chondrites based mainly on petrographic criteria), but technically this is a petrographic type classification, as it based on assignments one makes by using a microscope.
Differentiation - This is the process whereby materials separate as a result of melting processes. Usually minerals of different densities will separate from each other during igneous processes. The Earth is a differentiated object - it has a dense metallic core, a less dense rocky mantle, and a low density rocky crust. Differentiated meteorites are samples of igneous rocks that experienced such melting processes.
Figure from: http://garage.physics.iastate.edu/astro120/lectlist.html
- showing differentiation as a result of melting -
dense material sinks, light material floats towards the surface.
Igneous rock - an igneous rock is a rock that forms by solidification of a melt (magma). Atoms in the melt come together to form growing mineral grains in a process termed crystallization. Examples of igneous rocks are silicate-rich achondrites, and more metal-rich meteorites such as stony-iron meteorites (mesosiderites and pallasites) and iron meteorites. Weisberg et al. refer to stony-iron and iron meteorites as metal-rich achondrites, but most workers reserve the term achondrites for silicate-rich igneous meteorites.
Metamorphic rock - a metamorphic rock is a rock that forms from a previously existing rock by the rearrangement and growth of minerals in the solid state (no melting), in response to changes in pressure, temperature, or fluid composition. Metamorphic rocks are mainly produced at high temperatures and often at high pressures. Meteorite examples of metamorphic rocks are chondrites of all types except type 3 (types 4-6 were heated to progressively higher temperatures during metamorphism, types 1-2 were aqueously altered at low temperatures during mild heating), winonaites, and mesosiderites.
Sedimentary rock - a sedimentary rock is a rock that forms at the surface of a planetary body by processes such as weathering, compaction and cementation. Sedimentary rocks on planetary bodies are always produced under conditions of relatively low pressure and low temperature at the surface of the body. Chondrites can be regarded as a special and unique type of sedimentary rock, with their constituents formed not at the surface of a planetary body but rather as freely floating objects in space.
Refractory element - an element that remains in the solid state (rather than going into a vapor) at relatively high temperatures. The opposite of a refractory element is a volatile element, which is an element that prefers to be in a gaseous state at relatively high temperatures. Elements can have different degrees of volatile tendency. We will say more about this later.
Lithophile element - an element that tends to form chemical bonds with oxygen, typically forming silicate or oxide compounds (minerals).
Siderophile element - an element that tends to bond with iron or other siderophile elements to form metallic alloy compounds (minerals). In meteorites, siderophile elements are concentrated in Fe-Ni-rich alloys (Fe-Ni-metal).
Chalcophile element - an element that tends to bond with sulfur, typically forming sulfide compounds (minerals). In meteorites, chaclophile elements are concentrated in sulfides such as troilite (FeS).
Figure from http://www.science.uwaterloo.ca/earth/waton/5a.html
Periodic table of the elements showing chemical affinities.
The solidus is the minimum temperature at which the rock begins to melt. Most rocks begin to melt above 1000 degrees Celsius. The liquidus is the temperature at which the rock is entirely molten.
Oxidation vs. reduction - oxidation is the addition of oxygen to a chemical compound. Reduction is the removal of oxygen from a chemical compound. More generally, reduction involves a decrease in positive valence and oxidation involves an increase in positive valence of an ion. For meteorites, the oxidation state of iron (Fe) can vary from Feo (no charge, reduced), to Fe2+ (somewhat oxidized), to Fe3+ (highly oxidized).
Fractionation - the process by which elements or isotopes become separated from one another, such as during differentiation, condensation, etc.
Isotope - these are different forms of a particular element which differ in the number of neutrons. For example, in order for an atom to be an oxygen atom, it MUST by definition have 8 protons in its nucleus, but the number of neutrons can be variable. Stable isotopes of oxygen are oxygen-16 (8 protons, 8 neutrons), oxygen-17 (8 protons, 9 neutrons), and oxygen-18 (8 protons, 10 neutrons). Many elements have mutliple stable isotopes.
Figure from http://www.ims.uaf.edu/isotopes/class-images/isotope-table.jpg
- this type of diagram is called a chart of the nuclides, and plots the
number of protons (chemical identity) vs. the number of neutrons.
Notice that oxygen has three stable isotopes and five unstable (radioactive)
isotopes shown on the diagram.
The standard 3-isotope plot for oxygen shows a measure of 17O/16O on the y-axis relative to a measure of 18O/16O on the x-axis. These measures are expressed in "del" notation, where the isotope ratios are given relative to a standard. The formula for del 18O is:
where SMOW stands for Standard Mean Ocean Water (a reference standard for oxygen isotopes). This formula describes the 18O/16O ratio of a sample compared to the SMOW standard. A similar formula can be written for the proportion of oxygen-17 to oxygen-16 (del 17O).
Standard 3-isotope oxygen plot showing the compositional
fields for various meteorites.
Chondrite groups listed by capital letters; HED, Mes, Pal = HED meteorites, mesosiderites, pallasites;
Lod, Aca = lodranite and acapulcoite; Ure = ureilite; Bra, Win = brachinite, winonaite.
Mass-dependent fractionation of oxygen - This is the kind of variation between the three oxygen isotopes that is caused by differences in mass between the three isotopes. Oxygen-18 is 18/16 times heavier than oxygen-16, and 18/17 times heavier than oxygen-17. Processes such as evaporation or condensation of water will cause a mass fractionation of oxygen isotopes, which means that whatever change is produced for 17O/16O (the two isotopes differ in 1 atomic mass unit) will result in about half the change observed for 18O/16O (the two isotopes differ in 2 atomic mass units).
Non-mass dependent fractionation of oxygen - This is the kind of variation between the three oxygen isotopes that cannot be explained by differences in mass between the three isotopes. Processes such as nuclear fusion reactions in stars, or the rapid formation of ozone in the laboratory, will cause such a fractionation.
For a good primer on oxygen isotopes in meteorites, see:
Thermoluminscence (TL) - the process by which light is emitted from substances upon heating. Useful for studying metamorphic effects in meteorites (especially the weakly metamorphsoed type 3 chondrites), among other things.
Breccia/brecciation - a breccia is a rock made up of angular fragments of one or more pre-existing rocks. The broken pieces are known as clasts, and can be either made of an individual mineral (mineral clasts) or pieces of rock (known as rock or lithic clasts). The pre-existing rocks can be broken up into fragments by a number of processes, including impact (shock), shearing along a fault, and explosive volcanic eruptions. Brecciation is the process of forming a breccia. For meteorites, brecciation is due to shock (impact). (A mineral is a naturally occurring crystalline material that has definite structural and chemical properties; rocks contain two or more minerals or multiple grains of the same mineral.)
Figure from: http://www.gpc.peachnet.edu/~janderso/physical/sedrx.htm
- showing a breccia.
Backscattered electron (BSE) image - this is an image created by a scanning electron microscope (SEM) that depends on the number of electrons bounced back to a detector from the surface of a sample, usually rendered in monochrome with areas of high backscatter showing as bright and low backscatter showing as dark. BSE images depend mainly on the average atomic number of a material (the higher the atomic number, the more backscatter).
Example of a BSE image showing plagioclase ("plag",
dark grey) and pyroxene ("px", light grey) in a howardite.
Image obtained with the PSU Geology department's SEM. The black area at upper right is epoxy.
Fayalite (Fa) content in olivine - defined
as Fa = 100 x Fe/(Fe+Mg) in olivine. Similarly, ferrosilite (Fs)
and wollastonite (Wo) content in pyroxene are defined as Fs = 100 x Fe/(Fe+Mg+Ca),
Wo = 100 x Ca/(Fe+Mg+Ca).