Thin Section Petrography
Lane Frisch
In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond saw and ground optically flat. It is then mounted on a glass slide and then ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method uses the Michel-Lvy interference colour chart to determine thickness, typically using quartz as the thickness gauge because it is one of the most abundant minerals.
When placed between two polarizing filters set at right angles to each other, the optical properties of the minerals in the thin section alter the colour and intensity of the light as seen by the viewer. As different minerals have different optical properties, most rock forming minerals can be easily identified. Plagioclase for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning planes. The large blue-green minerals are clinopyroxene with some exsolution of orthopyroxene.
Thin sections are prepared in order to investigate the optical properties of the minerals in the rock. This work is a part of petrology and helps to reveal the origin and evolution of the parent rock.
In thin section, when viewed in plane polarized light (PPL), quartz is colorless with low relief and no cleavage. Its habit is either fairly equant or anhedral if it infills around other minerals as a cement. Under cross polarized light (XPL) quartz displays low interference colors and is usually the defining mineral used to determine if the thin section is at standardized thickness of 30 microns as quartz will only display up to a very pale yellow interference color and no further at that thickness, and it is very common in most rocks so it will likely be available to judge the thickness.[1]
In thin section, quartz grain provenance in a sedimentary rock can be estimated. In crossed polarized light, the quartz grain can go extinct all at once, called monocrystalline quartz, or in waves, called polycrystalline quartz. The extinction in waves is called undulose extinction and indicates dislocation walls in mineral grains. Dislocation walls are where dislocations, intracrystalline deformation via movement of a dislocation front within a plane, organize themselves into planes of sufficient quantity. They change the crystallographic orientation across the walls, so for example in quartz, the two sides of the wall will have slightly different extinction angles and thus result in undulose extinction.[2] Since undulose extinction requires dislocation walls to have developed, and these occur more easily at higher pressures and temperatures, quartz grains with undulose extinction indicate metamorphic rock provenance for that grain. Those grains that are monocrystalline quartz are more likely to have been formed by igneous processes. Differing sources suggest the extent to which this proxy for provenance can be used. Some note the trend for immature sandstones to have less polycrystalline quartz grains compared to mature sandstones, which have grains that have passed through many sedimentary cycles.[3] Quartz grains derived from previous sedimentary sources are determined by looking for authigenic, or grown in place, overgrowths of silica cement over the grain.[4]
The above descriptions of quartz in thin section are usually enough to identify it. Minerals with similar appearance may include plagioclase, although it can be distinguished by the distinctive twinning in crossed-polarized light and cleavage in plane-polarized light, and cordierite, although it can be distinguished by twinning or inclusions in the grain. However, for certainty, other distinguishing features of quartz include the fact that it is uniaxial, it has a positive optic sign, length-slow sign of elongation, and zero degree extinction angle.[5]
Using over 400 colour figures of a diverse range of artefact types and archaeological periods from 50 countries worldwide, this book outlines the mineralogical, chemical and microstructural composition of ancient ceramics and provides comprehensive guidelines for their scientific study within archaeology.
Thin section petrography, geochemistry, scanning electron microscopy and X-ray diffraction are key scientific methods used to investigate the raw materials, origins and production technology of archaeological pottery, ceramic building materials, ancient refractories and plaster. Using over 400 colour figures of a diverse range of artefact types and archaeological periods from 50 countries worldwide, this book outlines the mineralogical, chemical and microstructural composition of ancient ceramics and provides comprehensive guidelines for their scientific study within archaeology. The core of the book is dedicated to the versatile approach of ceramic petrography. This is complimented by a detailed account of the principles of bulk instrumental geochemistry, as well as the SEM microanalysis and XRD characterisation of ceramics. The book is intended as a reference manual for research as well as a course text for specialist training on scientific ceramic analysis.
JSC is happy to announce the recommencement of our Lunar and Meteorite Petrographic Thin Section Program. We would like to extend a thank you to our borrowers for their patience with any necessary changes due to COVID-19. As we work diligently to adapt to our new normal we ask that you email any requests to
js-cur-edu-...@mail.nasa.gov.
Petrographic Thin Section Packages containing 12 polished thin sections of samples from either the Lunar or Meteorite collections are provided to colleges and universities having a curriculum in the geosciences. Each set of 12 slides is accompanied by a sample disk of representative lunar or meteorite samples, embedded in acrylic disks suitable for classroom use, and teaching materials. The thin section package is intended for use in college and university courses in petrology and microscopic petrography for advanced geology students. The loan period is very strict and limited to two weeks. For further information or questions, please contact the Lunar and Meteorite Petrographic Thin Section Education Program Specialist.
Each Package includes a lunar thin section set consisting of 12 polished sections of rocks and soils, specifically selected to represent the lunar collection. The samples represented in this set are listed on the table below. Click on the sample number to see a description of the sample from the Lunar Petrographic Thin Section Set Study Guide, written by Dr. Charles Meyer along with photos and links to associated resources and references.
This manual is a comprehensive illustrated overview of the minerals and textures seen in polarized light microscopy of cultural artefacts. It focuses on the specific applications of thin-section petrography to the study of cultural materials made of stone (as found in architecture, sculpture, tools etc.) and ceramic objects (earthenware, stoneware, porcelain, brick and tile, etc.). The reader can learn the techniques used to identify and characterize such materials; to differentiate between them; to monitor the extent of their deterioration; to determine where they may have originated; and to interpret their fabrication, decoration, and history of use. The author has selected the most representative cultural materials for which thin-section petrography has been used as a major research tool and conducted a thorough survey of the literature indicating landmark publications from the past and more recent scholarship.
This book is accompanied by a CD-ROM that illustrates all the photomicrographs available in the book, but in addition allows the viewer to study each image in both plane polarized and crossed polarized light.
micROCKscopic.ro makes Optical Mineralogy and Petrology be easier to examine a thin section and to understand the characteristic properties of each mineral/rock without a petrographic microscope, which is known to be very expensive. The application is mainly addressed to geoscience students/geologists as a guide in individual or supervised laboratory work.
The study of rocks in thin section is the most effective way of giving the undergraduate or graduate student a true realization of the mineralogical constitution of rocks. Not until he has studied rocks in this way does the student actually apprehend the significance of such rock names as andesite, granite, etc. Thin section study thus performs a very important educational function in giving body to the terminology of rocks.
This website project (micROCKscopic.ro) has been planned with the purpose of providing a brief start in Optical Mineralogy and Petrology, descriptions of the rock-forming minerals encountered in the more common rocks, properties of the most common minerals under the petrographic microscope to assist in the identification of these minerals, and description of the common rock types. All of these accompanied by a vast collection of photomicrographs taken in plane-polarized light/parallel light (PPL) and cross-polarized light/crossed nicols (XPL).
Only the commoner rock-forming minerals are described in the mineralogical section. The minerals listed comprise over 99% of the constituents of the common rocks so that for most routine petrographic work no other source will be necessary.
The petrographic section of the website also has a number of novel features. It is divided into igneous, sedimentary, and metamorphic rocks in a conventional way. Each rock is systematically discussed under the following headings: definition and classification, photomicrographs of the whole rock under the microscope, rock-forming minerals under the microscope (essential and non-essential minerals), and varieties.
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