Thin section

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Three thin sections of rock
Thin sections under a petrographic microscope
carbonate vein in mica
rich rock. In cross-polarized light on left, plane-polarized light on right.

In

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-Lévy 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.

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.

A photograph of a rock in thin section is often referred to as a

photomicrograph
.

Thin sections are also used in the microscopic study of bones, metals and ceramics.

Quartz in thin section

Photomicrograph of a thin section of gabbro
Photomicrograph of a thin section of a limestone with ooids. The largest is approximately 1.2 mm in diameter.

Description

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]

Determining provenance

In thin section, quartz grain

authigenic, or grown in place, overgrowths of silica cement over the grain.[4]

Other distinguishing features

The above descriptions of quartz in thin section are usually enough to identify it. Minerals with similar appearance may include

optic sign, length-slow sign of elongation, and zero degree extinction angle.[5]

A sigma clast as seen in an ultra thin section. The uneven coloring is an artifact from uneven polishing.

Ultra-thin sections

Fine-grained rocks, particularly those containing minerals of high

microstructure of fine-grained carbonates such as the Lochseitenkalk mylonite in which the matrix grains are less than 5 μm in size.[6] This method is also sometimes used in the preparation of mineral and rock specimens for transmission electron microscopy and allows greater accuracy in comparing features using both optical and electron imaging.[7]

Gallery

See also

References

  1. ^ "Rock Thin Sections (Petrographic Thin Section Preparation)". Kemet. Retrieved 2018-05-15.
  2. OCLC 946008550
    .
  3. ^ Blatt, H.; Christie, J.M. (1963). "Undulatory Extinction in Quartz of Igneous and Metamorphic Rocks and Its Significance in Provenance Studies of Sedimentary Rocks". AAPG Datapages.
  4. OCLC 52127337.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  5. ^ "quartz". www.mtholyoke.edu. Retrieved 2018-05-15.
  6. ^ Badertscher, N.P. & Burkhard, M. 2000. Brittle±ductile deformation in the Glarus thrust Lochseiten (LK) calc-mylonite, Terra Nova, 12, 281-288
  7. ^ Barber, D.J. 1981. Demountable polished extra-thin sections and their use in transmission electron microscopy. Mineralogical magazine,44, 357-359
  • Shelley, D. Optical Mineralogy, Second Edition. University of Canterbury, New Zealand.

External links

Wikimedia Commons has media related to Rocks in thin section.
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