In optical mineralogy

Optical mineralogy

Optical mineralogy is the study of minerals and rocks by measuring their optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrographic microscope...

 and petrography

Petrography

Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the...

, a thin section is a laboratory

Laboratory

A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...

 preparation of a rock

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

, mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

, soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

, pottery, bones, or even metal sample for use with a polarizing petrographic microscope

Petrographic microscope

A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks...

, electron microscope

Electron microscope

An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

 and electron microprobe

Electron microprobe

An electron microprobe , also known as an electron probe microanalyzer or electron micro probe analyzer , is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials...

. 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

Microscope slide

A microscope slide is a thin flat piece of glass, typically 75 by 25 mm and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is placed or secured on the slide, and then both are inserted together in the microscope for viewing...

 and then ground smooth using progressively finer abrasive grit until the sample is only 30 μm

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 thick. The method involved using the Michel-Lévy interference colour chart. Typically quartz is used as the gauge to determine thickness as 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

Plagioclase

Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

 for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning

Crystal twinning

Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals....

 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

Petrology

Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....

 and helps to reveal the origin and evolution of the parent rock.

Ultra-thin sections

Fine-grained rocks, particularly those containing minerals of high birefringence

Birefringence

Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...

, such as calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

, are sometimes prepared as ultra-thin sections. An ordinary 30 μm

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 thin section is prepared as described above but the slice of rock is attached to the glass slide using a soluble cement such as canada balsam

Canada balsam

Canada balsam, also called Canada turpentine or balsam of fir, is a turpentine which is made from the resin of the balsam fir tree of boreal North America...

 (soluble in ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

) to allow both sides to be worked on. The section is then polished on both sides using a fine diamond

Diamond

In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

 paste until it has a thickness in the range of 2-12 μm

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

. This technique has been used to study the microstructure

Rock microstructure

Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...

 of fine-grained carbonates such as the Lochseitenkalk mylonite

Mylonite

Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...

 in which the matrix grains are less than 5 μm

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 in size. This method is also sometimes used in the preparation of mineral and rock specimens for transmission electron microscopy

Transmission electron microscopy

Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...

 and allows greater accuracy in comparing features using both optical and electron imaging.

External links

• Thin section preparation techniques
• Thin sections of ceramics
  Ceramography
  Ceramography is the art and science of preparation, examination and evaluation of ceramic microstructures. Ceramography can be thought of as the metallography of ceramics. The microstructure is the structure level of approximately 0.1 to 100 µm, between the minimum wavelength of visible light and...