Petrography
Encyclopedia
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 outcrop and include megascopic description of hand specimens. However, the most important tool for the petrographer is the petrographic microscope
. The detailed analysis of minerals by optical mineralogy
in thin section
and the micro-texture and structure are critical to understanding the origin of the rock. Electron microprobe
analysis of individual grains as well as whole rock chemical analysis by atomic absorption or X-ray fluorescence
are used in a modern petrographic lab. Individual mineral grains from a rock sample may also be analyzed by X-ray diffraction when optical means are insufficient. Analysis of microscopic fluid inclusions
within mineral grains with a heating stage on a petrographic microscope provides clues to the temperature and pressure conditions existent during the mineral formation.
With a small bottle of acid
to test for carbonate of lime, a knife to ascertain the hardness
of rocks and minerals, and a pocket lens to magnify their structure, the field geologist is rarely at a loss to what group a rock belongs. The fine grained species are often indeterminable in this way, and the minute mineral components of all rocks can usually be ascertained only by microscopic examination. But it is easy to see that a sandstone or grit consists of more or less rounded, water-worn sand grains and if it contains dull, weathered particles of feldspar, shining scales of mica or small crystals of calcite these also rarely escape observation. Shales and clay rocks generally are soft, fine grained, often laminated and not infrequently contain minute organisms or fragments of plants. Limestones are easily marked with a knife-blade, effervesce readily with weak cold acid and often contain entire or broken shells or other fossils. The crystalline nature of a granite or basalt is obvious at a glance, and while the former contains white or pink feldspar, clear vitreous quartz and glancing flakes of mica, the other shows yellow-green olivine, black augite, and gray stratiated plagioclase.
Other simple tools include the blowpipe (to test the fusibility of detached crystals), the goniometer
, the magnet, the magnifying glass and the specific gravity balance.
, produced by the lower Nicol prism
, or more recently polarising films), fracture characteristics of the grains, refractive index (in comparison to the mounting adhesive, typically Canada Balsam
), and optical symmetry (birefringent
or isotropic
). In toto, these characteristics are sufficient to identify the mineral, and often to quite tightly estimate its major element composition.
The process of identifying minerals under the microscope is fairly subtle, but also mechanistic - it would be possible to develop an identification key
that would allow a computer to do it. The more difficult and skilful part of optical petrography is identifying the interrelationships between grains and relating them to features seen in hand specimen, at outcrop, or in mapping.
for analysis is a common approach. It may
be performed with a powerful, adjustable-strength electromagnet.
A weak magnetic field attracts magnetite, then haematite and other iron ores. Silicates that contain iron follow
in definite order—biotite, enstatite, augite, hornblende,
garnet, and similar ferro-magnesian minerals are successively
abstracted. Finally, only the colorless, non-magnetic compounds, such as muscovite, calcite, quartz, and feldspar remain. Chemical methods also are useful.
A weak acid dissolves calcite from crushed limestone, leaving only dolomite,
silicates, or quartz. Hydrofluoric acid attacks feldspar before quartz and, if used cautiously, dissolves these
and any glassy material in a rock powder before it dissolves augite or hypersthene.
Methods of separation by specific gravity have a still wider application. The simplest of these is levigation—treatment
by a current of water. Levigationis extensively employed in mechanical
analysis of soils and treatment of ores, but is not so successful with rocks, as their components do not, as a rule,
differ greatly in specific gravity. Fluids are used that do not attack most rock-forming minerals, but have a high specific gravity. Solutions of potassium mercuric iodide (sp. gr. 3.196), cadmium
borotungstate (sp. gr. 3.30), methylene iodide (sp. gr. 3.32), bromoform
(sp. gr. 2.86), or acetylene bromide (sp. gr. 3.00) are the principal fluids employed. They may be diluted (with water, benzene, etc.) or concentrated by evaporation.
If the rock is granite consisting of biotite (sp. gr. 3.1), muscovite
(sp. gr. 2.85), quartz (sp. gr. 2.65), oligoclase (sp. gr. 2.64), and
orthoclase (sp. gr. 2.56), the crushed minerals float in methylene iodide. On gradual dilution with benzene they
precipitate in the order above. Simple in theory, these methods are tedious in practice, especially as it is common
for one rock-making mineral to enclose another. However, expert handling of fresh
and suitable rocks yields excellent results.
grains is an elegant and valuable means of discriminating between mineral components of fine-grained rocks.
Thus, the presence of apatite in rock-sections is established by covering a bare rock-section with ammonium molybdate solution. A turbid yellow precipitate forms over the crystals of the mineral in question (indicating the presence of phosphates). Many
silicates are insoluble in acids and cannot be tested in this way, but others are partly dissolved, leaving a film of gelatinous
silica that can be stained with coloring matters, such as the aniline dyes (nepheline, analcite, zeolites, etc.).
Complete chemical analysis of rocks are also widely used and important, especially in describing new species. Rock analysis has of late years (largely under the influence of the chemical laboratory of the United States Geological Survey) reached a high pitch of refinement and complexity. As many as twenty or twenty-five components may be determined, but for practical purposes a knowledge of the relative proportions of silica, alumina, ferrous and ferric oxides, magnesia, lime, potash, soda and water carry us a long way in determining a rock's position in the conventional classifications.
A chemical analysis is usually sufficient to indicate whether a rock is igneous or sedimentary, and in either case to accurately show what subdivision of these classes it
belongs to. In the case of metamorphic rocks it often establishes whether the original mass was a sediment or of volcanic origin.
It diminishes with weathering and, generally, highly crystalline rocks have higher specific gravity than wholly or partly vitreous rocks of the same chemical composition. The specific gravity of the commoner rocks
ranges from about 2.5 to 3.2.
use petrography to identify mineral components in pottery
. This information ties the artifacts to geological source areas for the clay used and the rock fragments (usually called "temper" or "aplastics") potters often add to modify the clay's properties. This provides insight into how potters selected and used local and non-local resources, and helps archaeologists determine whether pottery found in a particular location was locally produced or traded from elsewhere. In turn, this kind of information (in combination with other evidence) can support inferences about settlement patterns, group and individual mobility, and social contacts or trade networks. In addition, an understanding of how certain minerals are altered at specific temperatures can allow archaeological petrographers to infer aspects of the ceramic
production process itself, such as minimum and maximum temperatures reached in the original firing of the pot.
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....
that focuses on detailed descriptions of rocks
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...
. Someone who studies petrography is called a petrographer. The 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...
content and the textural
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...
relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the outcrop and include megascopic description of hand specimens. However, the most important tool for the petrographer is the 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...
. The detailed analysis of minerals by 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...
in thin section
Thin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
and the micro-texture and structure are critical to understanding the origin of the rock. 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...
analysis of individual grains as well as whole rock chemical analysis by atomic absorption or X-ray fluorescence
X-ray fluorescence
X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...
are used in a modern petrographic lab. Individual mineral grains from a rock sample may also be analyzed by X-ray diffraction when optical means are insufficient. Analysis of microscopic fluid inclusions
Fluid inclusions
thumb|250px|Trapped in a time capsule the same size as the diameter of a human hair, the ore-forming liquid in this inclusion was so hot and contained so much dissolved solids that when it cooled, crystals of halite, sylvite, gypsum, and hematite formed. As the samples cooled, the fluid shrank more...
within mineral grains with a heating stage on a petrographic microscope provides clues to the temperature and pressure conditions existent during the mineral formation.
Macroscopic characters
The macroscopic characters of rocks, those visible in hand-specimens without the aid of the microscope, are very varied and difficult to describe accurately and fully. The geologist in the field depends principally on them and on a few rough chemical and physical tests; and to the practical engineer, architect and quarry-master they are all-important. Although frequently insufficient in themselves to determine the true nature of a rock, they usually serve for a preliminary classification, and often give all the information needed.With a small bottle of acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
to test for carbonate of lime, a knife to ascertain the hardness
Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...
of rocks and minerals, and a pocket lens to magnify their structure, the field geologist is rarely at a loss to what group a rock belongs. The fine grained species are often indeterminable in this way, and the minute mineral components of all rocks can usually be ascertained only by microscopic examination. But it is easy to see that a sandstone or grit consists of more or less rounded, water-worn sand grains and if it contains dull, weathered particles of feldspar, shining scales of mica or small crystals of calcite these also rarely escape observation. Shales and clay rocks generally are soft, fine grained, often laminated and not infrequently contain minute organisms or fragments of plants. Limestones are easily marked with a knife-blade, effervesce readily with weak cold acid and often contain entire or broken shells or other fossils. The crystalline nature of a granite or basalt is obvious at a glance, and while the former contains white or pink feldspar, clear vitreous quartz and glancing flakes of mica, the other shows yellow-green olivine, black augite, and gray stratiated plagioclase.
Other simple tools include the blowpipe (to test the fusibility of detached crystals), the goniometer
Goniometer
A goniometer is an instrument that either measures an angle or allows an object to be rotated to a precise angular position. The term goniometry is derived from two Greek words, gōnia, meaning angle, and metron, meaning measure....
, the magnet, the magnifying glass and the specific gravity balance.
Microscopic characteristics
When dealing with unfamiliar types or with rocks so fine grained that their component minerals cannot be determined with the aid of a hand lens, a microscope is used. Characteristics observed under the microscope include colour, colour variation under plane polarised light (pleochroismPleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
, produced by the lower Nicol prism
Nicol prism
A Nicol prism is a type of polarizer, an optical device used to produce a polarized beam of light from an unpolarized beam. See polarized light. It was the first type of polarizing prism to be invented, in 1828 by William Nicol of Edinburgh...
, or more recently polarising films), fracture characteristics of the grains, refractive index (in comparison to the mounting adhesive, typically 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...
), and optical symmetry (birefringent
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...
or isotropic
Isotropy
Isotropy is uniformity in all orientations; it is derived from the Greek iso and tropos . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary...
). In toto, these characteristics are sufficient to identify the mineral, and often to quite tightly estimate its major element composition.
The process of identifying minerals under the microscope is fairly subtle, but also mechanistic - it would be possible to develop an identification key
Identification key
In biology, an identification key is a printed or computer-aided device that aids the identification of biological entities, such as plants, animals, fossils, microorganisms, and pollen grains...
that would allow a computer to do it. The more difficult and skilful part of optical petrography is identifying the interrelationships between grains and relating them to features seen in hand specimen, at outcrop, or in mapping.
Separation of components
Separation of the ingredients of a crushed rock powder to obtain pure samplesfor analysis is a common approach. It may
be performed with a powerful, adjustable-strength electromagnet.
A weak magnetic field attracts magnetite, then haematite and other iron ores. Silicates that contain iron follow
in definite order—biotite, enstatite, augite, hornblende,
garnet, and similar ferro-magnesian minerals are successively
abstracted. Finally, only the colorless, non-magnetic compounds, such as muscovite, calcite, quartz, and feldspar remain. Chemical methods also are useful.
A weak acid dissolves calcite from crushed limestone, leaving only dolomite,
silicates, or quartz. Hydrofluoric acid attacks feldspar before quartz and, if used cautiously, dissolves these
and any glassy material in a rock powder before it dissolves augite or hypersthene.
Methods of separation by specific gravity have a still wider application. The simplest of these is levigation—treatment
by a current of water. Levigationis extensively employed in mechanical
analysis of soils and treatment of ores, but is not so successful with rocks, as their components do not, as a rule,
differ greatly in specific gravity. Fluids are used that do not attack most rock-forming minerals, but have a high specific gravity. Solutions of potassium mercuric iodide (sp. gr. 3.196), cadmium
borotungstate (sp. gr. 3.30), methylene iodide (sp. gr. 3.32), bromoform
(sp. gr. 2.86), or acetylene bromide (sp. gr. 3.00) are the principal fluids employed. They may be diluted (with water, benzene, etc.) or concentrated by evaporation.
If the rock is granite consisting of biotite (sp. gr. 3.1), muscovite
(sp. gr. 2.85), quartz (sp. gr. 2.65), oligoclase (sp. gr. 2.64), and
orthoclase (sp. gr. 2.56), the crushed minerals float in methylene iodide. On gradual dilution with benzene they
precipitate in the order above. Simple in theory, these methods are tedious in practice, especially as it is common
for one rock-making mineral to enclose another. However, expert handling of fresh
and suitable rocks yields excellent results.
Chemical analysis
In addition to naked-eye and microscopic investigation, chemical research methods of are of great practical importance to the petrographer. Crushed and separated powders, obtained by the processes above, may be analyzed to determine chemical composition of minerals in the rock qualitatively or quantitatively. Chemical testing, and microscopic examination of minutegrains is an elegant and valuable means of discriminating between mineral components of fine-grained rocks.
Thus, the presence of apatite in rock-sections is established by covering a bare rock-section with ammonium molybdate solution. A turbid yellow precipitate forms over the crystals of the mineral in question (indicating the presence of phosphates). Many
silicates are insoluble in acids and cannot be tested in this way, but others are partly dissolved, leaving a film of gelatinous
silica that can be stained with coloring matters, such as the aniline dyes (nepheline, analcite, zeolites, etc.).
Complete chemical analysis of rocks are also widely used and important, especially in describing new species. Rock analysis has of late years (largely under the influence of the chemical laboratory of the United States Geological Survey) reached a high pitch of refinement and complexity. As many as twenty or twenty-five components may be determined, but for practical purposes a knowledge of the relative proportions of silica, alumina, ferrous and ferric oxides, magnesia, lime, potash, soda and water carry us a long way in determining a rock's position in the conventional classifications.
A chemical analysis is usually sufficient to indicate whether a rock is igneous or sedimentary, and in either case to accurately show what subdivision of these classes it
belongs to. In the case of metamorphic rocks it often establishes whether the original mass was a sediment or of volcanic origin.
Specific gravity
Specific gravity of rocks is determined by means of a balance and pycnometer. It is greatest in rocks with the most magnesia, iron, and heavy metals—least in rocks rich in alkalis, silica, and water.It diminishes with weathering and, generally, highly crystalline rocks have higher specific gravity than wholly or partly vitreous rocks of the same chemical composition. The specific gravity of the commoner rocks
ranges from about 2.5 to 3.2.
Archaeological applications
ArchaeologistsArchaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...
use petrography to identify mineral components in pottery
Pottery
Pottery is the material from which the potteryware is made, of which major types include earthenware, stoneware and porcelain. The place where such wares are made is also called a pottery . Pottery also refers to the art or craft of the potter or the manufacture of pottery...
. This information ties the artifacts to geological source areas for the clay used and the rock fragments (usually called "temper" or "aplastics") potters often add to modify the clay's properties. This provides insight into how potters selected and used local and non-local resources, and helps archaeologists determine whether pottery found in a particular location was locally produced or traded from elsewhere. In turn, this kind of information (in combination with other evidence) can support inferences about settlement patterns, group and individual mobility, and social contacts or trade networks. In addition, an understanding of how certain minerals are altered at specific temperatures can allow archaeological petrographers to infer aspects of the ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
production process itself, such as minimum and maximum temperatures reached in the original firing of the pot.
External links
- Atlas of Rocks, Minerals, and Textures Petrographical description of rocks and minerals