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. The classification of rocks is based on the information acquired during the petrographic analysis. Petrographic descriptions start with the field notes at the outcrop and include macroscopic 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, X-ray fluorescence, and laser-induced breakdown spectroscopy 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.

History

Petrography as a science began in 1828 when Scottish physicist William Nicol invented the technique for producing polarized light by cutting a crystal of Iceland spar, a variety of calcite, into a special prism which became known as the Nicol prism. The addition of two such prisms to the ordinary microscope converted the instrument into a polarizing, or petrographic microscope. Using transmitted light and Nicol prisms, it was possible to determine the internal crystallographic character of very tiny mineral grains, greatly advancing the knowledge of a rock's constituents.

During the 1840s, a development by Henry C. Sorby and others firmly laid the foundation of petrography. This was a technique to study very thin slices of rock. A slice of rock was affixed to a microscope slide and then ground so thin that light could be transmitted through mineral grains that otherwise appeared opaque. The position of adjoining grains was not disturbed, thus permitting analysis of rock texture. Thin section petrography became the standard method of rock study. Since textural details contribute greatly to knowledge of the sequence of crystallization of the various mineral constituents in a rock, petrography progressed into petrogenesis and ultimately into petrology.

It was in Europe, principally in Germany, that petrography advanced in the last half of the nineteenth century.

Methods of investigation

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 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.[1]

Microscopic characteristics

LvMS-Lvm
Photomicrograph of a volcanic sand grain; upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.

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 (pleochroism, 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.

Separation of components

Separation of the ingredients of a crushed rock powder to obtain pure samples 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, which is 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.[1]

Chemical analysis

In addition to naked-eye and microscopic investigation, chemical research methods 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 minute 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.[1]

Specific gravity

Specific gravity of rocks is determined by use of a balance and pycnometer. It is greatest in rocks containing the most magnesia, iron, and heavy metal while least in rocks rich in alkalis, silica, and water. It diminishes with weathering. Generally, the specific gravity of rocks with the same chemical composition is higher if highly crystalline and lower if wholly or partly vitreous. The specific gravity of the more common rocks range from about 2.5 to 3.2.[1]

Archaeological applications

Archaeologists use petrography to identify mineral components in pottery. This information ties the artifacts to geological areas where the raw materials for the pottery were obtained. In addition to clay, potters often used rock fragments, usually called "temper" or "aplastics", to modify the clay's properties. The geological information obtained from the pottery components provides insight into how potters selected and used local and non-local resources. Archaeologists are able to determine whether pottery found in a particular location was locally produced or traded from elsewhere. This kind of information, along with other evidence, can support conclusions about settlement patterns, group and individual mobility, social contacts, and 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 during the original firing of the pot.

See also

References

  1. ^ a b c d  One or more of the preceding sentences incorporates text from a publication now in the public domainFlett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. 21 (11th ed.). Cambridge University Press. pp. 323–333.

External links

Ceramic petrography

Ceramic petrography (or ceramic petrology) is a laboratory-based scientific archaeological technique that examines the mineralogical and microstructural composition of ceramics and other inorganic materials under the polarised light microscope in order to interpret aspects of the provenance and technology of artefacts.Samples are ground to a thickness of 0.03 mm and mounted on a glass slide. The approach relies heavily on the geological principles of optical mineralogy, thin section petrography and soil micromorphology. It combines these with an appreciation of the craft of ceramic manufacture and interprets data within an archaeological framework.

Ceramic petrography is used in academic archaeological research and commercial archaeology to address a range of issues. A common goal is tracing the movement of pottery and associated trade through provenance determination. The principle of provenance ascription with ceramic petrography relies on the fact that "the mineral and rock inclusions within a paste are a reflection of the geology of the source area of the ceramics" and that potters did not transport ceramic raw materials over significant distances.An equally important concern is the nature of ancient ceramic production and its meaning in terms of the knowledge, skills, identity and traditions of potters. As synthetic materials, ceramics are "sensitive indicators of human decision making and materials interaction". By examining microstructural evidence for processes such as clay paste preparation, forming and firing, ceramic petrographers can reconstruct the steps involved in the production of ceramic artefacts.

Ceramic petrography originated in the American Southwest with the work of Anna O. Shepard but has mainly been developed in the Old World in the later half of the 20th century. Other early studies include the work of David Peacock and his students in the UKCeramic petrography continues to be applied to the interpretation of British ceramics and is used heavily in the prehistoric Aegean. In the USA the approach is less popular, though important contributions have been made in the area of quantitative petrography. Other attempts to extend ceramic petrography include the use of automated image analysis, the palaeontological analysis of microscopic fossils within ceramic thin sections and the combined statistical classification of petrographic and chemical data from artefacts.Thin section archaeological petrography can be applied to a range of other artefact types in addition to ceramics; these include plaster, mortar, mudbricks and lithic implements.Academic papers on ceramic petrography are often published in journals such as Archaeometry, Journal of Archaeological Science and Geoarchaeology, as well as edited volumes. Petrographic research is often presented at the International Symposium on Archaeometry, the European Meeting on Ancient Ceramics and the meetings of the Ceramic Petrology Group.

Diabase

Diabase ( ) or dolerite or microgabbro is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic glass). Diabase is the preferred name in North America, while dolerite is the preferred name in the rest of English-speaking world, where sometimes the name diabase is applied to altered dolerites and basalts. Some geologists prefer the name microgabbro to avoid this confusion.

Eleanora Knopf

Eleanora Frances Bliss Knopf (July 15, 1883 – January 21, 1974) was a geologist who worked for the United States Geological Survey (USGS) and did research in the Appalachians during the first two decades of the twentieth century. She studied at Bryn Mawr College, and earned a bachelor's degree in chemistry, a master's degree in geology, and a Ph.D. in geology in 1912. After completing her Ph.D., she accepted a position at the USGS, where she met and married the geologist Adolph Knopf, a professor at Yale University. She was the first American geologist to use the new technique of petrography which she pioneered in her life's work - the study of Stissing Mountain.

Flood basalt

A flood basalt is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Flood basalt provinces such as the Deccan Traps of India are often called traps, after the Swedish word trappa (meaning "stairs"), due to the characteristic stairstep geomorphology of many associated landscapes. Michael R. Rampino and Richard Stothers (1988) cited eleven distinct flood basalt episodes occurring in the past 250 million years, creating large volcanic provinces, lava plateaus, and mountain ranges. However, more have been recognized such as the large Ontong Java Plateau, and the Chilcotin Group, though the latter may be linked to the Columbia River Basalt Group. Large igneous provinces have been connected to five mass extinction events, and may be associated with bolide impacts.

Glass with embedded metal and sulfides

Glass with embedded metal and sulfides (GEMS) are tiny spheroids in cosmic dust particles with bulk compositions that are approximately chondritic. They form the building blocks of anhydrous interplanetary dust particles (IDPs) in general, and cometary IDPs, in particular. Their compositions, mineralogy and petrography appear to have been shaped by exposure to ionizing radiation. Since the exposure occurred prior to the accretion of cometary IDPs, and therefore comets themselves, GEMS are likely either solar nebula or presolar interstellar grains. The properties of GEMS (size, shape, mineralogy) bear a strong resemblance to those of interstellar silicate grains as inferred from astronomical observations.

Igneous petrology

Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general. The modern study of igneous rocks utilizes a number of techniques, some of them developed in the fields of chemistry, physics, or other earth sciences. Petrography, crystallography, and isotopic studies are common methods used in igneous petrology.

Industry (archaeology)

Not to be confused with industrial archaeology, the archaeology of (modern) industrial sites.

In the archaeology of the Stone Age, an industry or technocomplex is a typological classification of stone tools.

An industry consists of a number of lithic assemblages, typically including a range of different types of tools, that are grouped together on the basis of shared technological or morphological characteristics. For example, the Acheulean industry includes hand-axes, cleavers, scrapers and other tools with different forms, but which were all manufactured by the symmetrical reduction of a bifacial core producing large flakes. Industries are usually named after a type site where these characteristics were first observed (e.g. the Mousterian industry is named after the site of Le Moustier). By contrast, Neolithic axeheads from the Langdale axe industry were recognised as a type well before the centre at Great Langdale was identified by finds of debitage and other remains of the production, and confirmed by petrography (geological analysis). The stone was quarried and rough axe heads were produced there, to be more finely worked and polished elsewhere.

As a taxonomic classification of artefacts, industries rank higher than archaeological cultures. Cultures are usually defined from a range of different artefact types and are thought to be related to a distinct cultural tradition. By contrast, industries are defined by basic elements of lithic production which may have been used by many unrelated human groups over tens or even hundred thousands of years, and over very wide geographical ranges. Sites producing tools from the Acheulean industry stretch from France to China, as well as Africa. Consequently, shifts between lithic industries are thought to reflect major milestones in human evolution, such as changes in cognitive ability or even the replacement of one human species by another. Therefore, artefacts from a single industry may come from a number of different cultures.

James B. Stoltman

James B. Stoltman (6 February 1935- 11 September 2019) was an American archaeologist who specialized on the American Mid-West.

He was a professor at the University of Wisconsin-Madison, where he focused on Great Lakes archaeology and research physical ceramic analysis on material from various parts of the world. " Jim Stoltman has been a pioneer in ceramic petrography in the US, on the tracks of Anna Shepard, studying and confirming her work." One of his contributions was developing a method for analyzing ceramic temper.

The archaeology laboratory at his university is named after him.

Lamprophyre

Lamprophyres (Greek λαμπρός (lamprós) = "bright" and φύρω (phýro) = to mix) are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel and chromium.

Lamprophyres occur throughout all geologic eras. Archaean examples are commonly associated with lode gold deposits. Cenozoic examples include magnesian rocks in Mexico and South America, and young ultramafic lamprophyres from Gympie in Australia with 18.5% MgO at ~250 Ma.

Mount Fee

Mount Fee is a volcanic peak in the Pacific Ranges of the Coast Mountains in southwestern British Columbia, Canada. It is located 13 km (8.1 mi) south of Callaghan Lake and 21 km (13 mi) west of the resort town of Whistler. With a summit elevation of 2,162 m (7,093 ft) and a topographic prominence of 312 m (1,024 ft), it rises above the surrounding rugged landscape on an alpine mountain ridge. This mountain ridge represents the base of a north-south trending volcanic field which Mount Fee occupies.

The mountain consists of a narrow north-south trending ridge of fine-grained volcanic rock and small amounts of fragmental material. It is 1.5 km (0.93 mi) long and 0.5 km (0.31 mi) wide with nearly vertical flanks. Mount Fee has two main summits, the southern tower of which is the highest. The summits are separated by a U-shaped crevice that gives them a prominent appearance.

Mount Harker

Mount Harker is a mountain peak located east of Willis Glacier in the Saint Johns Range, Victoria Land, Antarctica. The mountain was first mapped by the Terra Nova Expedition (1910–1913) led by Robert Falcon Scott. The mountain is named for Alfred Harker (1859–1939), an English geologist who specialised in petrology and petrography.

Natural History Museum, Vienna

The Natural History Museum Vienna (German: Naturhistorisches Museum Wien) is a large natural history museum located in Vienna, Austria. and one of the most important natural history museums worldwide.

The NHM Vienna is one of the largest museums and non-university research institutions in Austria and an important center of excellence for all matters relating to natural sciences. The museum's 39 exhibition rooms cover 8,460 square meters and present more than 100,000 objects. It is home to 30 million objects available to more than 60 scientists and numerous guest researchers who carry out basic research in a wide range of topics related to human sciences, earth sciences, and life sciences.

Petrology

Petrology (from the Ancient Greek: πέτρος, romanized: pétros, lit. 'rock' and λόγος, lógos) is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together because they both contain heavy use of chemistry, chemical methods, and phase diagrams. Sedimentary petrology is, on the other hand, commonly taught together with stratigraphy because it deals with the processes that form sedimentary rock.Lithology was once approximately synonymous with petrography, but in current usage, lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks while petrography is the speciality that deals with microscopic details.

In the petroleum industry, lithology, or more specifically mud logging, is the graphic representation of geological formations being drilled through, and drawn on a log called a mud log. As the cuttings are circulated out of the borehole they are sampled, examined (typically under a 10× microscope) and tested chemically when needed.

Rapakivi granite

Rapakivi granite is a hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). The name has come to be used most frequently as a textural term where it implies plagioclase rims around orthoclase in plutonic rocks. Rapakivi is Finnish for "crumbly rock", because the different heat expansion coefficients of the component minerals make exposed rapakivi crumbly.Rapakivi was first described by Finnish petrologist Jakob Sederholm in 1891. Since then, southern Finland's rapakivi granite intrusions have been the type locality of this variety of granite.

Roland Duer Irving

Roland Duer Irving (April 27, 1847 – May 30, 1888) was an American geologist. He was born in New York city and graduated from Columbia College School of Mines in 1869 as a mining engineer. In 1879, he received his Ph.D., also from Columbia.

Soon after his graduation he became assistant on the Ohio geological survey, and in 1870 was elected professor of geology, mining, and metallurgy in the University of Wisconsin. In 1879 the title of his chair was changed to that of geology and mineralogy. He became assistant state geologist of Wisconsin in 1878, and continued as such until 1879. During 1880-1882 he was one of the United States census experts, and in 1882 was made geologist in charge of the Lake Superior division of the United States Geological Survey. His specialty is the micro-petrography of the fragmental rocks and crystalline schists, and pre-Cambrian stratigraphy and the genesis of some of the so-called crystalline rocks. He is considered to be one of the pioneers of petrography in the United States.He was the father of John Duer Irving, another noted geologist and editor of the journal Economic Geology from 1905-1918.

Rudolf Koechlin

Rudolf Koechlin (11 November 1862 – 11 February 1939) was an Austrian mineralogist.

Koechlin was born and died in Vienna. He studied mineralogy, crystallography, petrology and geology at the University of Vienna, obtaining his doctorate in 1887 with a thesis on manganite, polianite and pyrolusite. At Vienna, his instructors included Gustav Tschermak and Albrecht Schrauf. In 1884 he began work as a volunteer in the mineralogical-petrography department of the Naturhistorisches Hofmuseum in Vienna. In 1897 he became a "custos-adjunct", later named a curator first-class (1912), and in 1920, was appointed director of the mineralogical-petrography department.His scientific research largely dealt with minerals found in the Tauern region in Austria, e.g. bornite, euclase and sphene as well as the salt minerals glauberite and simonyite from the salt mine at Hallstatt. The mineral koechlinite is named in his honor.He was the author of around 70 scientific papers and made important contributions towards publication of the "Mineralogisches Taschenbuch" (first edition, 1911). From 1910 to 1932, he was a staff member of the Wiener Mineralogische Gesellschaft. In 1922, he became a correspondent member of the Austrian Academy of Sciences.

Sintra Natural History Museum

The Sintra Natural History Museum (Portuguese: Museu de História Natural de Sintra) is a museum of natural history located in the historic center of the village of Sintra. The museum has both at national and international level due to the quality and rarity of many of its exhibits.

Thin section

In optical mineralogy and petrography, a thin section (or petrographic 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 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 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 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.

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

Tholeiitic magma series

The tholeiitic magma series, named after the German municipality of Tholey, is one of two main magma series in igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma into a more evolved, silica rich end member. The International Union of Geological Sciences recommends that tholeiitic basalt be used in preference to the term "tholeiite" (Le Maitre and others, 2002).

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