Petrography is that branch of petrology which 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 analyses 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.
Methods of investigation
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 which is really 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, waterworn sand-grains and if it contains dull, weathered particles of felspar, 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 felspar, clear vitreous quartz and glancing flakes of mica, the other will show yellow-green olivine, black augite and grey 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.
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
, 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
). In toto
, these characteristics are sufficient to identify the mineral, and often to quite tightly estimate it's 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
The separation of the ingredients of a crushed rock powder
from one to another in order to obtain pure samples suitable
for analysis is also extensively practised. It may
be effected by means of a powerful electro-magnet
the strength of which can be regulated as desired.
A weak magnetic field will attract magnetite, then haematite
and other ores of iron. Silicates containing iron will follow
in definite order and biotite, enstatite, augite, hornblende,
garnet and similar ferro-magnesian minerals may be successively
abstracted, at last only the colorless, non-magnetic
compounds, such as muscovite, calcite, quartz and felspar, will
remain. Chemical methods also are useful. A weak acid will
dissolve calcite from a crushed limestone, leaving only dolomite,
silicates or quartz. Hydrofluoric acid will attack felspar before
quartz, and if employed with great caution will dissolve these
and any glassy material in a rock powder before dissolving
augite or hypersthene. Methods of separation by specific
gravity have a still wider application. The simplest of these
by a current of water, it is extensively employed in the mechanical
analysis of soils and in the treatment of ores, but is not so
successful with rocks, as their components do not as a rule
differ very greatly in specific gravity.
Fluids are used which do not attack the majority of the rock-making
minerals and at the same time 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
media employed. They may be diluted (with water, benzene, etc.)
to any desired extent and again concentrated by evaporation.
If the rock be a 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 will all float in
methylene iodide; on gradual dilution with benzene they will be
precipitated in the order given above. Although simple in theory
these methods are tedious in practice, especially as it is common
for one rock-making mineral to enclose another. But expert handling of fresh
and suitable rocks yields excellent results.
In addition to naked-eye and microscopic investigations
chemical methods of research are of the greatest practical
importance to the geographer. The crushed and separated powders,
obtained by the processes described above, may be
analyzed and thus the chemical composition of the minerals
in the rock determined qualitatively or quantitatively. The
chemical testing of microscopic sections and minute
grains by the help of the microscope is a very
elegant and valuable means of discriminating between
the mineral components of fine-grained rocks. Thus the
presence of apatite in rock-sections is established by covering
a bare rock-section with solution of ammonium molybdate;
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 which can be stained with coloring matters such as the
aniline dyes (nepheline, analcite, zeolites, etc.).
Complete chemical analyses of rocks are also widely made use of
and are of the first importance, especially when new species are under
description. 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 will carry us a long way in determining the position
to which a rock is to be assigned in any of the conventional classifications.
A chemical analysis is in itself usually sufficient to indicate
whether a rock is igneous or sedimentary and in either case to show
with considerable accuracy to what subdivision of these classes it
belongs. In the case of metamorphic rocks it often establishes
whether the original mass was a sediment or of volcanic origin.
The specific gravity of rocks is determined in the usual way by
means of the balance and the pycnometer. It is greatest in those
rocks which contain most magnesia, iron and heavy
metals, least in rocks rich in alkalis, silica and water.
It diminishes with weathering, and generally those rocks
which are highly crystalline have higher specific gravity than those
which are wholly or partly vitreous when both have the same
chemical composition. The specific gravity of the commoner rocks
ranges from about 2.5 to 3.2.
Petrography is used by archaeologists
to identify the mineral components in pottery
. This information is then usually used to tie the artifacts to geological source areas for both the clay used and the rock fragments (usually called "temper" or "aplastics") often added by potters to modify the properties of the clay. This information provides insight into how potters were selecting and using local and nonlocal resources, as well as allowing archaeologists to 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 be used to build 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.