Petrography

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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.

[edit] Archaeological applications

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.