petrology, petrography, petrogenesis

petrology, petrography, petrogenesis Petra is the Greek for rock. Petrology is the science of rocks; petrography and petrogenesis are subdisciplines: petrography is concerned with the description of rocks; petrogenesis with the ways in which they are formed.

Three major divisions of rocks are recognized in geology: igneous, metamorphic, and sedimentary. Each of these terms carries petrogenetic connotations. Igneous rocks are those that have formed from molten magma. Metamorphic rocks, as the name implies, are those that have formed by the modification in the solid of an existing rock by the effects of heat and pressure. Most sedimentary rocks are formed at the surface of the Earth by deposition of particles that have been eroded or weathered from pre-existing rocks, which can be igneous, metamorphic, or sedimentary. Some sedimentary rocks are formed of particles deposited by or from a fluid by chemical or biological precipitation.

There are some rocks where more than one set of processes has been active in their formation. For example, igneous cumulates are formed from the accumulation of dense minerals that have crystallized out from cooling igneous melts. Fluid dynamic processes operate upon crystals settling out of a melt in the same way as on sand grains being deposited in water; sedimentary textures can thus be formed in an igneous rock. Another example of an igneous rock with a sedimentary texture is a tuff: a rock formed by the accumulation of volcanic ash. Metamorphic rocks that have formed from igneous or sedimentary rocks can still display relict igneous or sedimentary textures.

Petrography

Petrography, the description of rocks, includes the description of hand specimens and the examination of thin sections under the microscope.

Hand-specimen description includes observation of the colour, texture, grain-size, and mineral content of the rock. Where the rock contains mineral grains it may be possible to determine the hardness, lustre and streak of the minerals. Hardness is measured using Moh's scale of hardness, which is based on a ten-point scale whose points are defined by a series of minerals. Two tests that can be applied when a complete set of test minerals is not available are a fingernail (hardness 2.5) and the blade of a pen-knife (4.5). Streak is the colour of the mark produced by rubbing the mineral across the surface of unglazed porcelain or ground glass. Lustre describes the surface appearance of the mineral.

The use of thin sections greatly aids the description of rocks and the identification of minerals. A thin section is a slice of rock 30 micrometres thick affixed to a glass plate or slide with a resin called Canada balsam. Most rocks and minerals are transparent in slices as thin as this and can be viewed in transmitted light at various magnifications. Many minerals have properties that vary with the direction in which the light passes through the crystal. Rocks are viewed on a petrological microscope in polarized light on a rotating stage so that the various colours and refractive indices become apparent as the stage is rotated. The Canada balsam upon which the rock is mounted has a refractive index of 1.538. Minerals whose refractive index is higher than this appear to stand out from the surface of the slide like hills and are said to have positive relief; minerals of negative relief appear as pits in the slide. Minerals that are not transparent even when sliced so thinly are described as opaque. Opaque minerals are examined in reflected light, i.e. by episcopy. For exact determinations of the mineral composition from thin section, analytical tools such as an ion-probe or an electron microscopy are used. The proportions of matrix and minerals in a rock can be measured by point counting.

Petrogenesis

Igneous petrogenesis is the process or set of processes by which igneous rocks are formed. The basis of the formation of any igneous rock is melting to form a magma. Igneous petrogenesis includes not only the generation of a melt but also the subsequent modification of the magma composition. The petrogenesis of a rock cannot generally be observed directly and must be deduced from petrographic observations of the rock itself or experiments on synthetic materials designed to stimulate the rock-forming process; i.e. experimental petrology.

Judith M. Bunbury

Bibliography

Dud'a,, Rudolf, and Rejl,, and Lubos (1986) Minerals of the world. Spring Books, Twickenham, Middlesex.
Deer, W. A.,, Howie, R. A.,, and and Zussman, K. (1985) Rock forming minerals (2nd edn). Geological Society, London.