Knowra Petrography Petrography Petrography is the systematic description and classification of rocks, especially by examining their minerals, textures, and relationships in thin sections under a microscope.
Thin section : A slice of rock ground to roughly 30 micrometres thick for examination with transmitted light. Its standard thickness lets light pass through minerals for petrographic study.
Mineral : A naturally occurring solid with an ordered atomic structure and a characteristic chemical composition. Minerals are the principal constituents identified and described in most rocks.
QAPF diagram : A double-triangle diagram that classifies many coarse-grained igneous rocks by proportions of quartz, alkali feldspar, plagioclase, and feldspathoids. It converts modal mineral estimates into standardized names for plutonic igneous rocks.
Petrology : The study of rocks, including their origins, composition, structure, and geological history. Petrography describes and classifies rocks within the broader study of their formation and history.
Polarizing microscope : A microscope fitted with polarizing optics for examining how materials interact with polarized light. Its crossed polars reveal optical behavior that helps distinguish rock-forming minerals.
Modal composition : The proportions of minerals or other components actually present in a rock, usually expressed by volume. Modal composition supports quantitative description and many rock classifications.
Streckeisen classification : A widely used system for naming igneous rocks from modal mineral proportions or chemical composition. Petrographic data provide the mineral proportions required by its modal classification.
Petrochemistry : The study of the chemical composition of rocks and its geological significance. It quantifies bulk chemistry, whereas petrography emphasizes visible constituents and textures.
Optical mineralogy : The study of minerals through their interactions with visible light, especially under a polarizing microscope. Its observations supply the optical criteria used to identify minerals in thin sections.
Igneous rock : Rock formed by the cooling and solidification of magma or lava. Its mineral assemblages and crystal textures are central subjects of petrographic classification.
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