1 Sep In order to create a sustainable classification of igneous rocks which all geologists might use, an international body was set up by the IUGS: the. 18 Mar A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. 1/17/ Petrology-Spring , Goeke. basic system which classifies rocks on the basis of where they form. work by the IUGS Subcommission on the Classification of Igneous Rocks or Streckeissen .

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In such rocks, there is often glass or the minerals are too small to be readily identified. For volcanic rocks, the following table can be used. Most are what we term genetic.

General Classification of Igneous Rocks

Please help improve this article by adding citations to reliable sources. Still, because chemical composition can vary continuously, there are few natural breaks to facilitate divisions between different rocks.

Chemical composition cannot be easily determined in the field, making classification based on chemistry impractical. Other minerals may and almost certainly occur in these rocks as well but they have no significance in this classification scheme. The classification of igneous rocks has been the subject of frequent debate and voluminous iugs classification of igneous rocks.

A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. Still, because of the history of the science, many of these rock iugs classification of igneous rocks are firmly entrenched in the literature, so the student must be aware of all of these names, or at least know where to look to find out what the various rocks names mean.


These are rocks that contain just enough silica that quartz does not appear, and just enough silica that one of the silica undersaturated minerals does not appear.

The IUGS systematics of igneous rocks

But color can be misleading when applied to rocks of the same composition but different grain size. For instance, the classification of minerals is based on common anoins since minerals sharing common anions often have similar physical properties i.

For plutonic rocks, the IUGS system of classification can be used. By using this site, you agree to the Terms of Use and Privacy Policy.

An exact name can be given only if the mineralogical composition is known, which cannot be gocks in the field. If a magma is oversaturated with respect to Silica then a silica mineral, such as quartz, cristobalite, tridymite, or coesite, should precipitate from the magma, and be present in the rock. Rock classifications also seek to reduce complexity.

Not really, these are the minerals that should crystallize from a dacite magma, but don’t because the crystallization history is classificatiob by rapid cooling iugs classification of igneous rocks the surface. General Classification of Igneous Rocks. For example, aphanitic rocks are or volcanic origin while phaneritic rocks are plutonic.

QAPF diagram – Wikipedia

Which of the following minerals, if found in a rock, would indicate that the rock is undersaturated with respect to silica? Next, let’s look at a silica oversaturated composition, such as one just to the left of the point labeled ‘P’ in the diagram. While most peralkaline rocks are also alkaline, alkaline rocks are not necessarily peralkaline. Chemical analyses can be obtained, and a chemical classification, such as the LeBas et al. If there is an excess of Alumina over that required to form feldspars, we iugs classification of igneous rocks that the rock is peraluminous.


This taxonomic classification was designed to simplify the complexity of nature by lumping together living species iugs classification of igneous rocks shared common traits.

In those days each new rock described by a geologist could have shown characteristics different than the rocks that had already been described, so there was a tendency to give the new and different rock a new itneous. Alumina Al 2 O 3 Saturation After silica, alumina is the second most abundant oxide constituent in igneous rocks. Iugs classification of igneous rocks in the days of geology there were few rocks described and classified.

QAPF diagram

F and Q for chemical reasons cannot exist together in one plutonic rock. The silica saturation concept can thus be used to divide rocks in silica undersaturated, silica saturated, and silica oversaturated iugs classification of igneous rocks.

Because of the limitations of the various criteria that can used to classify igneous rocks, geologists use an approach based on iugs classification of igneous rocks information obtainable at various stages of examining the rocks. Silica Undersaturated Rocks – In these rocks we should find minerals that, in general, do not occur with quartz.

Thus, if we find any of these minerals in a rock, with an exception that we’ll see in a moment, then we can expect the rock to be silica undersaturated.