felsic and mafic rocks

      division of igneous rocks (mafic rock) on the basis of their silica content. Chemical analyses of the most abundant components in rocks usually are presented as oxides (oxide) of the elements; igneous rocks typically consist of approximately 12 major oxides totaling over 99 percent of the rock. Of the oxides, silica (SiO2) is usually the most abundant. Because of this abundance and because most igneous minerals are silicates, silica content was used as a basis of early classifications; it remains widely accepted today. Within this scheme, rocks are described as felsic, intermediate, mafic, and ultramafic (in order of decreasing silica content).

      In a widely accepted silica-content classification scheme, rocks with more than 65 percent silica are called felsic; those with between 55 and 65 percent silica are intermediate; those with between 45 and 55 percent silica are mafic; and those with less than 45 percent are ultramafic. Compilations of many rock analyses show that rhyolite and granite are felsic, with an average silica content of about 72 percent; syenite, diorite, and monzonite are intermediate, with an average silica content of 59 percent; gabbro and basalt are mafic, with an average silica content of 48 percent; and peridotite is an ultramafic rock, with an average of 41 percent silica. Although there are complete gradations between the averages, rocks tend to cluster about the averages. In general, the gradation from felsic to mafic corresponds to an increase in colour index (dark-mineral percentage).

      The fine-grained or glassy nature of many volcanic rocks makes a chemical classification such as the felsic-mafic taxonomy very useful in distinguishing the different types. Silica content is especially useful because the density and refractive index of natural glasses have been correlated with silica percentage; this makes identification possible in the absence of chemical data. For similar determinations, glasses can also be prepared in the laboratory from crystalline rocks.

      The influence of silica content on the particular minerals that crystallize from a rock magma is a complex interaction of several parameters, and it cannot be assumed that rocks with the same silica content will have the same mineralogy. Silica saturation is a classification of minerals and rocks as oversaturated, saturated, or undersaturated with respect to silica. Felsic rocks are commonly oversaturated and contain free quartz (SiO2), intermediate rocks contain little or no quartz or feldspathoids (undersaturated minerals), and mafic rocks may contain abundant feldspathoids. This broad grouping on the basis of mineralogy related to silica content is used in many modern classification schemes.

* * *


Universalium. 2010.

Look at other dictionaries:

  • Felsic — is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. The term combines the words feldspar and silica. Felsic minerals are… …   Wikipedia

  • Mafic — is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words magnesium and ferric .[1] Most mafic minerals are dark in color and the relative density is greater than 3. Common… …   Wikipedia

  • felsic rock — Igneous rock dominated by the light coloured, silicon and aluminum rich minerals feldspar and quartz. The presence of these minerals gives felsic rock its characteristic light gray colour. The presence of small amounts of dark minerals rich in… …   Universalium

  • mafic rock — In geology, any igneous rock dominated by the silicates pyroxene, amphibole, olivine, and mica. These minerals are high in magnesium and ferrous iron, and their presence gives mafic rock its characteristic dark colour. It is usually contrasted… …   Universalium

  • felsic — /fel sik/, adj. Geol. (of rocks) consisting chiefly of feldspars, feldspathoids, quartz, and other light colored minerals. Cf. mafic. [1910 15; FEL(DSPAR) + S(ILICA) + IC] * * * …   Universalium

  • mafic — /ˈmæfɪk/ (say mafik) adjective of some igneous rocks and their constituent minerals, composed dominantly of the magnesium rock forming silicates. Compare felsic …   Australian English dictionary

  • Emily Ann and Maggie Hays nickel mines — The Emily Ann and Maggie Hays nickel deposits are situated approximately 150km west of the town of Norseman, Western Australia, within the Lake Johnston Greenstone Belt [Hill R.E.T, Barnes S.J., Gole M.J., and Dowling S.E., 1990. Physical… …   Wikipedia

  • Oldest dated rocks — The oldest dated rocks on Earth, as an aggregate of minerals that have not been subsequently melted or disaggregated by erosion, are from the Archean Eon. Such rocks are exposed on the surface in very few places.[1] There is some controversy… …   Wikipedia

  • Case hardening of rocks — Case hardening is a weathering phenomenon of rock surface induratoin. It is observed commonly in: felsic alkaline rocks, such as nepheline syenite, phonolite and trachyte; pyroclastic rocks, as pyroclastic flow deposit, fine air fall deposits and …   Wikipedia

  • igneous rock — Any of various crystalline or glassy, noncrystalline rocks formed by the cooling and solidification of molten earth material (magma). Igneous rocks comprise one of the three principal classes of rocks, the others being metamorphic and sedimentary …   Universalium

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.