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felsic

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differentiation index

Overview page. Subjects: Earth Sciences and Geography.

The sum of the normative constituents (see cipw norm calculation) Q + Ab + Or + Ne + Kp + Lc in an igneous rock, where Q = quartz, Ab = albite, Or = orthoclase, Ne = nepheline, Kp =...

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intermediate rock

Overview page. Subjects: Environmental Science — Earth Sciences and Geography.

Igneous rocks that have a silica content between the extremes of the mafic and felsic rocks, and weather down to produce quite fertile soils. Intermediate between basic and acidic rocks.

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lamprophyre

Overview page. Subjects: Earth Sciences and Geography.

A dark-coloured, strongly porphyritic, intrusive igneous rock, containing abundant euhedral phenocrysts of biotite and/or amphibole which can be accompanied by phenocrysts of olivine,...

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lithophysae

Overview page. Subjects: Earth Sciences and Geography.

Concentric shells of aphanitic material enclosing a hollow core, which form rounded masses a few centimetres in diameter within glassy or aphanitic, felsic igneous rocks. The hollow,...

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mafic

Overview page. Subjects: Environmental Science — Earth Sciences and Geography.

Applied to any igneous rock which has a high proportion of pyroxene and olivine, such that its colour index is between 50 and 90 (i.e. it is dark coloured).

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orbicular texture

Overview page. Subjects: Earth Sciences and Geography.

Concentric shells of contrasting mineralogy and texture which form spherical masses of diameter 2–15 cm in basic and felsic plutonic igneous rocks.

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peralkaline

Overview page. Subjects: Earth Sciences and Geography.

A chemical classification applied to felsic igneous rocks in which there are more molecules of (Na2O + K2O) than of Al2O3. Rocks with such a chemistry crystallize alkali-rich ferromagnesian...

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porphyry

Overview page. Subjects: Art.

A hard igneous rock containing crystals of feldspar in a fine-grained, typically reddish groundmass, used in ancient Egypt as a building stone. The word comes (in late Middle English) via...

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ryolite

Overview page. Subjects: Environmental Science.

A relatively viscous, extrusive type of granite; a typical felsic rock.

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