Intermediate Rock Composition and Examples

Intermediate igneous rocks are medium in silica (52-63 wt.%) and have nearly equal amounts of felsic and mafic minerals. Examples are andesite and diorite.

Felsic minerals are light-colored minerals like quartz, feldspars, muscovite, feldspathoids, and conundrums. These minerals have light-colored elements like silicon, aluminum, potassium, and sodium.

On the other hand, mafic minerals are iron-rich and dark-colored. Examples include pyroxene, biotite, hornblende, and olivine.  

Some authors will call these rocks mesocratic. It implies they have approximately equal light and dark minerals.

Note: The term intermediate is not an IUGS-recognized term. We use it in the field to refer to rocks whose composition falls between felsic and mafic rocks.

Also, this term is not specific to intrusive or extrusive rocks and doesn’t refer to a certain texture. 

Intermediate rocks may have an aphanitic texture (fine-grained) if volcanic or coarse-grained if intrusive. Also, they can have other textures like porphyritic, pegmatitic, glassy, scoriaceous, etc.

Intermediate rock examples

The main examples of intermediate rocks are diorite, andesite, monzonite, and microdiorite. Others are trachyandesite, latite, basalt-andesite, and phonotephrite.

Some rocks like phonolite, trachyte, trachyandesite, nepheline syenite, and tonalite may be felsic to intermediate. Therefore, their classification depends on the specimen you have.

Composition

Chemically, intermediate rocks have 52-63 wt.% silica and a medium amount of iron, magnesium, sodium, potassium, and calcium.

Mineralogically, they have approximately equal amounts of plagioclase and mafic minerals like pyroxene, hornblende, and biotite. Also, some may have a smaller amount of alkali (potassium feldspar), and sometimes muscovite, quartz, or its tridymite form.

Lastly, these rocks will have accessory minerals like zircon, apatite, magnetite, ilmenite, garnet, sulfides, etc. However, the exact accessory mineral will depend on which rock you have.

Formation

Intermediate rocks form from somewhat evolved, moderately viscous magma. These magmas may be silica-oversaturated or undersaturated, but never strongly.

Usually, these intermediate magmas originate from the crystal fractional mantle-derived magmas (basaltic) or partial melting of subcrustal rocks.

However, contamination and magma mixing are possible ways to have such a composition.

Lastly, these magmas erupt explosively, effusively, or both. It depends on the volatile content they have.

Where are intermediate rocks found?

Intermediate rocks occur at convergent plate boundaries, especially above subduction zones. The famous Ring of Fire is one such place.

Also, these rocks can form in continental rifts and hotspots but in smaller amounts. However, they are rare in oceanic hotspots or mid-ocean ridges.  

More about these rocks

These rocks have densities between mafic and felsic rocks. Their color is mesocratic, with a typical color index M of 30 to 60.

Intermediate rocks crystallize at 800 (1472°F) to 1000°C (1832°F). This temperature is below mafic and above felsic.