Skip to content

Geology is the Way

Monzonite

Monzonite is an intermediate plutonic igneous rock containing almost equal proportions of alkali feldspar and sodic plagioclase and minor amounts of quartz or feldspathoids. Its name derives from the Monzoni range in the Fassa Valley (Dolomites, Northern Italy), where this rock is widely exposed. The monzonite field is located exactly in the center of the QAPF diagram and it is defined by plagioclase (or alkali feldspar) content over feldspars between 35 and 65%. Quartz normally represent between 0 and 20% of the felsic minerals, a much lower content than in granites. Monzonites may also lack quartz completely and rather contain up to 10% of feldspathoids over the felsic minerals. The presence of quartz (Q) or feldspathoids (F) allows to distinguish three varieties of monzonites: quartz monzonite (Q = 5 – 20%), monzonite (Q = 0 – 5%), and foid-bearing monzonite (F = 0 – 10%), in which the term ‘foid’ can be substituted with the most abundant feldspathoid present (e.g. leucite-bearing monzonite). Monzonites are characterized by a relatively high content of mafic minerals, normally in the range of 15 and 45% of the rock volume, which typically consist of hornblende, biotite, or pyroxene. The extrusive counterparts of monzonites are latites and trachyandesites.
Monzonites are relatively uncommon rock types that occur as marginal bodies associated with more acid (granites and granodiorites) and mafic (gabbros) intrusions. In some cases, they host skarns and porphyry copper deposits. Monzonites were also found on the Moon by the Apollo missions.

Larvikite is a variety of monzonite or augite syenites containing ternary feldspars (i.e. feldspars with significant proportions of the K-, Na-, and Ca- end members), dark amphibole, titanian augite, and lepidomelane, associated with minor quartz or nepheline and Fe-olivine. The name derives from the Larvik intrusive complex (Oslo Igneous Province, Norway).

larvikite

Larvikite, a variety of monzonite from Larvik, Norway, showing reddish to bluish perthitic feldspars, associated with titanium augite and sodic amphibole. © Dietmar Down Under.

Monzonite
Plutonic igneous rock
Felsic minerals:
alkali feldspar
plagioclase
quartz
feldspathoids
Mafic minerals:
hornblende
biotite
pyroxene

QAPF classification:
Q = 0 – 20% or F < 10%
Plagioclase/feldspars = 35 – 65%
Colored varieties:
• leucomonzonite (M < 15%)
• melamonzonite (M > 45%)
Other varieties: quartz monzonite, foid-bearing monzonite, larvikite
Extrusive equivalent: latite, trachyandesite

monzonite

Sample of a monzonite dyke. The white mass consists of alkali feldspar and plagioclase, while the black part are mafic minerals. Bearpaw Mountains, Montana, USA. © James St. John.

monzonite

Quartz monzonite containing alkali feldspar and plagioclase (white to iridescent), quartz (grey) and biotite (black, metallic). Butte, Montana, USA. © James St. John.

larvikite

Iridescence is a common feature of larvikite and the main reason why they are used as ornamental stones (with the name of Blue Pearl Granite). The feldspars in larvikite are iridescent because they contain microscopic unmixings of alkali feldspar and plagioclase (perthites and antiperthites). The interaction between light and these fine-grained lamellae produces the characteristic grey-blue luster (adularescence). Larvik, Norway. © James St. John.

References

        

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

Do you like this page?
en_USEnglish