Monzonite

In short

Monzonite is a medium- to coarse-grained rock formed when magma cools slowly deep in the crust. It holds pinkish-white alkali feldspar and milky plagioclase in about equal amounts, with almost no quartz. Grey and speckled, it looks like granite and is often sold as “granite”. Norway's larvikite, with its silver-blue sheen, is also a monzonite. In Türkiye it occurs around Kırşehir.

Photos

How to recognise it

1. 1

   It is a grey, speckled rock in which visible pinkish-white alkali feldspar and milky white plagioclase occur in about equal amounts.

2. 2

   Dark minerals such as biotite (black, shiny flakes), hornblende or pyroxene are scattered through it; glassy grey quartz grains are absent or very few.

3. 3

   In the kind called larvikite, a polished surface shows a silver-blue sheen from the feldspar crystals.

4. 4

   To tell monzonite from syenite, diorite and granodiorite you have to count feldspar proportions; a firm identification by eye is hard.

Myths and misconceptions

• Myth“Blue Pearl granite” is real granite.

  FactThe Norwegian stone sold as “Blue Pearl granite” is larvikite, a quartz-poor variety of monzonite. It differs from granite in having almost no quartz.

• MythEvery grey, speckled, coarse-grained plutonic rock is granite.

  FactMonzonite, syenite, diorite and granodiorite look like granite too. Monzonite has at most 5% quartz, whereas granite has more than 20%.

Did you know?

Where is it found?

• Predazzo and Monte Monzoni, Trentino, Italy · Type localityThe Dolomites, home of the Monzoni mountains that gave the rock its name. The Predazzo and Monzoni plutons, emplaced in the Triassic about 238 million years ago, are the type locality of monzonite and played a leading role in the birth of petrography.
• Larvik, Vestfold, Norway · Major depositLarvikite (sold as “Blue Pearl granite”) is mainly quarried here. It is a 298-292 million-year-old monzonite variety of the Larvik Batholith in the Oslo Rift and shows a silver-blue schiller when polished.
• Bingham Canyon Mine, Utah, USA · Major depositA porphyry copper deposit formed by a quartz monzonite porphyry intruded into sedimentary rocks. The pit, 4 km wide and more than 1,200 metres deep, has been in production since 1906.
• Little Cottonwood Canyon, Utah, USA · Historic sourceThe Salt Lake Temple was built with blocks of quartz monzonite, granite and granodiorite quarried from the Little Cottonwood stock near the mouth of the canyon; the stock is about 30 million years old.
• Stone Mountain, Georgia, USA · Known localityA quartz monzonite dome about 300-350 million years old; a mass of rock rising 251 metres above its surroundings with a base more than 8 km around.
• Kaman area, Baranadağ, Kırşehir, Türkiye · Known localityThe Baranadağ quartz monzonite, one of the Central Anatolian granitoids of the Kırşehir Massif, is a unit of the first plutonic phase of the region.

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →