monzonite

—monzonitic /mon'zeuh nit"ik/, adj.

/mon"zeuh nuyt'/, n.

any of a group of granular igneous rocks having approximately equal amounts of orthoclase and plagioclase feldspar, intermediate in composition between syenite and diorite.

[1880-85; < G Monzonit, named after Monzoni, mountain in Tyrol; see -ITE¹]

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Type of igneous rock that contains abundant and approximately equal amounts of plagioclase and potash feldspar, as well as other minerals.

The type region is Monzoni, Italy, in the Italian Tirol; similar rocks have been described from Montana, Norway, Sakhalin Island, and other localities. Monzonite is not rare, but it generally occurs in rather small, heterogeneous masses mixed with diorites, pyroxenites, or gabbros.

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▪ mineral

      intrusive igneous rock that contains abundant and approximately equal amounts of plagioclase and potash feldspar; it also contains subordinate amounts of biotite and hornblende, and sometimes minor quantities of orthopyroxene. Quartz, nepheline, and olivine, which are occasionally present, produce quartz, nepheline, and olivene monzonites. In the type region of Monzoni, Italy, in the Italian Tirol, these minerals rarely occur as major constituents; moreover, in Monzoni the plagioclase is often calcium-rich (labradorite) and commonly shows prominent zoning. Rocks similar to those of the type area have been described from Norway (akerite), Montana (yogoite, shonkinite), Sakhalin Island off the Pacific coast of Russia, and other localities throughout the world. Monzonite is not a rare rock type, but it generally occurs in rather small, heterogeneous masses associated with (and perhaps gradational to) diorites, pyroxenites, or gabbros.

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