Tonalite
Felsic to intermediate intrusive rock (quartz + plagioclase, little alkali feldspar)
A quartz-rich plutonic rock dominated by plagioclase feldspar with little alkali feldspar, closely related to granodiorite and quartz diorite.
- Mohs hardness
- 6-7
- Color
- Light to medium grey, speckled black and white
- Type
- igneous
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Overview
Tonalite is a coarse-grained intrusive igneous rock rich in quartz (more than 20%) and dominated by plagioclase feldspar, with very little alkali feldspar. This sets it apart from granite (which has abundant alkali feldspar) and links it closely to granodiorite and quartz diorite.
Named after Tonale Pass in the Italian Alps, tonalite typically contains biotite and hornblende as its dark minerals, giving it a salt-and-pepper appearance. Sodic, quartz-rich varieties grade into trondhjemite and plagiogranite.
Tonalite is an important member of the granitoid family and of the ancient TTG (tonalite-trondhjemite-granodiorite) suites that built early continental crust.
Formation & geology
Tonalite crystallizes slowly from water-bearing, intermediate-to-felsic magma deep in the crust, forming plutons and batholiths. It is especially common in subduction-related continental-margin arcs, where it forms alongside granodiorite, diorite, and granite.
Tonalitic rocks are major constituents of Archean cratons as part of the TTG suite, which represents some of Earth's oldest continental crust. Oceanic plagiogranite, a sodic tonalite/trondhjemite, also forms by differentiation of mafic magma in oceanic crust and ophiolites.
How to identify it
Tonalite is a light-to-medium grey, coarse-grained rock with a speckled appearance from abundant white-to-grey plagioclase, glassy quartz, and dark biotite and hornblende, but little or no pink alkali feldspar. Hardness is about 6-7 (quartz and feldspar).
Distinguish tonalite from granite by its scarcity of alkali feldspar (so it lacks granite's pink tones), from granodiorite by having even less alkali feldspar, and from quartz diorite by its slightly higher quartz content. A hand-lens check of feldspar types is the key test.
Uses & significance
Tonalite is quarried as a durable construction and dimension stone, used for aggregate, road base, facing, and sometimes countertops (often marketed loosely as granite). Some tonalite-related intrusions host copper and gold mineralization.
It has no gemstone or metaphysical tradition. Scientifically, tonalite is highly significant as part of the TTG suite that records the formation and growth of early continental crust.
Frequently asked questions
What is the difference between tonalite and granite?
Granite contains abundant alkali feldspar, while tonalite is dominated by plagioclase with little alkali feldspar, though both are quartz-rich.
Is tonalite the same as quartz diorite?
They are very similar; tonalite has more quartz (over 20%), and the two grade into one another.
What is the TTG suite?
Tonalite-trondhjemite-granodiorite, a group of sodic granitoids that make up much of Earth's oldest Archean continental crust.
What color is tonalite?
Usually light to medium grey with a black-and-white speckled look, lacking the pink tones of alkali-feldspar granite.
Tonalite guides
In-depth guides for identifying, valuing, and understanding Tonalite.
