Shonkinite
Mafic (melanocratic) alkaline rock; clinopyroxene-rich with alkali feldspar, often nepheline-bearing
A dark, mafic potassic alkaline rock rich in augite with alkali feldspar and often nepheline, classically forming the base of layered sills.
- Mohs hardness
- 5-6
- Color
- dark gray to black
- Type
- igneous
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Overview
Shonkinite is a dark, melanocratic (mineral-dark) alkaline intrusive rock dominated by clinopyroxene (augite/diopside) together with alkali feldspar and commonly olivine, biotite, and nepheline. It is essentially a mafic, pyroxene-rich relative of syenite, often classed as a feldspathoidal or potassic alkaline rock.
The rock is named from the Shonkin Sag laccolith and the Highwood Mountains of Montana, USA, where it forms a classic example of magmatic layering — shonkinite accumulating at the base beneath lighter syenite.
It is prized as a petrological teaching rock illustrating in-situ differentiation of alkaline magma.
Formation & geology
Shonkinite forms from potassium-rich alkaline magma that crystallizes and differentiates within shallow intrusions such as sills and laccoliths. Dense early-formed pyroxene and olivine settle and accumulate toward the base of the body, producing a dark, mafic shonkinite layer beneath more feldspar-rich syenite.
It is characteristic of continental alkaline provinces and rift-related potassic magmatism. The Highwood Mountains and Shonkin Sag laccolith of Montana are the classic localities, with comparable rocks in other alkaline complexes worldwide.
How to identify it
Look for a very dark, heavy, coarse-grained rock dominated by black-green augite with interstitial pale alkali feldspar, often visibly mafic-rich (melanocratic). Olivine and biotite may be present, and a greasy feldspathoid (nepheline) can occur.
The key feature is the high proportion of pyroxene combined with alkali feldspar — making it darker than typical syenite. It differs from gabbro in being alkaline and feldspar-poor-to-mixed with alkali feldspar and feldspathoids rather than dominated by plagioclase.
Look-alikes include pyroxenite, melteigite, and dark syenite; confirming alkali feldspar (and any nepheline) distinguishes shonkinite, generally via microscopy.
Uses & significance
Shonkinite has no significant commercial use, occasionally serving only as local crushed or dimension stone.
Its true importance is scientific and educational: the Shonkin Sag laccolith is a world-famous example of in-situ magmatic differentiation and crystal settling, making shonkinite a standard reference in petrology courses and research on layered alkaline intrusions.
It has no gemstone or metaphysical tradition.
Frequently asked questions
What is shonkinite made of?
Mainly augite (clinopyroxene) with alkali feldspar, and often olivine, biotite, and nepheline; it is a dark, mafic alkaline rock.
Where is shonkinite found?
Classically in the Highwood Mountains and Shonkin Sag laccolith of Montana, USA, and in other alkaline provinces.
Why is shonkinite important to geologists?
It illustrates magmatic differentiation, accumulating as a dark mafic layer at the base of layered alkaline sills and laccoliths.
How is shonkinite different from syenite?
Shonkinite is much darker and richer in pyroxene (melanocratic), whereas typical syenite is dominated by feldspar.
Shonkinite guides
In-depth guides for identifying, valuing, and understanding Shonkinite.
