Pulaskite
Alkali syenite; alkali feldspar with sodic pyroxene/amphibole and minor nepheline
A coarse-grained alkali syenite of perthitic feldspar with sodic pyroxene or amphibole and minor nepheline, from Pulaski County, Arkansas.
- Mohs hardness
- 5.5-6.5
- Color
- gray, greenish gray to pinkish gray
- Type
- igneous
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Overview
Pulaskite is a coarse-grained alkali syenite composed largely of perthitic alkali feldspar with sodic mafic minerals such as aegirine-augite or alkali amphibole, minor biotite, and small amounts of nepheline (and sometimes a little quartz in some descriptions). It is mildly to slightly silica-undersaturated, leaning toward the feldspathoidal syenite family.
The rock is named from Pulaski County, Arkansas, USA, part of the alkaline igneous province around Magnet Cove and Little Rock. It sits among the suite of nepheline syenites and related alkaline rocks.
Pulaskite is principally a petrological reference rock for alkaline plutonic suites.
Formation & geology
Pulaskite forms by slow crystallization of alkali-rich magma at depth, typically within alkaline intrusive complexes in rift or intraplate continental settings. The high alkali, low-silica character favors abundant alkali feldspar plus sodic mafics and minor nepheline.
It occurs in plutons and intrusive masses associated with nepheline syenites and other feldspathoidal rocks. The type area in Pulaski County, Arkansas, lies within an alkaline province also famous for Magnet Cove; comparable rocks occur in alkaline complexes elsewhere.
How to identify it
Look for a gray to greenish or pinkish, coarse-grained rock dominated by perthitic feldspar with dark green sodic pyroxene or amphibole and possibly a greasy gray feldspathoid (nepheline). Hardness is moderate to fairly high.
The key is recognizing an alkali syenite — feldspar-dominated, quartz-poor or quartz-free — with sodic mafic minerals and minor nepheline, marking its alkaline, mildly undersaturated nature.
Look-alikes include nordmarkite (more quartz-bearing), ordinary syenite, and nepheline syenite (more nepheline); the small nepheline content with abundant alkali feldspar distinguishes pulaskite, usually confirmed microscopically.
Uses & significance
Pulaskite has little direct commercial use, though as a feldspar-rich rock it could locally serve as dimension or crushed stone.
Its significance is scientific and educational: it is a named member of alkaline plutonic suites used in the classification and study of nepheline syenites and rift-related alkaline magmatism.
It has no gemstone or metaphysical tradition.
Frequently asked questions
What is pulaskite?
An alkali syenite of perthitic feldspar with sodic pyroxene or amphibole and minor nepheline, mildly silica-undersaturated.
Where does the name come from?
From Pulaski County, Arkansas, USA, within an alkaline igneous province near Magnet Cove.
How is pulaskite different from nepheline syenite?
Pulaskite has only minor nepheline with dominant alkali feldspar, whereas nepheline syenite contains substantially more nepheline.
Is pulaskite an alkaline rock?
Yes. Its sodic pyroxene/amphibole and nepheline content mark it as part of the alkaline syenite family.
Pulaskite guides
In-depth guides for identifying, valuing, and understanding Pulaskite.
