Pulaskite Identification Guide
Identify pulaskite, a quartz-free alkaline syenite, by its feldspar-dominated phaneritic texture and alkali mafic minerals.
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What Pulaskite Looks Like
Pulaskite is a coarse-grained alkaline syenite (first described from Fourche Mountain, Pulaski County, Arkansas). It is gray to greenish-gray, phaneritic (visibly crystalline), and dominated by alkali feldspar (orthoclase/perthite). It contains minor feldspathoids such as nepheline or sodalite, dark alkali pyroxene (aegirine) or amphibole (arfvedsonite), and little biotite — and crucially, essentially no quartz.
It often shows a trachytoid texture in which tabular feldspars are roughly aligned, giving a faint planar fabric.
Step-by-Step Field ID Checklist
- Grain size: Confirm a fully crystalline, coarse intrusive rock with interlocking grains.
- Dominant mineral: Most of the rock is pale feldspar (blocky, with cleavage flashes).
- Quartz check: Hunt for gray glassy quartz — pulaskite has little to none (key distinction from granite).
- Mafic minerals: Identify dark green-black needles/grains of aegirine or arfvedsonite.
- Feldspathoids: Look for greasy nepheline or blue sodalite among the feldspar.
Key Diagnostic Tests
- Hardness: feldspar ~6 dominates (scratches glass).
- Texture: phaneritic, often trachytoid (aligned feldspar laths).
- No acid reaction (silicate rock).
- Quartz absence: the defining negative test versus granite/nordmarkite.
- Color index: low to moderate (mostly light feldspar with scattered dark grains).
Common Look-Alikes and How to Tell Them Apart
- Granite: contains abundant gray glassy quartz; pulaskite is quartz-free or quartz-poor.
- Nordmarkite: a quartz-bearing alkali syenite — presence of quartz separates it from pulaskite.
- Nepheline syenite: contains more abundant feldspathoid; pulaskite is a transitional, feldspathoid-poor member.
- Larvikite: a feldspar-rich syenitic/monzonitic rock famous for blue schiller; pulaskite typically lacks the strong play of light.
- Diorite: contains plagioclase plus abundant hornblende/biotite and is darker; pulaskite is alkali-feldspar dominated with alkali mafics.
The diagnosis hinges on a phaneritic, alkali-feldspar-dominated rock with minor feldspathoids, alkali pyroxene/amphibole, and no quartz.
Where It Is Found
Pulaskite is found in alkaline intrusive complexes. The type area is Fourche Mountain / the Magnet Cove region of Arkansas, USA. Similar alkaline syenites occur in the Oslo rift (Norway) and other alkaline igneous provinces worldwide.
Frequently asked questions
How do you identify pulaskite?
Look for a coarse, gray to greenish-gray intrusive rock dominated by alkali feldspar with minor nepheline or sodalite and dark alkali pyroxene/amphibole, and essentially no quartz.
What is the difference between pulaskite and granite?
Granite contains abundant gray glassy quartz, while pulaskite is a quartz-free (or quartz-poor) alkaline syenite dominated by alkali feldspar with feldspathoids and alkali mafic minerals.
Where does the name pulaskite come from?
Pulaskite is named after Pulaski County, Arkansas, where it was first described from Fourche Mountain in the alkaline igneous province near Magnet Cove.
Does pulaskite contain quartz?
No — the absence of quartz is a defining feature. If a similar-looking syenite contains visible quartz, it is a quartz syenite such as nordmarkite rather than pulaskite.