Essexite Identification Guide
Identify essexite, a dark silica-poor intrusive rock (nepheline monzogabbro), by its mineralogy, texture, and field setting.
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What Essexite Looks Like
Essexite is a dark, coarse-grained alkaline intrusive (plutonic) rock, essentially a nepheline-bearing monzogabbro/monzodiorite. It is silica-undersaturated, so it contains feldspathoids instead of quartz.
- Color: dark gray to greenish-black, often mottled, with pale feldspar specks.
- Texture: phaneritic (visibly crystalline), coarse to medium grained.
- Minerals: abundant dark pyroxene (titanaugite) and amphibole, plus plagioclase, alkali feldspar, and nepheline; often biotite, olivine, and magnetite.
- Form: occurs as stocks, sills, dikes, and small intrusions.
Step-by-Step Field-ID Checklist
- Confirm it is coarse and intrusive. Interlocking visible crystals, not glassy or fine like a lava.
- Assess color index. It is dark and mineral-rich (lots of black pyroxene/amphibole).
- Look for feldspar. Pale plagioclase and feldspar grains contrast with the dark minerals.
- No quartz. Unlike diorite/gabbro with quartz, essexite is silica-poor; you should not see glassy gray quartz.
- Check setting. Associated with alkaline igneous complexes.
Key Diagnostic Tests
- Overall hardness: scratches glass (feldspar and pyroxene are 6-6.5).
- No acid fizz (it is a silicate igneous rock, not a carbonate).
- Specific gravity: moderately high (~2.9-3.1) due to abundant mafic minerals.
- Magnetism: often weakly magnetic from magnetite.
- Cleavage: seen in individual grains (two pyroxene cleavages near 90 degrees, feldspar cleavages); the rock as a whole fractures irregularly.
Common Look-Alikes and How to Tell Them Apart
- Gabbro/diorite: these are silica-saturated and may contain quartz; they lack nepheline. Essexite's feldspathoid content is the key chemical difference (best confirmed petrographically).
- Basalt: fine-grained volcanic equivalent; essexite is coarse and intrusive.
- Nepheline syenite: lighter colored and feldspar-dominated with far fewer dark minerals; essexite is darker and more mafic.
- Larvikite/monzonite: larvikite shows blue schiller feldspar; monzonite is more felsic and quartz-bearing.
Where It Is Typically Found
The type locality is Essex County, Massachusetts (USA). Other occurrences include the Czech Republic (Bohemian Massif), Scotland, Norway (Oslo Rift), Germany, and Brazil. It is associated with alkaline, rift-related magmatism. Dense essexite is famously used to make curling stones.
Frequently asked questions
What type of rock is essexite?
Essexite is a dark, coarse-grained alkaline intrusive (plutonic) igneous rock, classified as a nepheline-bearing monzogabbro or monzodiorite. It is silica-undersaturated and contains feldspathoids instead of quartz.
How do you tell essexite from gabbro?
Both are dark and coarse, but gabbro is silica-saturated and may contain quartz, while essexite is silica-poor and contains nepheline. Confirming the feldspathoid usually requires a thin section.
Is essexite used for curling stones?
Yes. Dense, tough, fine-textured essexite, notably from Wales and Scotland, has traditionally been used to make curling stones because it resists chipping and absorbs impact well.
Does essexite contain quartz?
No. Essexite is silica-undersaturated, so it contains nepheline rather than free quartz. Seeing glassy gray quartz argues against essexite and toward a normal gabbro or diorite.