Nordmarkite Identification Guide
How to recognize nordmarkite, a quartz-bearing alkali-feldspar syenite, by its feldspar-dominated texture, sparse quartz, and field setting.
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What Nordmarkite Looks Like
Nordmarkite is a coarse- to medium-grained intrusive igneous rock — essentially an alkali-feldspar syenite carrying a small amount of quartz (roughly 5–20%), which places it just into the quartz-syenite field. Hand specimens are dominated by stubby to tabular alkali feldspar (perthitic microcline or orthoclase) in pale gray, buff, pinkish, or salmon tones, often with a subtle bluish or greenish sheen on cleavage faces. The overall look is light-colored and feldspar-rich, with only scattered dark minerals (hornblende, biotite, sometimes aegirine) and minor interstitial quartz filling gaps between the feldspars.
- Color: light gray to pink/salmon, locally greenish
- Luster: vitreous to pearly on feldspar cleavages
- Texture: holocrystalline, equigranular to weakly porphyritic; feldspars often show flat reflective faces
- Quartz: present but minor — clear to smoky, glassy, filling between feldspars
Step-by-Step Field ID Checklist
- Confirm it is intrusive: interlocking crystals all visible to the naked eye, no glass, no vesicles.
- Identify the dominant mineral as alkali feldspar: look for two cleavages near 90°, flat reflective faces, and perthite (fine intergrowth streaks).
- Estimate quartz content: find the glassy, cleavage-free grains. If quartz is clearly minor (a few percent up to ~20%) it is nordmarkite/quartz syenite; if quartz is abundant (>20%), reconsider granite.
- Note scarcity of dark minerals: nordmarkite is leucocratic with sparse mafics.
- Check for plagioclase: alkali feldspar should dominate; little to no chalky white plagioclase with fine striations.
Key Diagnostic Tests
- Hardness: feldspar and quartz both ~6–7; the rock scratches glass.
- Cleavage vs fracture: feldspar shows two good cleavages and step-like faces; quartz shows conchoidal fracture and no cleavage — this contrast separates the two main minerals.
- Streak: white/colorless overall.
- Acid: no reaction (silicate rock).
- Density: moderate (~2.6 g/cm³), lighter feel than gabbro or syenite rich in mafics.
Common Look-Alikes and How to Tell Them Apart
- Granite: the closest confusion. Granite has abundant quartz (>20%) and usually visible plagioclase. Nordmarkite has only minor quartz and is feldspar-dominated by alkali feldspar — estimate the glassy quartz grains; sparse quartz favors nordmarkite.
- Syenite (sensu stricto): true syenite has essentially no quartz. If you can find even a few percent of clear glassy quartz between feldspars, lean toward nordmarkite/quartz syenite.
- Larvikite: a related Oslo-region feldspathic rock, but larvikite is a monzonite/syenite famous for blue schiller (labradorescent feldspar) and is darker; nordmarkite is paler and quartz-bearing.
- Pink granite/alkali granite: distinguish by quartz abundance — alkali granite is quartz-rich.
Where Nordmarkite Is Found
Nordmarkite is the type rock of the Permian Oslo Rift (Oslo igneous province) in Norway, named for the Nordmarka district near Oslo, where it occurs in large alkaline plutons alongside larvikite and syenites. Comparable quartz-bearing alkali-feldspar syenites occur in other alkaline ring complexes and rift settings worldwide. In the field, expect it in association with other under- to slightly oversaturated alkaline rocks rather than ordinary calc-alkaline granite terrain.
Frequently asked questions
How can you tell if a rock is nordmarkite?
Look for a light-colored, coarse intrusive rock dominated by alkali feldspar (pink to gray, with two cleavages near 90°) and only a small amount of glassy quartz filling between the feldspars, plus sparse dark minerals. Minor quartz plus alkali-feldspar dominance is the signature.
What is the difference between nordmarkite and granite?
Both are coarse and feldspar-rich, but granite contains abundant quartz (over 20%) and usually noticeable plagioclase. Nordmarkite is a quartz-bearing alkali-feldspar syenite with only minor quartz, so estimating the proportion of glassy, cleavage-free quartz grains is the key test.
Is nordmarkite the same as syenite?
It is closely related. True syenite is quartz-free; nordmarkite is a quartz syenite, meaning it carries a few to about 20% quartz. Finding small glassy quartz grains between the feldspars distinguishes nordmarkite from ordinary syenite.
Where does nordmarkite come from?
It is named for the Nordmarka area near Oslo, Norway, and is a classic rock of the Permian Oslo Rift, where it occurs with larvikite and other alkaline intrusives. Similar quartz alkali-feldspar syenites occur in alkaline ring complexes elsewhere.