Nordmarkite
Alkali (quartz-bearing) syenite; perthitic alkali feldspar with minor quartz and sodic mafics
A light-colored alkali quartz syenite dominated by perthitic feldspar with minor quartz, from the Oslo igneous province of Norway.
- Mohs hardness
- 6-6.5
- Color
- light gray, pinkish to reddish gray
- Type
- igneous
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Overview
Nordmarkite is a pale, coarse-grained alkali syenite that carries a small amount of quartz, placing it on the boundary between syenite and granite. It is dominated by perthitic alkali feldspar with subordinate quartz and minor sodic mafic minerals such as aegirine or alkali amphibole, plus accessory biotite.
The rock is named from Nordmarka, near Oslo, Norway, part of the famous Oslo (Permian) igneous province, a classic alkaline rift-related complex. It is closely associated with larundite/larvikite and other alkaline rocks of the region.
Nordmarkite is mainly a petrological reference rock, though as a hard, attractive feldspar rock it can serve as building or decorative stone.
Formation & geology
Nordmarkite forms by slow crystallization of alkali-rich, mildly silica-saturated magma at depth, typically in continental rift settings. The melt is rich enough in alkalis that perthitic alkali feldspar dominates, with just enough excess silica to crystallize minor quartz.
It is part of the Oslo Rift igneous province, a Permian intracontinental rift complex that also produced larvikite, rhomb porphyry lavas, and nepheline syenites. Nordmarkite occurs as plutons and larger intrusive masses within this province and in analogous alkaline provinces elsewhere.
How to identify it
Look for a light gray to pinkish, coarse-grained rock dominated by glassy to slightly iridescent perthitic feldspar, with only minor visible quartz and scattered dark sodic minerals. Hardness is fairly high (around 6-6.5) due to feldspar and quartz.
The defining feature is being an alkali syenite with just a little quartz — more feldspar-dominated than granite but with some quartz, unlike quartz-free syenite. Sodic mafics (aegirine, arfvedsonite) rather than ordinary biotite/hornblende point to its alkaline nature.
Look-alikes include syenite, alkali granite, and larvikite; the small quartz content plus perthitic alkali feldspar distinguishes nordmarkite, often confirmed under the microscope.
Uses & significance
Nordmarkite has limited commercial use, though as a hard, feldspar-rich rock it can be quarried as dimension stone, paving, or crushed aggregate where available.
Its main significance is scientific: as a named rock of the Oslo igneous province, it is important in the study of alkaline rift magmatism and the classification of quartz-bearing alkali syenites.
It has no gemstone or metaphysical tradition.
Frequently asked questions
What is nordmarkite?
An alkali syenite that contains a small amount of quartz, dominated by perthitic alkali feldspar, from the Oslo region of Norway.
Where does the name come from?
From Nordmarka near Oslo, Norway, part of the Permian Oslo igneous (rift) province.
How is nordmarkite different from granite?
It contains much less quartz than granite and is dominated by alkali feldspar, sitting between syenite and granite.
What minerals make nordmarkite alkaline?
Sodic minerals such as aegirine or alkali amphibole, along with perthitic alkali feldspar, give it its alkaline character.
Nordmarkite guides
In-depth guides for identifying, valuing, and understanding Nordmarkite.
