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Larvikite is an intrusive igneous rock characterized by its large, iridescent feldspar crystals, typically anorthoclase, which exhibit a distinctive schiller effect (labradorescence or peristerescence). The rock is generally dark gray to black, with flashes of blue, silver, or green from the feldspar. It is a variety of monzonite or syenite, meaning it is composed predominantly of alkali feldspar and plagioclase feldspar, with minor amounts of mafic minerals like biotite, augite, hornblende, and sometimes olivine. The texture is typically coarse-grained.
How to Identify
- Color
- Dark gray to black groundmass with prominent, large (often 1-3 cm, sometimes larger) feldspar crystals that display a characteristic blue, silver, or green iridescence (schiller effect).
- Luster
- Vitreous to pearly on feldspar cleavage surfaces, duller on other minerals.
- Texture
- Phaneritic (coarse-grained) to porphyritic, with large, often euhedral to subhedral feldspar phenocrysts.
- Crystal Form
- Feldspar crystals are typically blocky, tabular, and often well-formed, exhibiting the characteristic iridescence.
- Cleavage
- Feldspar minerals exhibit two distinct cleavages at or near 90 degrees.
- Geological Environment
- Intrusive igneous bodies (plutons) within rift-related magmatic provinces. Specifically, it is found in the Larvik Plutonic Complex of the Permian Oslo Rift, Norway.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale (due to feldspar).
- Specific Gravity: 2.65 to 2.75 g/cm³.
- Crystal System: Monoclinic (for the dominant anorthoclase feldspar).
- Color: Dark gray to black with iridescent blue, silver, or green flashes.
- Luster: Vitreous to pearly.
- Transparency: Opaque to translucent (in thin sections).
- Fracture: Conchoidal to uneven.
- Cleavage: Good in two directions at nearly 90 degrees (characteristic of feldspars).
- Composition: Predominantly anorthoclase feldspar (a solid solution of albite, orthoclase, and anorthite), with minor amounts of plagioclase, biotite, augite, hornblende, and sometimes olivine, magnetite, ilmenite, and apatite.
Quick Check
- Color: Dark gray to black with iridescent blue/silver/green flashes.
- Luster: Vitreous to pearly on feldspar.
- Streak: White (due to feldspar content).
Physical Characteristics
- Crystal Habit: Typically anhedral to subhedral granular, with large, blocky feldspar crystals often showing euhedral forms.
- Cleavage Type: Perfect to good in two directions, typically {001} and {010} for feldspars, intersecting at approximately 90 degrees.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces, with a distinctive schiller (iridescent) effect.
Formation
Larvikite forms from the slow cooling and crystallization of magma deep within the Earth's crust. It is part of the Permian Oslo Rift igneous province, specifically associated with the Larvik Plutonic Complex. The distinctive schiller effect (labradorescence/peristerescence) is due to the exsolution lamellae of albite and orthoclase within the anorthoclase feldspar crystals, which are large enough to diffract light.
Usage
Primarily used as a dimension stone for building facades, countertops, floor tiles, monuments, and decorative items due to its attractive appearance and durability. It is also used for crushed stone aggregate in some regions.
Age Distribution
Permian period, approximately 292-297 million years ago
Where to Find
Larvik Plutonic Complex, Norway
The primary and type locality for Larvikite, located in the Vestfold and Telemark counties of southeastern Norway. Extensive quarries operate in this region.
Other Rift Zones (rarely)
Similar rocks with iridescent feldspars can occur in other anorogenic rift-related igneous provinces globally, but true Larvikite, as defined by its specific mineralogy and petrogenesis, is unique to the Oslo Rift.
Finding Tips
Observe Iridescence
The most distinctive feature is the strong blue, silver, or green schiller effect (labradorescence/peristerescence) from the large feldspar crystals. Rotate the sample under light to observe this play of color.
Examine Grain Size
Larvikite is typically coarse-grained, with individual feldspar crystals often visible to the naked eye and frequently exceeding 1 cm in size.
Check for Dark Minerals
Look for minor amounts of dark, mafic minerals (biotite, augite, hornblende) interspersed within the feldspar-rich matrix.
Consider Geological Context
If found in situ, it will be part of a large intrusive igneous body, often associated with other alkaline igneous rocks.
Similar Rocks
Syenite
Syenite
Also known as: Alkali Feldspar Syenite
Monzonite
Monzonite
Also known as: Quartz Monzonite, Granodiorite (if quartz content is higher)
Labradorite
Labradorite (Plagioclase Feldspar)
Also known as: Spectrolite (a high-quality variety)
Scientific Classification
- Mineral Class
- Silicate (specifically, Tectosilicate for feldspars)
- Group
- Feldspar Group (dominant mineral is anorthoclase, a member of the alkali feldspar series)
- Crystal System
- Monoclinic (for anorthoclase)
- Chemical Formula
- Complex, primarily (Na,K)AlSi3O8 for anorthoclase, with varying amounts of CaAl2Si2O8 (anorthite component) and other minor elements in accessory minerals.
- Composition
- Larvikite is an igneous rock composed mainly of alkali feldspar (anorthoclase, often perthitic), plagioclase feldspar, and minor mafic minerals such as biotite, augite, and hornblende. Accessory minerals include magnetite, ilmenite, apatite, and zircon. The defining characteristic is the presence of large, iridescent anorthoclase crystals.
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