How to identify Larvikite

To correctly identify Larvikite (Monzonite (feldspar group)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to pearly on feldspar cleavage surfaces, duller on other minerals.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained) to porphyritic, with large, often euhedral to subhedral feldspar phenocrysts.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Feldspar crystals are typically blocky, tabular, and often well-formed, exhibiting the characteristic iridescence.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Feldspar minerals exhibit two distinct cleavages at or near 90 degrees.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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