How to identify Granite with Mafic Inclusion

To correctly identify Granite with Mafic Inclusion (Granite (felsic igneous rock) with a mafic xenolith or enclave), 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

   Host granite: typically light-colored (white, grey, pink, red) due to quartz and feldspar. Mafic inclusions: dark-colored (black, dark grey, dark green) due to higher concentrations of mafic minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous for quartz and feldspar in the granite; dull to sub-vitreous for mafic minerals in the inclusion.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Host granite: phaneritic (coarse-grained), equigranular to porphyritic. Mafic inclusions: typically finer-grained (aphanitic to fine-grained phaneritic) than the host granite, often with a microgranular texture. The overall texture is heterogeneous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Anhedral to subhedral crystals of quartz and feldspar in granite; anhedral to subhedral crystals of plagioclase, hornblende, pyroxene, and biotite in inclusions. Mafic inclusions often have rounded or ovoid shapes.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Feldspars exhibit two good cleavages at or near 90 degrees. Micas (biotite, muscovite) exhibit one perfect basal cleavage. Amphiboles (hornblende) exhibit two cleavages at approximately 56 and 124 degrees. Pyroxenes exhibit two cleavages at approximately 90 degrees.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive igneous environments, specifically within large batholiths and plutons formed in continental arcs, collisional orogens, and post-collisional settings. Found in areas of active or past magmatism where felsic and mafic magmas coexisted or where granitic magmas assimilated country rock.

Key Facts

Hardness

6-7 (Mohs scale) for quartz and feldspar in granite; 5-6 for amphiboles and pyroxenes in inclusions.

Specific Gravity

2.6-2.7 g/cm³ for granite; 2.8-3.2 g/cm³ for mafic inclusions.

Crystal System

Quartz: Trigonal; Feldspars: Monoclinic/Triclinic; Micas: Monoclinic; Amphiboles: Monoclinic; Pyroxenes: Orthorhombic/Monoclinic.

Color

Polychromatic: light-colored host with dark-colored inclusions.

Luster

Vitreous to sub-vitreous.

Transparency

Opaque to translucent.

Fracture

Conchoidal (quartz), uneven to splintery (feldspars, mafic minerals).

Cleavage

Good to perfect in feldspars, micas, amphiboles, and pyroxenes; none in quartz.

Composition

Host granite: >20% quartz, 65-90% total feldspar (plagioclase + alkali feldspar), <20% mafic minerals. Mafic inclusion: <52% SiO2, rich in plagioclase, hornblende, pyroxene, biotite, olivine (rarely).

Physical characteristics

• Crystal Habit: Granular, anhedral to subhedral crystals in both granite and inclusions. Mafic inclusions often have a fine-grained, equigranular to porphyritic texture.
• Cleavage Type: Perfect basal (micas), good prismatic (feldspars, amphiboles, pyroxenes).
• Fracture Type: Conchoidal (quartz), uneven, splintery.
• Tenacity: Brittle.
• Luster Type: Vitreous, sub-vitreous, dull.

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